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Cosmology

COSMOLOGY

COSMOLOGY. During the fifteenth century, the cosmological systems of the Epicurean atomists, Plato, and the Stoics were known from antiquity, but the cosmology that was taught in universities throughout Europe was that of Aristotle, as augmented by Ptolemy. By the beginning of the eighteenth century a new cosmology, associated with the names of Copernicus, Kepler, Galileo, Descartes, and Newton, had almost completely replaced the earlier consensus. The present article considers the cosmologies of these main figures and reviews changes in historians' understanding of the causes of the scientific revolution.

ARISTOTLE'S COSMOS

Aristotle's cosmos was finite, spherical, and full. Its outer boundary was a sphere carrying the fixed stars. Its center was the Earth, and the sphere carrying the Moon divided the cosmos into a terrestrial portion and a celestial portion. The region beneath the Moon consisted of four elements, each endowed with the tendency to return to its natural place by a motion along a radius of the cosmos. The element Earth tended to seek the center; water moved naturally to a sphere surrounding the central globe of Earth; air sought a sphere concentric to water, and fire, which in its pure form was quite transparent, would naturally move to the region above the air and beneath the Moon. The general structure of the world reflected its elementary constitution, with most earth covered by water and both inner elements covered by air. Only the sphere of fire was not directly observable, although it was a theoretical necessity. Mixing and transmutation created complex combinations of elements, such as people, plants, and animals. Changes in the proportions of the four elements explained terrestrial change, especially growth and decay.

By contrast, the heavens consisted of a single element, ether, which was already in its natural place, and moved naturally in a circle, at constant speed, around the central earth. Deprived of the opportunity for transmutation or mixing of elements, the heavens were incapable of physical change. The order of the heavenly bodies was determined partly by observation and partly by convention. Eclipses and occultations made it clear that the Moon was the closest heavenly body and the fixed stars were the most distant. Mars, Jupiter, and Saturn could be ordered according to their periods of return, with the longest being the farthest away. However, the periods of return for the remaining planets and the Sun were not distinguishable. The locations of the five known planets were divided by the zone occupied by the Sun, and, beyond the Moon, an ordering of Mercury, followed by Venus, followed by the sun became conventional.

The heavens consisted of nested concentric shells. A single heavenly body was confined within and carried by each shell. Physically, the heavenly bodies were believed to be denser regions in the ether. During the fifteenth and sixteenth centuries, followers of Averroes (Ibn Rushd) and Ptolemy violently disagreed over the inner structure of these shells.

In the Almagest Ptolemy had introduced a system of moving circles carrying other circles to explain the details of planetary motion. In the Planetary Hypotheses he introduced a corresponding set of physical models, which Arabic commentators presented as sets of hollow orbs carrying smaller spheres within them. These, in turn, carried individual planets. Ptolemaic astronomers assumed that the orb clusters for different planets fitted perfectly inside one another, and were thereby able to calculate the distances of planets, including the Sun, and their relative sizes. But most importantly, Ptolemy's mathematical apparatus allowed the calculation of planetary positions with an accuracy sufficient, for example, to predict eclipses of the Sun and Moon, and approximate conjunctions and other planetary alignments important in astrology. These models were presented in Georg Peurbach's Theoricae novae planetarum (c. 1474), which rapidly became a standard text. Averroists objected to the eccentric circles and epicycles used by their rivals on the grounds that they were not strictly centered on the Earth. They proposed that planets were carried by a series of nested orbs, exactly concentric to the Earth, but, as late as the 1530s, attempts to construct predictive models failed. Copernicus was exposed to both viewpoints during his education.

THE NEW COSMOLOGIES

Motivated by a desire to establish an absolute order for the planets, Copernicus moved the center of the cosmos to the Sun (On the Revolutions of the Heavenly Spheres, 1543). In other respects, his cosmology was conservative. He continued to assume that the planets were carried by orbs and that the sphere of fixed stars was the boundary of a finite universe, although his shift of center created large and inexplicable gaps between orbs, and especially between the outermost planet, Saturn, and the fixed stars. These gaps were later explained by Kepler using the geometrical construction introduced in the Mysterium Cosmographicum (1596). The immediate reaction, led by astronomers at the Lutheran University of Wittenberg, was to adapt Copernicus's new models to an Earth-centered system and to reject his cosmology on physical and scriptural grounds.

To remove Aristotle's cosmology, it was necessary to undermine his account of the construction of the heavens. Two major factors began this process: the revival of Stoic physics and precise observations of comets. Aristotle had taught that comets, which appeared and vanished at irregular intervals, must be long-lasting fires in the region below the Moon, because there could be no change in the heavens. In 1572 a nova suggested that change did occur in the heavens. Attempts to measure comets' distances placed them in the heavens. At the same time, the revival of Stoic physics suggested that the heavens might be filled by a continuous fluid rather than Aristotle's solid spheres. Tycho Brahe in Denmark and Michael Maestlin in Germany both measured precise distances for a comet that appeared in 1577. Both concluded that the comet had moved through a series of Aristotle's Earth-centered spheres and that any spheres must be centered on the Sun. Maestlin became a Copernican, later teaching his ideas to Johannes Kepler. But Brahe was unable to accept the motion of the Earth and developed a new cosmology in which the Earth remained the center, the Moon and Sun circled the Earth, and the remaining planets circled the Sun. To avoid the overlap his system created between the orbs of Mars and the Sun, Brahe adopted fluid heavens in which celestial spheres were no more than geometrical boundaries.

Today, Johannes Kepler is credited with discovering the three laws of planetary motion that bear his name, but his innovations were not generally accepted until Isaac Newton showed that they followed from his own theory. Kepler introduced the modern concept of an orbit, located the cause of planetary motion in the Sun, and replaced the circles of traditional astronomy with ellipses, but he continued to regard the fixed stars as the boundary of a finite universe. Like Tycho, he adopted a theory that made the substance of the heavens a fluid. The unprecedented accuracy of his astronomical tables advertised the importance of his insights after his death in 1630.

Galileo Galilei, by contrast, preserved many features of traditional cosmology. He never adopted Kepler's ellipses and denied that comets were celestial objects. However, his telescopic discoveries offered a host of new observational evidence supporting Copernicus. Jupiter's moons showed that the Averroists were wrong in demanding a single center of rotation for the cosmos. Sunspots and the observation of terrestrial features on the Moon showed that the heavens were not changeless and suggested that a single physics should embrace both heavens and Earth. The cycle of phases displayed by Venus showed that it, at least, circled the Sun. It was possible to accommodate all of these innovations in a modified Aristotelian scheme (as postulated by Du Chevreul in 1623), but the motions of comets and their implications for the substance of the heavens were unaccounted for. In the climate created by the Catholic Church's condemnation of Copernicanism in 1616 and 1633, Tycho Brahe's system became the most attractive option to anyone wishing to reconcile religious orthodoxy, traditional physics, and new astronomical discoveries. Jesuits exported it to China, and it was taught in Northern European universities into the eighteenth century.

Galileo's later work helped revive the ancient theory that matter was composed of atoms, a viewpoint that was being developed by Beeckman, Gassendi, and Descartes. The latter delayed publishing an atomistic cosmology because of Galileo's condemnation. In Le Monde, finished in 1633, but not published until 1664, Descartes described a cosmos filled by vortices of atoms. Stars naturally formed at the center of each vortex, while matter falling onto their surface caused sunspots. A large enough quantity of infalling material formed a crust over the entire star, which then became free of its vortex and wandered through the heavens, appearing as a comet. When finally captured by another vortex, the comet became a planet. Descartes therefore explained many new discoveries in a single scheme that was inherently heliocentric, although the sun was now just one among many vortex centers scattered throughout space.

Newton's synthesis (1689) provided a detailed mathematical physics that unified the heavens and the Earth. The planets were now held in place not by vortices, but by universal gravitation. Comets were divided into returning and nonreturning, and the reappearance of Halley's comet in 1758 was a highly visible success. With the general acceptance of Newton's system, cosmology assumed a form that persisted until the early twentieth century. As with Descartes, the Sun was identified as a star. The planets with their attendant satellites were bound to the Sun, but were not unique; other stars were assumed to be the centers of other planetary systems. Comets were definitely celestial, although only the determination of the numerical value of Newton's Universal Gravitational Constant allowed the recognition of their diminutive mass in comparison to planets or stars. Newton's First Law required that inertial motion continue indefinitely and implied a universe that was infinite in space.

THE NATURE OF THE SCIENTIFIC REVOLUTION

The changes in cosmology just described have often been taken as the centerpiece of an event known as the scientific revolution, usually described as the replacement of Aristotle's scientific system with modern mathematical physics, based on experimental evidence. But recent historiography has tended to emphasize continuity with earlier achievements. It is now clear that the modern conception of experiment developed over a long period, with important changes beginning in the sixteenth century with the work of astronomers and early mathematical physicists. Kepler's unification of physics and mathematical astronomy became an important precedent, although it was more important with hindsight, after the development of new mathematical techniques for doing physics by Descartes, Newton, and their contemporaries. The work of Boyle and other members of the early Royal Society, as well as members of similar institutions in France and Italy, also contributed, although the modern conception of experiment did not emerge until the power of the new mathematical methods had been reconciled with the empiricism advocated by Bacon, a process that continued from Newton's career through the development of mathematical physics in France during the Enlightenment. Galileo's use of experiment resembles the earlier, rather than the later, concept. He was clearly not the originator of the experimental method, and modern research also demonstrates that his ideas on physics and scientific method in general were transformations of existing ideas rather than complete novelties.

Recent historians also give a more equal role to noncanonical sciences such as alchemy and astrology in the development of modern science. Alchemy clearly contributed to the replacement of Aristotle's theory of the terrestrial elements. Astrology remained important as the main motive for the study of astronomy and cosmology because of applications including medical diagnosis and treatment, weather prediction, and political planning. Although most practitioners followed the great Lutheran reformer and educator Philipp Melanchthon in believing that the heavens predisposed rather than compelled terrestrial events, casting horoscopes was a professional skill prized by the patrons of Tycho Brahe, Kepler, and Galileo. Alchemy was gradually transformed, first into the phlogiston theories of Stahl and his contemporaries, and then into the modern discipline of chemistry at the hands of Lavoisier. The disappearance of astrology lacks a generally agreed explanation. In England, at least, its public suppression may have had less to do with the development of the new science and new scientific societies after the Civil War than with the fact that its supporters were on the losing side after the Restoration of Charles II.

The supposed warfare between science and religion is now recognized to be largely a fiction of late-nineteenth-century historiography. Both Catholic and Protestant churches were active in supporting and sometimes opposing the new science. During the sixteenth century, for example, followers of Melanchthon arranged for the publication of Copernicus's work and actively spread his ideas, although, initially, they accepted his mathematical astronomy and rejected his cosmology. The trial of Galileo in 1633 cannot be attributed solely to his defense of Sun-centered cosmology. Other factors may include the dynamics of patronage (Galileo's patron Ciampoli offended the pope; other supporters had died) and internal church politics (the potential rebellion of a Spanish faction over the pope's handling of the Counter-Reformation). The condemnation of Copernicanism, and especially the outbreak of the Thirty Years' War in 1618, created new difficulties, but the Jesuit order of the Catholic Church remained at the forefront of scientific research. Kepler and Newton both saw their religious beliefs as integral to, rather than separable from, their scientific work.

The importance of new career paths and new scientific institutions has qualified earlier accounts of the scientific revolution. Copernicus was a lowly member of the Catholic hierarchy, who, until almost the end of his life, pursued his research essentially in private. His earliest supporters were university teachers, like Melanchthon's followers at Wittenberg and Maestlin at Tübingen. But his most important successors were courtiers whose research was supported by patronage. Tycho Brahe was financed by the king of Denmark, and later the Holy Roman emperor, who also supported his successor Kepler. Galileo moved from a university post to the court of the Medici in Florence, where he did his most important work. The first scientific societies appeared during the seventeenth century and provided new avenues of scientific communication, including published proceedings and journals, and new forms of support for scientists. In later life, Newton dominated the Royal Society of London. But the acceptance of Newton's system in Germany, and especially in France, followed the adoption of the new science as an intellectual fashion by the upper classes throughout Europe. This process depended upon the ascendancy of another social forum, the salon, where, for the first time since antiquity, women made major contributions to science.

The scientific revolution was not the work of a few great men, nor the result of changes that occurred only in the mathematical sciences, or in sciences that still exist today. It was not the result of the sudden appearance of the modern conception of experiment, nor did it come about because of any early separation between science and religion. There are profound differences between the content, method, and structure of the sciences from the origin to the close of the early modern period, but these changes are now regarded as the result of a complex combination of intellectual, theological, social, and institutional causes.

See also Alchemy ; Aristotelianism ; Astrology ; Bacon, Francis ; Boyle, Robert ; Brahe, Tycho ; Charles II (England) ; Copernicus, Nicolaus ; Descartes, René ; Enlightenment ; Galileo Galilei ; Gassendi, Pierre ; Kepler, Johannes ; Lavoisier, Antoine ; Medici Family ; Melanchthon, Philipp ; Newton, Isaac ; Scientific Revolution ; Stoicism ; Thirty Years' War (16181648)

BIBLIOGRAPHY

Primary Sources

Aiton, E. J. "Peurbach's Theoricae Novae Planetarum :A Translation with Commentary." Osiris, 2nd series, 3 (1987): 544.

Brahe, Tycho. De Mundi Eetherei Recentioribus Phaenomenis. Uraniborg, 1588. Tycho's book on comets and his new cosmic scheme.

Chevreul, Jacques du. Sphaera. Paris, 1623. Contains an Aristotelian cosmic scheme that accommodates all Galileo's telescopic discoveries. (See also Ariew, below.)

Copernicus, Nicolaus. On the Revolutions of the Heavenly Spheres. Translated by A. M. Duncan. New York, 1976. Translation of De Revolutionibus Orbium Coelestium (1543).

Galilei, Galileo. Dialogue Concerning the Two Chief World Systems, Ptolemaic & Copernican. Translated by Stillman Drake. Berkeley, 1967. English translation of Dialogo sopra i due massimi sistemi del mondo, Tolemaico e Copernicano (1632), the work for which Galileo was condemned.

Goldstein, Bernard R. "The Arabic Version of Ptolemy's Planetary Hypotheses." Transactions of the American Philosophical Society 57 (1967), Part 4. Presents Ptolemy's physical models.

Descartes, René. The World and Other Writings. Translated by Stephen Gaukroger. Cambridge, U.K., 1998. English versions of Le monde de Mr Descartes; ou, Le traite de la lumiere. Paris, 1664.

Ptolemy's Almagest. Translated by G. J. Toomer. New York, 1984. Ptolemy's main work on mathematical astronomy. (See also Goldstein, above.)

Secondary Sources

Aiton, E. J. The Vortex Theory of Planetary Motions. London, 1972. Cartesian cosmology.

Ariew, Roger. Descartes and the Last Scholastics. Ithaca, N.Y, 1999. Presents Descartes in the context of Aristotelian responses to the new philosophy and science, including the work of Du Chevreul.

Barker, Peter, and Roger Ariew, eds. Revolution and Continuity: Essays in the History and Philosophy of Early Modern Science. Washington, D.C., 1991. Appraises the alleged discontinuity between medieval and modern science.

Biagioli, Mario. Galileo Courtier: The Practice of Science in the Culture of Absolutism. Chicago, 1993.

Dear, Peter. Revolutionizing the Sciences: European Knowledge and its Ambitions, 15001700. Princeton, 2001. Sound introduction that balances the contributions of canonical and noncanonical sciences.

Densmore, Dana. Newton's Principia: The Central Argument. Santa Fe, N.M., 1995. Translation, with notes, and expanded proofs of key mathematical arguments in Principia Mathematica (1687).

Osler, Margaret J., ed. Rethinking the Scientific Revolution. Cambridge, U.K., 2000. New historiography for early modern science.

Sutton, Geoffrey V. Science for a Polite Society: Gender, Culture, and the Demonstration of Enlightenment. Boulder, Colo., 1995. The social framework of Cartesian and Enlightenment science.

Westman, Robert S. "The Astronomer's Role in the Sixteenth Century: A Preliminary Survey." History of Science 18 (1980): 105147. Classic study of the transition from university support to patronage support in early modern science.

Peter Barker

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Cosmology

Cosmology

Cosmology is the study of the origin and evolution of the universe. In the last half of the twentieth century, astronomers made enormous progress in understanding cosmology. The discovery that the universe apparently began at a specific point in time and has continued to evolve ever since is one of the most revolutionary discoveries in science.

The History of the Universe: In the Beginning

The universe began in what astronomers dubbed the "Big Bang"an initial event, after which the universe began to expand. Current estimates place the Big Bang at about 13 to 15 × 109 years ago. During the first seconds after the Big Bang, the universe was extremely hot and dense. The physics needed to understand the universe in these early stages is very speculative because it is impossible to recreate these conditions in an experiment today to check the predictions of the theory. Before 10-44 seconds after the Big Bang, the four fundamental forces of naturegravity, the electromagnetic force, and the strong and weak nuclear forceswere unified into a single force. At 10-44 seconds, gravity separated from the others; at 10-34 seconds, the strong force became separated; and at 10-11 seconds, the weak force separated from the electromagnetic force.

During this period the universe began a sudden burst of exponential expansionfaster than the speed of light. This expansion is called "inflation" and explains why the universe we observe is so uniform. Temperatures were so hot (1027 K) before inflation that the familiar particles that make up atoms today (protons and neutrons ) were not stablethe universe was a hot soup of quarks (particles that are hypothesized to make up baryons), leptons (electrons and neutrinos), photons, and other exotic particles.

The History of the Universe: Formation of the Elements, Stars, and Galaxies, and the Cosmic Microwave Background

As the universe expanded after inflation it continued to cool. For the first three minutes conditions everywhere were similar to those at the center of stars today, and fusion of protons into deuterium, helium, and lithium took place. Most of the helium we see today in stars is believed to have been produced during these early minutes. The universe was an extremely opaque plasma, and photons dominated the mass density and dynamical evolution of the universe. When the universe cooled sufficiently to allow the free electrons to recombine with the hydrogen and helium nuclei, suddenly the opacity dropped, and the photons were free to stream through space unimpeded. These photons are seen today as the cosmic microwave background, a bath of light that is seen in all directions today. The experimental detection of the cosmic microwave background was one of the great triumphs of the Big Bang theory. Recombination and the subsequent production of the cosmic microwave background occurred about 180,000 years after the Big Bang.

At this point the matter distribution of the universe was still fairly uniform, with only small density fluctuations from place to place. As the universe expanded, the slightly overdense regions began to collapse. Sheets and filaments in the gas formed, which drained into dense clumps where star formation began. Eventually, these protogalactic fragments merged and galaxies and quasars formed. The universe began to look like it does today.

The Future of the Universe: Einstein's Biggest Blunder or Most Amazing Prediction?

Cosmologists predict the future of the universe as well as study its past. Whether the universe will expand forever or eventually slow down, turn around, and recollapse depends on how fast the galaxies are moving apart today and how much gravity there is to counter the expansionquantities that in principle can be measured.

German-born American physicist Albert Einstein (1879-1955) described the modern theory of gravity, general relativity. He used the idea that space could be curved to reformulate English physicist and mathematician Isaac Newton's (1642-1727) theory of gravity. In general relativity, the mass of an object curves the space around it, and parallel lines no longer go on forever without intersecting. In many textbooks the curvature of space is represented by a sphere or a saddle shapebut in reality, space is three-dimensional, and the "curvature" is not in a particular direction. Einstein wrote down what are called "field equations" that described how the curvature of space can be calculated from mass and energy. When he solved the equations he realized that even if the universe is infinite, isotropic (the same in all directions), and homogeneous (the same density everywhere), it would not be static. Depending on the geometry, it would expand or contract. American astronomer Edwin P. Hubble (1889-1953) had not yet discovered that the universe expands, so in 1917 Einstein added a "parameter" lambda, called the cosmological constant, to the field equations. Later, when Hubble showed that the universe is expanding, and that there was no need to add a cosmological constant to the field equations, Einstein called the cosmological constant "the biggest blunder of my life."

Were Einstein alive to day, he would be amazed to learn about recent observations that suggest that the cosmological constant is not zero and that the expansion is accelerating. In this case, the curvature of space is not so easily related to the dynamical evolution of the universe. At the beginning of the twenty-first century, theorists had not come up with a theory for the origin of a non-zero lambda that has testable predictions. Certainly, more observations are called for to confirm or refute this result.

Nonetheless, the conditions in the universe in the distant future can be described, given the physics that is understood today. If the universe is closed, then the Hubble expansion will eventually stop, and the universe will then collapse. If the density of the universe is, for the sake of argument, about twice the critical density for closing the universe, then the expansion stops about 50 billion years after the Big Bang. At about 85 billion years after the Big Bang, the density of the universe will again be about what it is today. At this point, the nearby galaxies will appear to move toward us, more distant galaxies will be standing still, and the very distant galaxies will be moving away. Eventually, the galaxies will all touch, and the universe will continue to contract and heat. Soon the stars will be cooler than the universe as a whole, so radiation will not be able to flow out of them, and they will explode. As a result, 100 billion years after the Big Bang will come the big crunch. At this point the universe may become a black holeor it may bounce, and cycle again.

If the universe is open or flat, the Hubble expansion goes on forever. Physical processes that take such a long time that they are irrelevant in today's universe will eventually have time to occur. After 1 trillion (1012) years, star formation will have used up all the available gas, and no new stars will form. Stellar remnants such as white dwarfs, neutron stars , and black holes will remain. After 1018 years, galaxies will evaporatetheir stars will disperse into space. After 1040 years, protons and neutrons will decay into positrons and electrons. After that, only black holes will exist. The black holes will eventually evaporate by Hawking radiation. At 10100 years after the Big Bang, all of the black holes, even the supermassive ones in quasars, will be gone. The universe will be very black and cold indeed.

Conclusion

The questions asked by cosmologists are some of the most simple and yet most profound questions intelligent creatures can ask. What is the origin of this beautiful and complex universe we live in, and what is its ultimate fate? Amazing progress was made over the last hundred years in cosmology, but clearly many important parts of the story are yet to be discovered.

see also Age of the Universe (volume 2); Einstein, Albert (volume 2); Galaxies (volume 2); Hubble Constant (volume 2); Hubble, Edwin P. (volume 2); Shapley, Harlow (volume 2); What is Space? (volume 2).

Jill Bechtold

Bibliography

Guth, Alan H., and Alan P. Lightman. The Inflationary Universe: The Quest for a New Theory of Cosmic Origins. Reading, MA: Addison-Wesley Publishing, 1998.

Hogan, Craig J., and Martin Rees. The Little Book of the Big Bang: A Cosmic Primer. New York: Copernicus Books, 1998.

Livio, Mario, and Allan Sandage. The Accelerating Universe: Infinite Expansion, the Cosmological Constant, and the Beauty of the Cosmos. New York: John Wiley & Sons, 2000.

Rees, Martin J. Just Six Numbers: The Deep Forces that Shape the Universe. New York:Basic Books, 2001.

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Cosmology

Cosmology

Cosmology is the study of the origin, evolution, and structure of the universe. This science grew out of mythology, religion, and simple observations and is now grounded in mathematical theories, technological advances, and space exploration.

Ancient astronomers in Babylon, China, Greece, Italy, India, and Egypt made observations without the assistance of sophisticated instruments. One of their first quests was to determine Earth's place in the universe. In a.d. 100, Alexandrian astronomer Ptolemy suggested that everything in the solar system revolved around Earth. His theory, known as the Ptolemaic system (or geocentric theory), was readily accepted (especially by the Christian Church) and remained largely unchallenged for 1,300 years.

A Sun-centered solar system

In the early 1500s, Polish astronomer Nicolaus Copernicus (14731543) rose to challenge the Ptolemaic system. Copernicus countered that the Sun was at the center of the solar system with Earth and the other planets in orbit around it. This sun-centered theory, called the Copernican system (or heliocentric theory), was soon supported with proof by Danish astronomer Tycho Brahe (15461601) and German astronomer Johannes Kepler (15711630). This proof consisted of careful calculations of the positions of the planets. In the early 1600s, Kepler developed the laws of planetary motion, showing that the planets follow an ellipse, or an oval-shaped path, around the Sun. He also pointed out that the universe was bigger than previously thought, although he still had no idea of its truly massive size.

The first astronomer to use a telescope to study the skies was Italian Galileo Galilei (15641642). His observations, beginning in 1609, supported the Copernican system. In the late 1600s, English physicist Isaac Newton (16421727) introduced the theories of gravity and mass, explaining how they are both responsible for the planets' motion around the Sun.

Over the next few centuries, astronomers and scientists continued to make additions to people's knowledge of the universe. These included the discoveries of nebulae (interstellar clouds) and asteroids (small, rocky chunks of matter) and the development of spectroscopy (the process of breaking down light into its component parts).

Modern cosmology

During the first two decades of the twentieth century, physicists and astronomers looked beyond our solar system and our Milky Way galaxy, forming ideas about the very nature of the universe. In 1916, German-born American physicist Albert Einstein (18791955) developed the general theory of relativity, which states that the speed of light is a constant and that the curvature of space and the passage of time are linked to gravity. A few years later, Dutch astronomer Willem de Sitter (18721934) used Einstein's theory to suggest that the universe began as a single point and has continued to expand.

Words to Know

Asteroid: Relatively small, rocky chunk of matter that orbits the Sun.

Copernican system: Theory proposing that the Sun is at the center of the solar system and all planets, including Earth, revolve around it.

Gravity: Force of attraction between objects, the strength of which depends on the mass of each object and the distance between them.

Light-year: Distance light travels in one solar year, roughly 5.9 trillion miles (9.5 trillion kilometers).

Mass: Measure of the total amount of matter in an object.

Nebula: Cloud of interstellar gas and dust.

Ptolemaic system: Theory proposing that Earth is at the center of the solar system and the Sun, the Moon, and all the planets revolve around it.

Radiation: Energy in the form of waves or particles.

Spectroscopy: Process of separating the light of an object (generally, a star) into its component colors so that the various elements present within that object can be identified.

Speed of light: Speed at which light travels in a vacuum: 186,282 miles (299,728 kilometers) per second.

In the 1920s, American astronomer Edwin Hubble (18891953) encountered observable proof that other galaxies exist in the universe besides our Milky Way. In 1929, he discovered that all matter in the universe was moving away from all other matter, proving de Sitter's theory that the universe was expanding.

Creation of the universe

Astronomers have long been interested in the question of how the universe was created. The two most popular theories are the big bang theory and the steady-state theory. Belgian astrophysicist Georges-Henri Lemaître (18941966) proposed the big bang theory in 1927. He suggested that the universe came into being 10 to 15 billion years ago with a big explosion. Almost immediately, gravity came into being, followed by atoms, stars, and galaxies. Our solar system formed 4.5 billion years ago from a cloud of dust and gas.

In contrast, the steady-state theory claims that all matter in the universe has been created continuously, a little at a time at a constant rate, from the beginning of time. The theory, first elaborated in 1948 by Austrian-American astronomer Thomas Gold, also states that the universe is structurally the same all over and has been forever. In other words, the universe is infinite, unchanging, and will last forever.

Astronomers quickly abandoned the steady-state theory when microwave radiation (energy in the form of waves or particles) filling space throughout the universe was discovered in 1964. The existence of this radiationcalled cosmic microwave backgroundhad been predicted by supporters of the big bang theory. In April 1992, NASA (National Aeronautics and Space Administration) announced that its Cosmic Background Explorer (COBE) satellite had detected temperature fluctuations in the cosmic microwave background. These fluctuations indicated that gravitational disturbances existed in the early universe, which allowed matter to clump together to form large stellar bodies such as galaxies and planets. This evidence all but proves that a big bang is responsible for the expansion of the universe.

Continued discoveries

At the end of the twentieth century, astronomers continued to revise their notion of the size of the universe. They repeatedly found that it is larger than they thought. In 1991, astronomers making maps of the universe discovered great "sheets" of galaxies in clusters and super-clusters filling areas hundreds of millions of light-years in diameter. They are separated by huge empty spaces of darkness, up to 400 million light-years across. And in early 1996, the Hubble Space Telescope photographed at least 1,500 new galaxies in various stages of formation.

In the late 1990s, while studying a certain group of supernovas, astronomers discovered that older objects in the group were receding at a speed similar to younger objects. In a "closed" universe, the expansion of the universe should slow down as it ages. Older supernovas should be receding more rapidly than younger ones. This is the theory that astronomers used to put forth: that at some future point the universe would stop expanding and then close back in on itself, an inverted big bang. However, with this recent finding, astronomers tend to believe that the universe is "open," meaning that the universe will continue its outward expansion for billions of years until everything simply burns out.

Creationism

Creationism is a theory about the origin of the universe and all life in it. Creationism holds that Earth is perhaps less than 10,000 years old, that its physical features (mountains, oceans, etc.) were created as a result of sudden calamities, and that all life on the planet was miraculously created as it exists today. It is based on the account of creation given in the Old Testament of the Bible.

Because Creationism is not based on any presently held scientific principles, members of the scientific community dismiss it as a possible theory on how the universe was created. However, people who strongly believe in Creationism feel that it should be taught as a part of science education. The heated debate between the two sides continues.

[See also Big bang theory; Dark matter; Doppler effect; Galaxy; Redshift; Relativity, theory of ]

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cosmology

cosmology, area of science that aims at a comprehensive theory of the structure and evolution of the entire physical universe.

Modern Cosmological Theories

Present models of the universe hold two fundamental premises: the cosmological principle and the dominant role of gravitation. Derived by Hubble, the cosmological principle holds that if a large enough sample of galaxies is considered, the universe looks the same from all positions and in all directions in space. The second point of agreement is that gravitation (or an antigravitation force, called dark energy) is the most important force in shaping the universe. According to Einstein's general theory of relativity, which is a geometric interpretation of gravitation, matter produces gravitational effects by actually distorting the space about it; the curvature of space is described by a form of non-Euclidean geometry. A number of cosmological theories satisfy both the cosmological principle and general relativity. The two main theories are the big-bang hypothesis and the steady-state hypothesis, with many variations on each basic approach.

The Steady-State Theory

According to the steady-state theory, now of historical interest only, the universe expands, but new matter is continuously created at all points in space left by the receding galaxies. The theory implies that the universe has always expanded, with no beginning or end, at a uniform rate and that it always will expand and maintain a constant density.

The Big-Bang Theory

According to big-bang theories, at the beginning of time, all of the matter and energy in the universe was concentrated in a very dense state, from which it "exploded," with the resulting expansion continuing until the present. This "big bang" is dated between 10 and 20 billion years ago, most likely c.13.75 billion years ago. In this initial state, the universe was very hot and contained a thermal soup of quarks, electrons, photons, and other elementary particles. The temperature rapidly decreased, falling from 1013 degrees Kelvin after the first microsecond to about one billion degrees after three minutes. As the universe cooled, the quarks condensed into protons and neutrons, the building blocks of atomic nuclei. Some of these were converted into helium nuclei by fusion; the relative abundance of hydrogen and helium is used as a test of the theory. After many millions of years the expanding universe, at first a very hot gas, thinned and cooled enough to condense into individual galaxies and then stars.

Several spectacular discoveries since 1950 have shed new light on the problem. Optical and radio astronomy complemented each other in the discovery of the quasars and the radio galaxies. It is believed that the energy reaching us now from some of these objects was emitted not long after the creation of the universe. Further evidence for the big-bang theory was the discovery in 1965 that a cosmic background noise is received from every part of the sky. This background radiation has the same intensity and distribution of frequencies in all directions and is not associated with any individual celestial object. It has a blackbody temperature of 2.7°K (-270°C) and is interpreted as the electromagnetic remnant of the primordial fireball, stretched to long wavelengths by the expansion of the universe. More recently, the analysis of radiation from distant celestial objects detected by artificial satellites and orbiting observatories has given additional evidence for the big-bang theory.

Development of Modern Cosmology

The earliest pre-Ptolemaic theories assumed that the earth was the center of the universe (see Ptolemaic system). With the acceptance of the heliocentric, or sun-centered, theory (see Copernican system), the nature and extent of the solar system began to be realized. The Milky Way, a vast collection of stars separated by enormous distances, came to be called a galaxy and was thought to constitute the entire universe with the sun at or near its center. By studying the distribution of globular star clusters the American astronomer Harlow Shapley was able to give the first reliable indication of the size of the galaxy and the position of the sun within it. Modern estimates show it to have a diameter of about 100,000 light-years with the sun toward the edge of the disk, about 28,000 light-years from the center.

During the first two decades of the 20th cent. astronomers came to realize that some of the faint hazy patches in the sky, called nebulae, are not within our own galaxy, but are separate galaxies at great distances from the Milky Way. Willem de Sitter of Leyden suggested that the universe began as a single point and expands without end. After studying the red shift (see Doppler effect) in the spectral lines of the distant galaxies, the American astronomers Edwin Hubble and M. L. Humason concluded that the universe is expanding, with the galaxies appearing to fly away from each other at great speeds. According to Hubble's law, the expansion of the universe is approximately uniform. The greater the distance between any two galaxies, the greater their relative speed of separation.

At the end of the 20th cent. the study of very distant supernovas led to the belief that the cosmic expansion was accelerating. To explain this cosmologists postulated a repulsive force, dark energy, that counteracts gravity and pushes galaxies apart. It also appears that the universe has been expanding at different rates over its cosmic history. This led to a variation of the big-bang theory in which, under the influence of gravity, the expansion slowed initially and then, under the influence of dark energy, suddenly accelerated. It is estimated that this "cosmic jerk" occurred five billion years ago, about the time the solar system was formed. This theory postulates a flat, expanding universe with a composition of c.70% dark energy, c.30% dark matter, and c.0.5% bright stars.

A number of questions must be answered, however, before cosmologists can establish a single, comprehensive theory. The expansion rate and age of the universe must be established. The nature and density of the missing mass, the dark matter and dark energy that is far more abundant than ordinary, visible matter, must be identified. The total mass of the universe must be determined to establish whether it is sufficient to support an equilibrium condition—a state in which the universe will neither collapse of its own weight nor expand into diminishing infinity. Such an equilibrium is called "omega equals one," where omega is the ratio between the actual density of the universe and the critical density required to support equilibrium. If omega is greater than one, the universe would have too much mass and its gravity would cause a cosmic collapse. If omega is less than one, the low-density universe would expand forever. Today the most widely accepted picture of the universe is an omega-equals-one system of hundreds of billions of galaxies, many of them clustered in groups of hundreds or thousands, spread over a volume with a diameter of at least 10 billion light-years and all receding from each other, with the speeds of the most widely separated galaxies approaching the speed of light. On a more detailed level there is great diversity of opinion, and cosmology remains a highly speculative and controversial science.

Bibliography

See D. W. Sciama, Modern Cosmology and the Dark Matter Problem (1993); J. D. Barrow, The Origin of the Universe (1994); P. Coles and F. Lucchin, Cosmology: The Origin and Evolution of Cosmic Structure (1995); M. S. Longair, Our Evolving Universe (1996); B. Green, The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory (2000); S. Hawking, The Universe in a Nutshell (2001); R. P. Kirshner, The Extravagant Universe: Exploding Stars, Dark Energy, and Accelerating Cosmos (2002); S. Singh, Big Bang: The Origin of the Universe (2005).

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Cosmology

Cosmology (Gk., kosmos + logos). Reflection on, and account of the world/universe as a meaningful whole, as embodying or expressing an order or underlying structure that makes sense: cosmogony is concerned with the coming into being of the cosmos, and cosmography with the description of its extent.

It is rare for religions to give a single cosmology or cosmogony purporting to be a description of the origin of the universe, in the way in which a scientific cosmology might aim to give a critically realistic account of the origin and nature of the universe. Religious cosmologies give accounts of origin and nature, but principally in order to display the cosmos as an arena of opportunity; and for that reason, a religion may offer, or make use of, many cosmogonies without making much attempt to reconcile the contradictions between them. It is this aesthetic and spiritual relaxation which allows religions to address cosmological issues from the point of view of accountability and responsibility (as at the present time over issues of ecology), not as competitors with a scientific account: thus the Vancouver Assembly of the World Council of Churches (see ECUMENISM) decided ‘to engage member churches in a conciliar process of mutual commitment (covenant) to justice, peace and the integrity of creation (subsequently known as JPIC)’; while this (especially the word ‘covenant’) depends on a particular understanding of creation, and thus of cosmogony, it has moved far beyond concerns about identifying the ‘correct’ account of the cosmos and its origins.

Judaism

Tanach (Jewish scripture) contains at least six different types of creation narrative, all of which are integrated to the overriding cult of Yahweh. The controlling accounts are those in Genesis: God created everything that exists in six days and rested on the seventh (1–2. 4). A second, more anthropocentric account (Genesis 2. 4–24), although differing in detail, also emphasizes that God is the origin of everything. The world is created solely in obedience to the divine will.

Christianity

Christians inherited the Jewish cosmology, but virtually from the outset (as early as Paul's letters) they associated Christ with the activity of the Father in creation. Furthermore, creation now has its end and purpose in him. Not surprisingly, therefore, Christian interest in cosmology and creation has seen them as a matter, not of technique, but of relationship—i.e. the relation of dependence which the created order has on its creator, not just for its origin, but for its sustenance. Thus God is the cause, not simply of things coming to be, but also of their being. The prevailing cosmography for millennia was one of a ‘three-decker’ universe (heaven above, earth in the middle, and hell below), but its ‘correction’ by modern cosmologies has not affected the more fundamental point of the earlier (or of any) religious cosmology which mapped the universe as an arena of opportunity. For that reason, a three-decker universe may well persist indefinitely in liturgy.

Islam

The Qurʾān strongly affirms God as creator and disposer of all that is. By a simple word, kun (‘Be’), he commands and it is (2. 117, 6. 73). God is al-Khāliq (the Creator, from khalaqa, ‘he created’), and has the power and authority to bring about all things as he disposes (qadar, Allāh). Everything that he has created is a sign (ayā), not only of God for those who have eyes to see, but also that God has power to continue his creative act in relation to humans by bringing them from the grave for judgement (e.g. 50. 6–11). The creation of a first man and first woman, and of the earth and seven heavens in two days, and of the cosmos in six, is described in such a way that, given the nature of the Qurʾān, any apparent conflict with other accounts (e.g. in the natural sciences) would have to be resolved in favour of the Qurʾān.

Hinduism

Vedic religion displays a clear sense of an ordered universe in which ṛta prevails. There are many different accounts of how the universe came into being, some implying agency, others emanation from a pre-existing state in which there is neither beginning nor end. Thus Śaṅkara understood the emanation as a progress from the subtle to the gross constituents of the world. But earlier than that, there had developed a sense of an unending process like a wave, with elements rising up into organized appearance, but then lapsing into a corresponding trough during ‘the sleep of Brahmā’, a period of dissolution (pralaya). It was thus possible that the cosmos arose from infinite space and consciousness, a belief expressed through Aditi. In truth, Indian religion accepted that the origin of the cosmos could not be known, but that the conditions of ordered life could be extremely well known. Cosmology lays out the terms for achieving that understanding—cosmology, again, as the arena of opportunity—while remaining agnostic about detail.

There was a greater confidence in cosmography. Vertically, the world was understood to be made up of seven continents (dvipas), ranged in circles with intervening oceans around the central point of Mount Meru. Vertically, if one takes a cross-section of the Brahmāṇḍa, one finds a series of layers. At the top are the lokas of the gods and high attainers; next are the planets, sky, and earth; then the underworlds, and finally the twenty-eight narakas or hells. See also CARDINAL DIRECTIONS.

Jainism and Buddhism

The Indian scepticism about the work of the gods or God in creating this cosmos was taken to a further extreme in both Jainism and Buddhism. The Jains inherited the triloka (see LOKA), and envisaged it as something like an hour-glass, squeezed in at the middle. Above (Urdhvaloka) are a series of heavens of increasing brightness, at the top of which is ‘the slightly curved place’ (Iśatpragbhara) where dwell the liberated and disembodied souls. In the middle is the Madhyaloka, which includes the continent inhabited by humans. Below is the Adholoka, a series of increasingly terrible hells—from which release is eventually certain, though the intervening time may be unimaginably long.

Buddhism inherited the same basic cosmography, but adapted it greatly. It envisages a series of levels, all of which are open to the process of reappearance: at the summit are the four realms of purely mental rebirth, (arūpaavacara); below them are the realms of pure form (rūpa-avacara), where the gods dwell in sixteen heavens, five of which are known as ‘pure abodes’ (suddhāvāsa), the remaining eleven of which arise out of the jhānas (meditational states). Lower still are the sense-desire heavens, including those of the Tāvatiṃsa gods (the thirty-three Vedic gods, the chief of whom, Indra, known as Sakka, has become a protector of Buddhism) and of the Tusita gods (where bodhisattvas spend their penultimate birth, and in which Maitreya now dwells). The world is simply a process, passing through cycles (kappa) of immense length.

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Cosmology

Cosmology

Cosmology is the study of the origin, structure and evolution of the universe.

The origins of cosmology predate the human written record. The earliest civilizations constructed elaborate myths and folk tales to explain the wanderings of the Sun , Moon, and stars through the heavens. Ancient Egyptians tied their religious beliefs to celestial objects and Ancient Greek and Roman philosophers debated the composition and shape of the Earth and the Cosmos. For more than 13 centuries, until the Scientific Revolution of the sixteenth and seventeenth centuries, the Greek astronomer Ptolemy's model of an Earth-centered Cosmos composed of concentric crystalline spheres dominated the Western intellectual tradition.

Polish astronomer Nicolaus Copernicus' (14731543) reassertion of the once discarded heliocentric (Sun-centered) theory sparked a revival of cosmological thought and work among the astronomers of the time. The advances in empiricism during the early part of the Scientific Revolution, embraced and embodied in the careful observations of Danish astronomer Tycho Brahe (15461601), found full expression in the mathematical genius of the German astronomer Johannes Kepler (15711630) whose laws of planetary motion swept away the need for the errant but practically useful Ptolemaic models. Finally, the patient observations of the Italian astronomer and physicist Galileo, in particular his observations of moons circling Jupiter and of the phases of Venus, empirically laid to rest cosmologies that placed Earth at the center of the Cosmos.

English physicist and mathematician Sir Isaac Newton's (16421727), important Philosophiae Naturalis Principia Mathematica (Mathematical principles of natural philosophy) quantified the laws of motion and gravity and thereby enabled cosmologists to envision a clockwork-like universe governed by knowable and testable natural laws. Within a century of Newton's Principia, the rise of concept of a mechanistic universe led to the quantification of celestial dynamics, that, in turn, led to a dramatic increase in the observation, cataloging, and quantification of celestial phenomena. In accordance with the development of natural theology, scientists and philosophers debated conflicting cosmologies that argued the existence and need for a supernatural God who acted as "prime mover" and guiding force behind a clockwork universe. In particular, French mathematician Pierre Simon de Laplace (17491827) argued for a completely deterministic universe, without a need for the intervention of God. Most importantly to the development of modern cosmology, Laplace asserted explanations for celestial phenomena as the inevitable result of time and statistical probability.

By the dawn of the twentieth century, advances in mathematics allowed the development of increasingly sophisticated cosmological models. Many advances in mathematics pointed toward a universe not necessarily limited to three dimensions and not necessarily absolute in time. These intriguing ideas found expression in the intricacies of relativity and theory that, for the first time, allowed cosmologists a theoretical framework upon which they could attempt to explain the innermost workings and structure of the universe both on the scale of the subatomic world and on the grandest of galactic scales.

As direct consequence of German-American physicist Albert Einstein's (18791955) relativity theory , cosmologists advanced the concept that space-time was a creation of the universe itself. This insight set the stage for the development of modern cosmological theory and provided insight into the evolutionary stages of stars (e.g., neutron stars, pulsars, black holes, etc.) that carried with it an understanding of nucleosythesis (the formation of elements) that forever linked the physical composition of matter on Earth to the lives of the stars.

Twentieth-century progress in cosmology has been marked by corresponding and mutually beneficial advances in technology and theory. American astronomer Edwin Hubble's (18891953) discovery that the universe is expanding, Arno A. Penzias and Robert W. Wilson's observation of cosmic background radiation, and the detection of the elementary particles that populated the very early universe all proved important confirmations of the Big Bang theory . The Big Bang theory asserts that all matter and energy in the universe, and the four dimensions of time and space were created from the primordial explosion of a singularity of enormous density, temperature , and pressure.

During the 1940s Russian-born American cosmologist and nuclear physicist George Gamow (19041968) developed the modern version of the big bang model based upon earlier concepts advanced by Russian physicist Alexander (Aleksandr Aleksandrovich) Friedmann (also spelled as Fridman, 18881925) and Belgian astrophysicist and cosmologist Abbé Georges Lemaître (18941966). Big bang based models replaced static models of the universe that described a homogeneous universe that was the same in all directions (when averaged over a large span of space) and at all times. Big bang and static cosmological models competed with each other for scientific and philosophical favor. Although many astrophysicists rejected the steady state model because it would violate the law of mass-energy conservation, the model had many eloquent and capable defenders. Moreover, the steady model was interpreted by many to be more compatible with many philosophical, social and religious concepts centered on the concept of an unchanging universe. The discovery of quasars and of a permeating cosmic background radiation eventually tilted the cosmological argument in favor of big bang-based models.

Technology continues to expand the frontiers of cosmology. The Hubble Space Telescope has revealed gas clouds in the cosmic voids and beautiful images of fledgling galaxies formed when the universe was less than a billion years old. Analysis of these pictures and advances in the understanding of the fundamental constituents of nature continue to keep cosmology a dynamic discipline of physics and the ultimate fusion of human scientific knowledge and philosophy.

See also Cosmic microwave background radiation; Stellar life cycle

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Cosmology

Cosmology


Many, perhaps all, early cosmologies or descriptions of the structure of the world were anthropocentric (focused on the role and fate of human beings) and they envisioned a universe subject to whims of gods. As such, cosmology and religion were closely intertwined.

From the ancient Greeks through the Middle Ages, over some two millennia, the geocentric cosmology or worldview of Aristotle (384322 b.c.e.) dominated much of the Western intellectual world. Circular and unalterable heavens rotated around the Earth, which was motionless in the center of the one and only world. Created during roughly the same period and in the same regions of the world, Aristotelian philosophy and Biblical accounts of cosmology and cosmogony are, not surprisingly, congruent in some respects. Aristotle's teleological explanations assumed that the world was fulfilling a purpose formed by a superhuman mind; Christian philosophy also is inherently meaningful and purposive.

During the Middle Ages, Aristotelian cosmology was subordinated to religious concerns. In the sixteenth century Nicolaus Copernicus (14731543) displaced the Earth, though not the solar system, from the center of the universe, and increasingly from the center of God's attention as well. In the seventeenth century Galileo Galilei (15641642) destroyed Aristotelian cosmology. The subsequent mechanical cosmology of Isaac Newton (16421727), though initially requiring God's intervention to keep the planets circling the sun, eventually replaced God completely with the universal law of gravity.

Early in the twentieth century, the American astronomer Harlow Shapley (18851972) showed that the solar system is not at the center of our galaxy, but off to the side, and that our galaxy is many times larger than previously contemplated. A few years later, Edwin Hubble (18891953) showed that our galaxy is but one of many island universes, and that the acentric universe is expanding. Each new cosmological discovery displaced humankind farther from the center of the universe and seemed to render humans less significant in an increasingly immense universe.

A contemporary resurgence of dialogue between scientific cosmology and religious thought late in the twentieth century involved yet another version of the traditional design argument for God. The Anthropic Principle noted that values of the fundamental constants of nature (the speed of light, Planck's constant, etc.) and the fundamental physical laws are "fine-tuned" to precisely what is needed for the evolution of life. As with earlier cosmologically based arguments for the existence of God, the Anthropic Principle has proven highly vulnerable to theory-change in science. The inflationary Big Bang cosmological model now explains much fine-tuning without recourse to God.

The history of the relationship between cosmology and religion, particularly in Western thought, has been enlivened by changes in cosmological understanding and beliefs. As the Earth has been increasingly displaced from the center of the universe and observed phenomena have been increasingly brought under the rule of natural physical laws, humankind's relationship with and understanding of God has required revisions.

See also Anthropic Principle; Biblical Cosmology; Big Bang Theory; Big Crunch Theory; Feminist Cosmology; Galileo Galilei; Geocentrism


Bibliograpy

danielson, dennis richard. the book of the cosmos: imagining the universe from heraclitus to hawking. cambridge, mass.: perseus, 2000.

gribbin, john. companion to the cosmos. london: weidenfeld & nicolson, 1996.

hetherington, norriss s. encyclopedia of cosmology: historical, philosophical, and scientific foundations of modern cosmology. new york and london: garland, 1993

north, john. the norton history of astronomy and cosmology. new york and london: norton, 1995.

norriss hetherington

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Cosmology

100. Cosmology

See also 25. ASTRONOMY ; 318. PLANETS ; 387. SUN .

cosmism
a 19th-century theory about cosmic evolution, developed from contemporary science, that regards the cosmos as self-existent and self-acting. cosmist , n.
cosmogony
1. a theory about the origin and the evolution of the universe.
2. the branch of astrophysics that studies the origin and evolution of specific astronomical systems and the universe as a whole.
3. cosmology. cosmogonist , n. cosmogonic , adj.
cosmography
1. the branch of astronomy that maps and describes the main features of the universe.
2. a description or representation of the main features of the universe. cosmographer , n. cosmographic , cosmographical, adj.
cosmology
1. the branch of astronomy that studies the overall structure of the physical universe.
2. the branch of philosophy that studies the origin, structure, and evolution of the universe, especially such characteristics as space, time, causality, and freedom. cosmologic, cosmological , adj. cosmologist , n.
cosmotheism
the concept that the universe and God are identical; pantheism. cosmotheist , n.
cosmozoism
the concept of the cosmos as alive.
creationism
the belief concerning the creation by a transcendant God of the universe, matter, and living organisms out of nothing. creationist , n.
geocentricism
1. the concept that the earth is the center of the universe.
2. Astronomy. the measurements or observations that are relative to the center of the earth. geocentric , adj.
heliocentricism
1. the concept that the sun is the center of the universe.
2. Astronomy. the measurements or observations that are relative to the center of the sun. Also heliocentricity. heliocentric , adj.
pancosmism
the theory that the totality of existence comprises only the physical universe in time and space. pancosmic , adj.
Pansatanism
a Gnostic theory that considered Satans to be the controlling will of the universe.
Spencerianism
the philosophical theory of Herbert Spencer that cosmic evolution is cyclic, controlled by mechanical forces which tend toward equilibrium and relative complexity until a peak is reached, after which dissolution occurs, the universe reverts to a simple state, and the cycle begins again. Spencerian , n., adj.
teleologism
the belief that purpose and design control the development of the universe and are apparent through natural phenomena. teleologist , n. teleology , n.
universology
the science of the universe. universologist , n.

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cosmology

cosmology The study of the origin and evolution of the universe. The current big bang cosmology derives the observable universe from a singular event 15–20 billion years ago. Previous hypotheses include the steady-state theory, in which the expansion of the universe was due to the continuous creation of matter. The most celebrated of earlier world views was the Ptolemaic system, in which the Earth was the centre of the universe; this was superseded by the Copernican revolution, which displaced the Earth from its central position.

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cosmology

cosmology Branch of scientific study that brings together astronomy, mathematics and physics in an effort to understand the make-up and evolution of the universe. Once considered the province of theologians and philosophers, it is now an all-embracing science, which has made great strides in the 20th century. The discovery by Edwin Hubble in the 1920s that galaxies are receding from each other promoted the Big Bang theory. Associated with this is the oscillating Universe theory. The other main theory is the steady-state theory.

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cosmology

cos·mol·o·gy / käzˈmäləjē/ • n. (pl. -gies) the science of the origin and development of the universe. ∎  an account or theory of the origin of the universe. DERIVATIVES: cos·mo·log·i·cal / ˌkäzməˈläjikəl/ adj. cos·mol·o·gist / -jist/ n.

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Cosmology

COSMOLOGY.

This entry includes two subentries:

Asia
Cosmology and Astronomy

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cosmology

cosmology •haji • algae • Angie •argy-bargy, Panaji •edgy, sedgy, solfeggi, veggie, wedgie •cagey, stagy •mangy, rangy •Fiji, gee-gee, squeegee •Murrumbidgee, ridgy, squidgy •dingy, fringy, mingy, stingy, whingy •cabbagy • prodigy • effigy • villagey •porridgy • strategy • cottagey •dodgy, podgy, splodgy, stodgy •pedagogy •Georgie, orgy •ogee • Fuji •bhaji, budgie, pudgy, sludgy, smudgy •bulgy •bungee, grungy, gungy, scungy, spongy •allergy, analogy, genealogy, hypallage, metallurgy, mineralogy, tetralogy •elegy •antilogy, trilogy •aetiology (US etiology), amphibology, anthology, anthropology, apology, archaeology (US archeology), astrology, biology, campanology, cardiology, chronology, climatology, cosmology, craniology, criminology, dermatology, ecology, embryology, entomology, epidemiology, etymology, geology, gynaecology (US gynecology), haematology (US hematology), hagiology, horology, hydrology, iconology, ideology, immunology, iridology, kidology, meteorology, methodology, musicology, mythology, necrology, neurology, numerology, oncology, ontology, ophthalmology, ornithology, parasitology, pathology, pharmacology, phraseology, phrenology, physiology, psychology, radiology, reflexology, scatology, Scientology, seismology, semiology, sociology, symbology, tautology, technology, terminology, theology, topology, toxicology, urology, zoology • eulogy • energy • synergy • apogee • liturgy • lethargy •burgee, clergy •zymurgy • dramaturgy

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Cosmology

COSMOLOGY

From Gr. κόσμος, world, and λόγος, knowledge or science, cosmology means literally the science of the world. Christian wolff, introducing the term in 1730, classified cosmology as special metaphysics. Based on reason alone and deduced from notions previously defined and explained in general metaphysics, it applies the conclusions of ontology to the consideration of the totality of existing things. In developing this metaphysics of bodies, Wolff was influenced more by leibniz than by aristotle. Leibniz's notion of "a striving and a resisting substance" led him to hold that there can be no body without motion and no substance without effort or force. For Wolff, however, matter and motor force are neither real beings nor distinct, substantial principles, but only "substance phenomena" whose common source are the elements from which bodies derive their substantiality. He regarded these elements as first principles containing the ultimate reasons for everything perceived in bodies, e.g., their composition, their apparent constituents, their changes, and their phenomena. Yet unlike Leibniz's monad, the elements have no power of representation and can influence one another in a physical way. Wolff's treatment strongly influenced the tradition of manuals in scholastic philosophy, with the result that the term cosmology came to be frequently used in scholasticism to designate the philosophy of nature. In modern scientific usage, cosmology also specifies that discipline which concerns itself with theories pertaining to the origin and structure of the universe.

See Also: philosophy of nature; universe, order of.

[j. v. burns]

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Cosmology

COSMOLOGY

The term cosmology stands for a family of related inquiries, all in some sense concerned with the world at large. Two main subgroups of uses may be distinguished: those belonging to philosophy and those belonging to science.

"Cosmology" has received wide currency as a name for a branch of metaphysics, ever since Christian von Wolff, in his Discursus Praeliminaris de Philosophia in Genere (1728), gave cosmology a prominent place in his classificatory scheme of the main forms of philosophical knowledge and distinguished this branch from ontology, theology, and psychology. (See Discourse on Philosophy in General, translated by R. J. Blackwell, Indianapolis, 1963, Para. 77). Despite the severe strictures that Immanuel Kant leveled against the pursuit of rational cosmology in his Critique of Pure Reason, the term has continued to enjoy a standard use among many philosophers. For example, it occupies a central place in the manuals of scholastic philosophy; these adhere, for the most part, to the Wolffian scheme of classification of the branches of metaphysics. The term has been used, too, by many philosophers not in the scholastic tradition; for example, A. E. Taylor in his Elements of Metaphysics (London, 1903) assigns to cosmology the task of considering "the meaning and validity of the most universal conceptions of which we seek to understand the nature of the individual objects which make up the experienced physical world, 'extension,' 'succession,' 'space,' 'time,' 'number,' 'magnitude,' 'motion,' 'change,' 'quality,' and the more complex categories of 'matter,' 'force,' 'causality,' 'interaction,' 'thinghood,' and so forth" (p. 43). Cosmology is sometimes understood even more broadly, as being synonymous with speculative philosophy in its most comprehensive sense. Thus in Alfred North Whitehead's Process and Reality (New York, 1929), whose subtitle is "An Essay in Cosmology," the attempt is made to construct a categorial scheme of general ideas "in terms of which every element of our experience can be interpreted" (p. 4).

In its second major use, the term cosmology designates a science in which the joint efforts of the observational astronomer and the theoretical physicist are devoted to giving an account of the large-scale properties of the astronomical or physical universe as a whole. The task of constructing models of the universe that are suggested by and tested by appeals to the observational findings of the astronomer distinguishes the enterprise of scientific cosmology from the a priori investigations of rational cosmology (as a branch of metaphysics) and the purely conceptual and categorial analyses of the speculative philosopher. Nevertheless, even scientific cosmology poses a number of philosophical questions. The sum of theseand they are principally methodological and epistemological in characterconstitutes the philosophy of scientific cosmology. The present entry is concerned with the philosophy of cosmology in this sense. Attention will be focused on a central theme in this area: the question whether cosmology must employ a method different from that employed in other empirical sciences because of its distinctive subject matter, namely, the universe as a unique system.

Description or Explanation?

Is the familiar distinction between description and explanation (or the corresponding one drawn between sciences still in the early stages that are primarily descriptive, and those that have progressed to the predominating use of the explanatory aspects of theory) a distinction that can be profitably applied in giving an account of the logic of cosmology? No simple and unqualified answer can be given. For, on the one hand, cosmology, in attempting to gain knowledge of the universe as a whole, certainly is not content to rest with the observational reports of the astronomer, and therefore cannot be classed with the descriptive sciences. On the other hand, in advancing to the level of theory, as cosmology in a qualified sense certainly does, it is not primarily concerned with the explanation of lawsas is the case with other explanatory sciences.

If by description is meant giving an account of some single event or object in observational terms, or (in an extended sense of "description") formulating a generalization (law) in observational terms which refers to the observable or measurable properties and relations of a class of events, then cosmology, which is interested in giving an account of the universe as a whole, is not engaged in description. Even if we recognize, as we must, the descriptive activities of observational cosmology as a branch of observational astronomy, these fall short of giving us an adequate account of the universe as a whole. All that astronomy can give us is a description of the domain of objects and events within the range of its most powerful instruments. At the present time, however, these instruments, have not reached the limit, if there is a limit, to what is in principle observable. Moreover, even if the universe were in some sense finite and wholly explorable by actually or theoretically available instruments, the statement that what is thus observationally explored is in fact the universe as a whole would not be warranted by observational evidence alone. Such a statement could not, therefore, be part of the description of the universe, insofar as this description is a report of what is found. The claim that the universe is open to complete inspection requires the support of theory. It is a statement which is not included in the description, but is a rider to the descriptionto the effect that the description as given is of the universe as complete; considerations other than purely observational ones are needed to support this claim.

If cosmology is not content with description, does it then aim at giving explanations? Here our answer must be qualified. In the case of ordinary empirical generalizations, where there are multiple instances of some phenomenon of which we have examined a limited number, we say that the law supported by this evidence may be used as a reliable rule of inference. Since the law applies to a kind of subject matter, or a type of phenomenon, it can be upheld as a useful means for predicting and explaining those instances that can be brought within its scope. But in cosmology, the primary goal is not to establish laws. The universe, by definition, is a unique object or system. Cosmology does not undertake to establish laws about universes; at best one can establish laws about the constituents of the universe. The relation that the observable portion of the universe bears to the universe is that of part to presumed whole, rather than that of instance to law. Hence, if to explain means to bring an instance under a law, this mode of explanation, which is a characteristic concern of other branches of physical science, does not characterize cosmology.

Can it be said, then, cosmology aims at giving explanations in the sense in which theories are employed to explain laws? Here our answer, once more, cannot be a straightforward "yes" or "no." The characteristic device employed by theoretical cosmology is a model of the universe, and a model in many respects functions precisely as a theory does. It is a conceptual construction that cannot be said to be a mere report of what is already found in observation, nor even an anticipated description of what might be found in future observations. Rather, it is a means for making the observational data themselves intelligible. However, the facts that the cosmologist wants to explain are not laws in the ordinary sense of the term, and so in this respect the purpose of a model of the universe is not identical with that of a theory in the ordinary sense.

Consider, for example, the question "Why is the apparent magnitude of galaxies correlated with their red shift?" This question asks for an explanation of an important datum of observation. The observed fact is sometimes called Hubble's "law," but it is a law only in the peculiar sense in which we refer to Kepler's laws as "laws." That is, Hubble's law tells us something about a particular distribution or process of a unique set of objects, namely the system of galaxies, just as Kepler's laws tell us something about the orbits of the planets in our solar system, not in any solar system. In general, however, laws of science are characterized by their universal form. They are unrestricted in scope and are not ostensibly tied to objects or events specifically located in some particular space-time region. Thus Newton's law of gravitation, for example, says that for any two bodies, the gravitational force that holds between them is inversely as the square of their distance apart and proportional to their masses. Now when we deal with the system of bodies and processes that constitute the unique configuration we call the universe, we are not dealing with any configuration of events and objects; we are dealing with the configuration actually observed and given.

An interesting and important question that can be raised here, however, is whether the unrestricted laws of ordinary physics are not themselves, in a more profound sense, relatively restricted, since they apply to bodies or phenomena within the ultimately unique configuration that constitutes the physical universe. From this point of view, the study of cosmology sets the environment and limiting framework for all other branches of physical science. Hence it is not unreasonable to expectas E. A. Milne, D. W. Sciama, and others have pointed outthat one may hope to understand the laws of physics themselves in terms of the unique background making up the universe studied in cosmology. Such a claim, however, is associated only with certain specific models, namely, the kinematic model as worked out by Milne and the steady state model as sketched by Hermann Bondi and Sciama, and therefore this idea of explaining all laws by a cosmological model cannot be held up as a working goal for all cosmological models. In fact, the majority of models developed within the framework of general relativity theory have not been designed to embody this feature.

Observation and Theory

The study of cosmology has two lines of attack, that of the observational astronomer and that of the theoretical physicist. One might say that both the observational astronomer and the theoretical cosmologist are studying the universe, though from different vantage points, or that one supplies observational data about the universe that the other undertakes to interpret; but this is, at best, only a sketch of the situation and is in some ways seriously misleading. For it will not do to say that both the astronomer and the theoretical cosmologist are studying the universe, as if the universe is laid out for identification before them and the only difference between them is in approach and method. If we look more closely at the study of cosmology, the situation is rather different.

The observational astronomer is not confronted with the universe as an observationally complete whole. Instead, he obtains observational clues from various instruments about a large population of identifiable subsystemsnamely, individual galaxies and clusters of galaxies. This population of observable entities is sometimes referred to as "the observable universe." However, this phrase is not to be understood in the sense that we have independent means for identifying the universe and that we wish to refer to it insofar as it is being observed. "The observable universe" is not the same as "the universe observed." What the astronomer reports on of relevance to cosmology is an observable population of galaxies and clusters of galaxies. These observational reports have to do with such matters as the spatial distribution of galaxies, their systematic motions, density, spectroscopic patterns, individual shapes, and stellar composition.

The population of subsystems that makes up the observable universe is now, as in a sense it must always be, a finite population. With the advance in the power and sensitivity of instruments, knowledge of the extent of this population and the refinement in the details of the reports about this population are improved. Although it is regarded as likely that further advances in observational resources will disclose a wider population of subsystems similar to those already observed, it must be remembered that it is always possible for further observations to disclose as basic constituents of the universe astronomical units of a higher degree of inclusiveness than galaxies or clusters of galaxies, or even entities of an altogether different type from those heretofore disclosed. Whatever may be the case in the future, it certainly is the case at present that what comes within the observational reaches of the astronomer is definitely not the universe as an absolute whole, if there is in fact such a whole.

When we say that the theoretical cosmologist studies the universe in order to understand it or make it intelligible, what is it that he studies? He does not study the universe in any direct way, if that means having before him a readily identified object which he tries to comprehend, for example by subsuming it under some law. Nor, as we have just seen, does the universe he studies consist of a complete population of entities about which the observational astronomer furnishes him detailed reports. The theoretical cosmologist is not given information about the universe as a whole, nor even about what lies beyond the immediate range of the astronomer's instruments.

What then does he study? A brief and simple answer is to say that he constructs a model of the universe and that he studies the way in which this model may be used to interpret the observational data already available. The cosmologist will use his model to interpret the data assembled by the observational astronomer and to guide the astronomer in the search for further data. Insofar as the use of theoretical models proves satisfactory, we may say that cosmology has helped us to understand the universe and to make it intelligible. This is not to be understood, however, as meaning that even at the end of a relatively successful course of inquiry, the cosmologist has been able to confront the universe directly as some kind of readily identifiable object, system, or class of objects. What is to be understood by "the universe," in short, can only be approached and identified through the use of models, not independently of them.

The Model and Its Construction

The kind of model that the cosmologist constructs is wholly conceptual rather than material. It consists of different sorts of symbols including ordinary language, mathematical language, diagrams, and charts, all of which will normally be employed in presenting a given model. A model of the universe is not something that can be directly visualized or completely represented in a pictorial diagram. Consider, for example, a typical model in which use is made of a geometric mode of representation according to which the galaxies are treated as a set of mathematical points that trace out a set of geodesic curves in space-time. In this case, the metric of this set of points is given by the general Robertson-Walker expression for the space-time interval (ds ):

where R (t ) is the expansion factor, k is a constant whose value determines whether space is Euclidean or non-Euclidean, c is the velocity of light, and r, θ, and ϕ are spatial coordinates. In addition to the specification of purely geometric or kinematic features, which are specified by introducing appropriate values for the curvature constant (k ) and the expansion factor (R (t )), a model will also require some assignment of specific dynamic or gravitational properties to the entities thus represented. Additional formulas will then be required, and these will normally involve relativity theory or some equivalent branch of physics. It is clear that however much a simple diagram making use of dots and lines may serve to give us a visual representation of what we are talking about, this hardly suffices to encompass all those additional features of the model not included in the diagram.

Although the cosmologist cannot inspect the original, the universe itself, he nevertheless undertakes to make a model of it. How is this done? The answer is to be found by noting the various clues and sources to which the cosmologist appeals in determining the properties to be assigned to his model. These are of two principal types: observational clues provided by the astronomer, and theoretical principles thought to be of relevance to the cosmologic problem.

observational clues

In general, the observational data the astronomer gathers aid the cosmologist by suggesting ways of assigning certain idealized properties to the model, by providing empirically ascertained values for the constants and variables in the model, and by offering tests for the adequacy of the model as a tool for predicting observable matters of fact.

Idealized properties

The kinds of entities and their properties that the astronomer observes suggest to the cosmologist the lines to follow in developing a simplified and idealized conception of the universe. Let us take some examples. The galaxies, though of enormous physical bulk, may be considered for purposes of the model as particles making up a continuous and perfect fluid. The advantage of treating the galaxies in this fashion is that it permits a great simplification of the problem, to which readily available mathematical tools of representation and calculation may be applied. Here, of course, the cosmologist adopts a technique that is universally adopted in other branches of physical science and with similar justification. If necessary, suitable corrections to this idealization can always be introduced when application is made of the model to "describe" the actual universe.

An important feature of the domain of galaxies already observed is their spatial distribution. The actual spatial distribution of the galaxies is roughly homogeneous and isotropic when fairly large volumes of space are considered. On a smaller scale, departures from homogeneity become more noticeable, in the clustering of the galaxies, for example. When still smaller volumes of space are investigated, homogeneity breaks down altogether. In general, then, the claim to the uniformity of distribution of galaxies can be upheld only if one takes a sufficiently large unit of volume, say 3.5 × 108 parsecs in diameter. Yet in constructing his model, and as a first approximation, the cosmologist will assign a complete homogeneity to his model of the universe. The expression "cosmological principle" is commonly used to designate this feature of spatial homogeneity. Models that satisfy this cosmological principle, and thus possess the feature of spatial homogeneity, are known as uniform model universes.

When put into mathematical language, a uniform model universe is one possessing a constant curvature at a given moment of time. In the language of general relativity theory, since the density and pressure of material that make up the model are the same in all volumes of space at a given time, whatever their size, a geometric representation of this fact will involve the use of one or another of the spaces of constant curvature. All segments of space of the entire universe will have the same curvature. Such a model clearly requires a process of idealizing and simplifying the spatial distribution of bodies actually observed. For if we were to use the language of geometry to describe the actually observed spatial distribution, we should have to note the actual local departures from homogeneity or constancy of curvature.

In constructing a model of the universe that embodies the feature of spatial homogeneity or constancy of curvature, it is not enough to specify what that curvature is at the present moment of cosmic time. A fully determined model requires (in addition to other features) that the spatial properties of the universe be specified for any point in its past or future. Here there are, broadly speaking, two possibilities. According to one, the spatial properties of the universe remain the same at all times; this view is upheld by those who adhere to the "perfect cosmological principle" and use it to define the properties of the steady state model. A second alternative is to adopt the cosmological principle in its more restricted form as designating merely spatial uniformity, as is the case with the orthodox cosmological models of general relativity. For such models, the entire history of the universe, from a spatial point of view, could be specified if one knew just one thingthe rate at which the distance between any two galaxies changes with time. In a universe characterized simply by the cosmological principle, since an observer would always find a spatially isotropic distribution of particles about him, the only basic feature subject to change is a temporally noticeable feature, namely, changes in the density of the distribution of particles. Such changes in the density might then serve to define a cosmic "clock."

Empirically obtained values

A second important function that the appeal to observational data serves in the construction of cosmological models is that of yielding empirically obtained values for some of the constants and variables of theory. For example, in relativistic uniform models of the expanding universe, the defining characteristics of a particular model need to be specified by assigning values for the following quantities: the cosmological constant (λ ), the temporal pattern of the universe as determined by evaluating the function R (t ), the values for the velocity factor and the acceleration factor in the velocity-distance relation that specify how the galaxies are moving, the density (ρ ) and the pressure (p ) of the material and energetic content that fills the universe, and the curvature constant (k ). Observational evidence is, at present, either not available at all or not accurate enough to give sufficiently precise determinations for all of these terms. The cosmologist must, therefore, use whatever data is available to eliminate those models that are incompatible with present observations and to suggest lines of inquiry that will help to narrow the field down to those models that can be further tested by observation.

One overall condition for the acceptance of a model is, of course, the consistency of the empirically obtained values it proposes. In a particular model, a particular combination of empirically assigned curvature and density values, for example, may lead to a calculated "age of the universe" that will be inconsistent with an independently obtained estimate for the lower bound of such an "age"; the estimated time scale of the universe will be too short. In general what is sought is a model all of whose empirically ascertainable values are mutually consistent within the available limits of accuracy.

Empirical tests

Finally, as a natural extension of the point just made, we see how the data obtained by the astronomer serve to test the calculated numerical values for quantities appearing in the cosmologist's equations or other qualitative predictions made on the basis of a given model. Thus whether the extremely remote galaxies at the horizon of the now observable population of galaxies have roughly the same characteristics as those that are nearer is an important question much discussed at the present time as a means for evaluating the rival claims of the steady state and evolutionary theories. The steady state theory claims that galaxies that are at the outer limits of observability should have roughly the same characteristics as those at lesser distances. According to various "evolutionary" models, those same remote galaxies, from which we receive light and other forms of radiation emitted billions of years ago, could, in effect, tell us about the earlier stages of the evolution of the universe. Since conditions at the time of emission were presumably different from what they are now, these very remote galaxies should display differences from those that are nearer to us in at least some of their properties, and these differences should give us valuable clues about the course of development of the universe as a whole. In this regard a number of delicate questions that are the subject of much controversy have arisen in current research.

theoretical ideas

A second major source of ideas in the construction of cosmological models is to be found in the conceptual resources of mathematical physics. Here there are two broad possibilities that confront the cosmologist.

Use of established principles

As a first possibility, the cosmologist may turn to some already established body of physical theory as expressed in fundamental principles and derived laws. Such theory will normally have already been found to be successful in dealing with a variety of physical problems of lesser scope than, and wide differences from, the purely cosmological problem. The cosmologist will nevertheless propose to see to what extent the same general body of ideas may be used when applied to the distinctive subject matter of cosmology. He will investigate to what extent the universe as a physical system has a detailed structure that may be articulated and specified by means of the selected physical theory. For example, he may use Newtonian mechanics to construct a model of the universe. Isaac Newton himself drew, in a general qualitative way, the cosmological consequences of using the inverse square law of gravitation as a guide. He argued that the universe, throughout its infinite space, must be filled by a more or less evenly distributed matter. For if all the matter that exists were to be confined to a finite "island" in an infinite "ocean" of space, it would have a center of mass toward which, in time, all matter would move by gravitational attraction. The fact is that no such motion is found, and Newton concluded, therefore, that matter is distributed uniformly throughout an infinite space.

At the present time, the primary and predominant source to which the cosmologist turns is the general theory of relativity as expressed in Albert Einstein's general field equations. These equations specify the relations between the space-time metric of any physical domain and its material or energetic content. The discovery of solutions to those field equations that are of special relevance to the cosmologic situation has led to the construction of several varieties of relativistic models. The other major use to which the field equations of general relativity have been put takes the form of the Schwarzschild solution. It was this solution that first afforded the opportunity for testing the predictive and explanatory powers of the theory as a whole. Karl Schwarzschild's solution is particularly applicable to a physical system such as we encounter in the solar system, namely, a single massive particle (the sun) in whose neighborhood we may study the behavior of much smaller masses (the planets) and light rays. The success of its predictions and explanations has been the primary basis for the confidence placed in the general theory.

To return then to cosmology: Within the broad class of homogeneous, or uniform, model universes we may distinguish the nonstatic models and the static models. Among the static models is Einstein's original model of 1917, which pictured the universe as finite and unbounded; in the light of the subsequent discovery of the mutual recession of the galaxies, it is no longer considered adequate. The nonstatic models include the ever-expanding-universe models that originate from zero or in some finite volume, and oscillating models that undergo alternate contractions and expansions. Within each of these groups, individuating characteristics for a particular model are to be found in the choice of values for the curvature, age, density, and cosmological constant. No single model has as yet been universally adopted.

Creation of new principles

The other broad possibility for furnishing theoretical ideas for cosmologic models is one in which the cosmologist, instead of appealing to already established principles or laws, for example those of relativistic mechanics, undertakes to create afresh basic principles thought to be of special relevance to the cosmologic problem. By way of illustration, there is the conflict of the 1930s and 1940s between the way in which E. A. Milne sought to establish his kinematic model and the more orthodox procedures of relativistic cosmology. Although Milne did use the formulas of special relativity, he did not take these over directly from Einstein's own presentation; Milne attempted instead to derive them from what he thought of as more basic and primitive postulates. These postulates, he claimed, state the conditions for the measurement of time and for the communication of results by different timekeepers and observers.

A more recent example of the same sort of procedure is the steady state model of the universe proposed by Bondi and Thomas Gold in 1948. In support of this model, it is argued that since the universe is unique, there is no reason to believe that the laws which apply to smaller-scale physical phenomena, for example in laboratory terrestrial physics, or even in the domain of gravitational phenomena in the solar system, need be expected to apply to the universe as a whole. Therefore, instead of taking such laws as the point of departure in investigating the physical properties of the universe as a whole, it is suggested that the cosmologist can actually enjoy a far greater freedom than is believed possible in orthodox relativistic cosmologies. Let the cosmologist adopt any "laws" or principles which he believes are appropriate to the study of the universe as a whole, even though these may not have been established or confirmed in other (smaller-scale) areas of physical phenomena. The important thing is to see whether using these laws and principles leads to confirmable empirical results and whether they help to increase and deepen our understanding of the universe.

Those who favor this view (Bondi and Gold among others) determine some of the major features of the steady state model by appeal to the specially introduced postulate known as the perfect cosmological principle. This principle was not in prior use in other branches of physics but was introduced because of its special relevance to cosmology. (Fred Hoyle's model of the steady state universe proceeds along more conventional lines, at least in this respect. Although it differs from the expanding universe models of general relativity in abandoning the principle of the conservation of matterin order to make possible the idea of the continuous creation of matterit appeals for its basic physical principles, although in modified form, to the field equations of general relativity.)

One general motive that seems to inspire the setting up of specially devised principles for cosmologic models is the desire to show that the science of cosmology is basic to all other physical sciences. Instead of appealing to other branches of physics for principles to be used in describing the features of the universe as a whole, it is thought desirable that one should be able, eventually, to show that the laws of ordinary physics can be linked with the properties of the universe as a whole. The universe would then disclose itself to be a unitary physical system within which it would be possible in principle to deduce ordinary physical laws from the principles of cosmology. Milne undertook to show how, for example, the inverse square law of gravitation, among other things, could be deduced from such more fundamental cosmological ideas. Similarly, within the framework of a steady state model, Sciama attempts to show how the local inertial properties of matter can be linked (as Ernst Mach originally proposed) with the distribution of masses in the universe at large.

From a logical point of view, there is no reason to discourage such efforts. On the contrary, the realization of such a goal would be of immeasurable significance for all of science, and one should in logic suspend judgment until such a program can be carried through with some fair degree of success.

Meanwhile, it is necessary to point out that some of the writers who favor this approach put methodological interpretations on the use and warrant for the specially devised principles that are not acceptable, whatever the eventual success or promise of the program as a whole. Thus Milne and Bondi, who support different models, are each concerned to stress what they take to be a special method for cosmologyas contrasted with other branches of physics. Milne, for example, thought of ordinary physics as employing an inductive method, whereas cosmology, he believed, should be based on a deductive method. Cosmology, he argued, should not employ the laws of ordinary physics to the extent that these are inductively warranted. This was his major complaint against what he took to be the faulty procedure of relativistic cosmologies founded on the "inductively established" principles of general relativity. In making this claim, Milne was in error, since the principles of general relativity theory are not, as he thought, ordinary inductive generalizations.

In fact, Milne's own appeal to "self-evidence" as the warrant for introducing his preferred cosmological principles must be rejected, for the appeal is groundless and fails to support the certainty and uniqueness which he claimed for his principles. In constructing a model of the universe, the cosmologist is engaged in setting up a theoretical tool for dealing with the facts of observation. Whether he gets his theoretical principles by "borrowing" them from some other branch of physics or whether he creates them especially for the problem at hand is of secondary importance to what he does with these principles once he has them and how he evaluates the results he achieves. There is a common method that characterizes cosmology regardless of the particular model being proposed or favored, and it is precisely the same method which is employed in other branches of physics. Moreover, the same criteria of evaluation need to be brought to bear in the appraisal of results in cosmology as in other areas of science. Far from including any appeal to self-evidence or to similar rationalistic demands, a satisfactory model requires the constant support provided by observational evidence.

Cognitive Worth of Models

Consideration of the goals set by scientific cosmology gives rise to a central philosophical question, that of determining the cognitive worth of any cosmological model. This is an epistemological question and may be put in terms of the traditional issues separating the realist and the conceptualist (or the instrumentalist). Should we say with the realist that cosmological models offer us an account of the structure of the independently existing universe, or, rather, should we say with the conceptualist that these models are simply useful means of presenting and interpreting observational data?

As a basis for clarifying the issue at hand, it will be helpful to point out a fundamental ambiguity in the use of the term universe itself. Employed without the qualifying adjectives "observed" or "observable," it may have at least two quite distinct senses. One meaning of universe is "that to which the observed universe belongs"; another is "that which is characterized by a cosmological model." So far as the realist is concerned, the two meanings are equivalent; in his view the universe defined by a cosmological model will be the same universe as the one described by the expression "that to which the observed universe belongs." But the realist's view of cosmological models cannot be assumed in advance to be the only tenable one. Thus the distinction suggested here has the merit of permitting us to keep this question open. If later a realist philosophy is accepted, the appropriate modifications can be made. Clearly, we do not need to commit ourselves to the position that everything properly said of "the universe" in the sense of "that which is characterized by a cosmological model" can also be said of "the universe" in the sense of "that to which the observed universe belongs." For example, we might want to attribute the property of being "a whole" or "an absolute totality" to the universe as characterized by a particular model, but not to the universe in the sense of that to which the observed universe belongs.

Cosmology aims at articulating the character of the universe as a whole. To that extent, then, it rests on the methodological postulate that the universe is a whole. The specific character of the whole will, of course, be variously described by various models. What remains fixed, however, is the assumption that the goal of cosmology is to characterize the universe as a whole. Therefore the statement "The universe is a whole" is in this context an analytic statement, a matter of definition. But note that it is a definition in which "the universe" is used to signify "that which is characterized by a cosmological model." Not only does cosmology require that, as a matter of definition, the universe be thought of as a whole (in the sense of being intelligible in the way that mathematical classes, geometrical relations, or physical systems are); it also postulates that the universe as a whole is unique or absolute. This means that there is just one such class, pattern, or system, and that all other physical processes or systems of lesser duration or spatial extent are to be taken only as parts of this all-embracing whole. Since each model will so define the universe, it would be a misuse of language to speak of a plurality of universes. Again, of course, the precise structure of this unique or absolute whole will, at least in some respects, vary from model to model.

But what if "the universe" means "that to which the observed universe belongs"? Is the statement "The universe is an absolute, unique whole" still analytic? To this the answer must be no. For when we use "the universe" in this sense, we move from methodology to ontology. In contrast to the case of the universe as defined by a cosmological model, we are no longer committed by the basic methodological postulate of cosmology to saying that the universe is a whole. True, in setting up a science, it may be necessary to presuppose the existence of some pervasive structure as the object of study. Yet such a presupposition need not be binding on what the universe is existentially. So long as "the universe" means simply "that to which the observed universe belongs," nothing in this meaning contains analytically the notion of its being a "whole" or an "absolute whole." Indeed, even if we grant that the observed universe is structured in some manner, this does not entail that the wider universe of which it is a part is also pervasively structured. Nor does the fact that we describe the observed universe as "part of" or "that which belongs to" something else require us to say that the universe to which it belongs is a unique or absolute whole. For our reliance on such terms as part, whole, and belong reveals merely that the mind, in reaching into the unfamiliar, must use analogies in order to relate the unfamiliar to what it already knows.

The universe as the "something more" than the observed universe may well be a complete, unique and intelligibly structured whole. But the claim that we are able to say so is something to which we need not commit ourselves. It is better left as an open question, since, strictly speaking, it is one on which we neither have nor can have any knowledge. Stipulating an affirmative answer by definition does not, of course, establish such knowledge.

See also Creation and Conservation, Religious Doctrine of; Einstein, Albert; Mach, Ernst; Newton, Isaac; Relativity Theory; Taylor, Alfred Edward; Time; Whitehead, Alfred North; Wolff, Christian.

Bibliography

historical surveys

De Sitter, W. Kosmos. Cambridge, MA: Harvard University Press, 1932.

Koyré, A. From the Closed World to the Infinite Universe. Baltimore: Johns Hopkins Press, 1957.

Kuhn, T. S. The Copernican Revolution. Cambridge, MA: Harvard University Press, 1957.

Munitz, M. K., ed. Theories of the Universe from Babylonian Myth to Modern Science. Glencoe, IL: Free Press, 1957.

systematic surveys

Bondi, H. Cosmology. 2nd ed. Cambridge, U.K.: Cambridge University Press, 1961.

Bondi, H. et al. Rival Theories of Cosmology. London: Oxford University Press, 1960.

Bonnor, W. B. The Mystery of the Expanding Universe. New York: Macmillan, 1964.

Couderc, P. The Expansion of the Universe. London: Faber and Faber, 1952.

Eddington, A. The Expanding Universe. Cambridge, U.K.: Cambridge University Press, 1933.

Hubble, E. P. Observational Approach to Cosmology. Oxford, 1937.

Hubble, E. P. The Realm of the Nebulae. New Haven, CT: Yale University Press, 1936.

Lemaitre, G. The Primeval Atom. New York: Van Nostrand, 1950.

McVittie, G. C. Fact and Theory in Cosmology. London: Eyre and Spottiswoode, 1961.

Milne, E. A. Modern Cosmology and the Christian Idea of God. Oxford: Clarendon Press, 1952.

Milne, E. A. Relativity, Gravitation, and World-Structure. Oxford: Clarendon Press, 1935.

Sciama, D. The Unity of the Universe. Garden City, NY: Doubleday, 1959.

Tolman, R. C. Relativity, Thermodynamics and Cosmology. Oxford: Clarendon Press, 1934.

Whitrow, G. J. The Structure and Evolution of the Universe. London: Hutchinson, 1959.

philosophical problems

Bondi, H. "Fact and Inference in Theory and in Observation." In Vistas in Astronomy, edited by A. Beer. London: Pergamon Press, 1955. Vol. I.

Bondi, H. "Philosophical Problems of Cosmology." In British Philosophy in the Mid-Century, edited by C. A. Mace. London: Allen and Unwin, 1957.

Davidson, W. "Philosophical Aspects of Cosmology." British Journal of the Philosophy of Science 13 (50) (August 1962).

Dingle, H. "The Philosophical Aspects of Cosmology." In Vistas in Astronomy, edited by A. Beer. London: Pergamon Press, 1955. Vol. I.

Finlay-Freundlich, E. "Cosmology." In International Encyclopedia of Unified Science Vol. I, No. 8. Chicago: University of Chicago Press, 1951.

Harré, R. "Philosophical Aspects of Cosmology." British Journal of the Philosophy of Science 13 (50) (August 1962).

Johnson, M. Time, Knowledge and the Nebulae. London: Faber and Faber, 1945.

McVittie, G. C. "Rationalism and Empiricism in Cosmology." Science 133 (21) (April 1961).

Monist 47 (1) (Fall 1962). Entire issue.

Munitz, M. K. The Mystery of Existence. New York: Appleton-Century-Crofts, 1965.

Munitz, M. K. Space, Time, and Creation. Glencoe, IL: Free Press, 1957.

Toulmin, S. "Contemporary Scientific Mythology." In Metaphysical Beliefs, edited by S. Toulmin, R. W. Hepburn, and A. MacIntyre. London: SCM Press, 1957.

Milton K. Munitz (1967)

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Cosmology

COSMOLOGY

Cosmological theories describe the physical structure of the universe. For cosmology in the Bible, see *Creation.

In the Talmud

According to R. Simeon b. Yoḥai, the earth and the heavens are like "a pot with a cover." This "cover" is the raki'a, the firmament. "The darkness of the firmament is that of a journey of 50 years. While the sun in the sky passes this journey of 50 years, a man can walk four miles." The distance between the firmament and the earth is the equivalent of a journey of 500 years (tj, Ber. 1:1, 2c). The firmament is composed of water and the stars of fire, but they dwell harmoniously together (tj, rh 2:5,58a). The heavens (shamayim) are an admixture of fire and water (esh and mayim) or made wholly of water (Sham mayim; Ḥag. 12a). Indeed, "the Holy One, blessed be He, took all the waters of the sea and with half He made the firmament and the other half the ocean. The firmament is like a pool, and above it is an arch" (Gen. R. 4:4 and 5). The earth is of the same thickness as the firmament (Gen. R. 4:5). Once every 1,656 years the firmament shakes on its foundations (Gen. R. 38:6).

There is however more than one firmament; according to R. Judah, there are two, according to Resh Lakish, seven (Ḥag. 12b). The sun and the moon are situated in the second firmament (Gen. R. 6:6). The above-quoted view of R. Simeon b. Yoḥai would imply that the world is wholly enclosed by the firmament. R. Joshua was also originally of the same opinion, that the world was "like a tent" enclosed on all sides, but later he came round to the view of R. Eliezer that it is like an exedra, closed on three sides only, but open on the north side, and it is from this opening that the north wind comes (bb 25b).

Originally the sun and the moon were both of the same size but God, realizing that "two kings cannot wear one crown," diminished the size of the moon. Thus what were originally "the two great luminaries" became "the greater luminary" and "the lesser luminary" of Genesis 1:16 (Ḥul. 60b). Eclipses of the sun are a sign of God's anger or displeasure (Suk. 29a). Beneath the earth is the abyss (tehom). There is a cavity which descends from the Holy of Holies to the abyss.

[Louis Isaac Rabinowitz]

In Medieval Jewish Philosophy

In medieval philosophy there were four types of cosmological theories: the Aristotelian-Ptolemaic, the neoplatonic, the Kalām theory, and the theory of the infinite universe.

aristotelian-ptolemaic

The medieval version of the Aristotelian-Ptolemaic cosmology asserts that the universe is a finite sphere whose center is the earth, around which nine other concentric spheres – the moon, the sun, the various planets, the stars, and the diurnal sphere – rotate. These spheres form a compact whole in which there are no gaps, or an inner vacuum, and around which there is nothing. The earth and the heavenly spheres differ in their composition. The latter are made up of a single element, ether, whose homogenous nature is free from change other than locomotion. The earth is composed of four elements, earth, water, air, and fire, whose continual transmutations make terrestrial substances subject to generation and corruption. Each of these moving spheres has a "soul," or internal moving force, which is set in motion by corresponding incorporeal substances, the Separate Intelligences. (According to some, "soul" and "intellect" are different aspects of the incorporeal substance.) According to *Maimonides, these incorporeal substances are identical with the angels (Maimonides, Guide of the Perplexed, 2:6). The ultimate source of motion is God, the Prime Mover, who "moves" the universe insofar as He is the most perfect substance, and therefore the object of love of all other substances (Aristotle, Metaphysics, 7:7; Maimonides, Guide 1:72; 2:1).

neoplatonic

Medieval neoplatonic cosmology employs several Aristotelian notions but tries to overcome the terrestrial-celestial dichotomy inherent in the Aristotelian theory. Indeed, in some neoplatonic philosophies there is a decidedly pantheistic or monistic tendency (cf. the Christian philosopher Scotus Erigena). Solomon Ibn *Gabirol is the most neoplatonic of the medieval Jewish philosophers. In attempting to demonstrate the essential unity of the universe he applied the Aristotelian form-matter framework to every part of the universe except God and the Divine Will. The result of this extension is that every level of being that emanates from God (see *Emanation) exhibits a common universal matter and universal form. Each level of being, however, is further characterized by a specific material nature and a particular formal structure. In this way both homogeneity and diversity are accounted for. Typical of monistic cosmologies, Gabirol's system tends to be static: the emanation of the lower stages of being from God is described in non-temporal terms. The origin of the universe, as well as motion, is explained by Ibn Gabirol as the effect of God's will, which seems to serve as the mediating link between God and the universe.

kalĀm

The *Kalām cosmology employs the model of a universe consisting of atoms in a vacuum. These indivisible particles combine, separate, and recombine, forming the universe by these movements. The Kalām version of atomism differs from its Greek antecedents in that it rejects any notion of an infinite magnitude (Maimonides, Guide, 1:73). Atomism had virtually no impact upon the mainstream of Jewish philosophy, although a number of Karaite philosophers accepted its doctrines (see *Atomism). Abu al-Barākāt *Ḥibat Allah of Baghdad was a profound atomist but he had no influence upon Jewish thought, probably because of his conversion to Islam in his late years. Indeed, the most important Jewish representative of the Kalām, *Saadiah Gaon, was not an atomist.

infinite universe – crescas

One aspect of classical atomism, however, is found in the cosmology of Ḥasdai *Crescas. His philosophy constitutes a vigorous critique of Aristotle's physics and cosmology. Crescas reverts to the atomistic hypothesis of an infinite vacuum in which our universe, and perhaps others, are located. (The possibility of a plurality of universes is also found in rabbinic literature, but is rejected by Maimonides and other medieval Aristotelians; cf. Gen. R. 3; Maimonides, Guide, 2:30). Although Crescas does not explicitly introduce atoms into his physics, his theory of matter exhibits atomistic aspects. For example, unlike Aristotle, Crescas sees matter as requiring no external principle for its motion: bodies have a natural tendency to move. Consequently, Crescas eliminates the artificial system of intelligences as causes of motion. Finally, he rules out the distinction between the composition of the earth and the heavens in favor of the notion of a common matter characteristic of all bodies celestial and terrestrial. In several important respects Crescas' cosmology anticipates some ideas of Galileo and Newton.

[Seymour Feldman]

bibliography:

Aristotle, Physics and On the Heavens; Munk, Mélanges; Maimonides, Guide of the Perplexed, ed. and tr. by S. Pines (1963); H.A. Wolfson, Crescas' Critique of Aristotle (1929); idem, in: paajr, 11 (1941), 105–63; A.C. Crombie, Medieval and Early Modern Science, 1 (1959), index; Hyman, in: Congrès International de Philosophie Médiévale, La filosofia della natura nel Medioevo (1966), 209–18. add. bibliography: J. Dillon, "Ibn Gabirol: The Sage among the Schoolmen," in: L.E. Goodman (ed.), Neoplatonism and Jewish Thought (1992), 77–110; P. Duhem, Medieval Cosmology (1985); I. Efros, The Problem of Space in Jewish Medieval Philosophy (1917); S. Feldman, "Platonic Themes in Gersonides' Cosmology," in: Salo W. Baron Jubilee Volume (1975), 383–405; G. Freudenthal, "Cosmogonie et physique chez Gersonide," in: rej, 145 (1986), 295–314; L.E. Goodman, "Maimonidean Naturalism," in: Neoplatonism in Jewish Thought (1992): 157–94; A. Hyman, "From What Is One and Simple Only What Is One and Simple Can Come to Be," in: L.E. Goodman (ed.), Neoplatonism in Jewish Thought (1992), 111–36; T. Rudavsky, "Philosophical Cosmology in Judaism," in: Early Science and Medicine (1997), 149–84; N. Samuelson, "The Role of the Elements and Matter in Gersonides' Cosmogony," in: G. Dahan (ed.), Gersonide en son temps (1991), 199*233; H. Wolfson, Crescas' Critique of Aristotle (1929).

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Cosmology

Cosmology

Evolution of cosmological thought

The expanding Universe

The big bang

Implications of the big bang

Trouble in paradise

Resources

Cosmology is the study of the origin, structure and evolution of the universe.

The origins of cosmology predate the human written record. The earliest civilizations constructed elaborate myths and folk tales to explain the wanderings of the sun, moon, and stars through the heavens. Ancient Egyptians tied their religious beliefs to celestial objects and Ancient Greek and Roman philosophers debated the composition and shape of the Earth and the Cosmos. For more than 13 centuries, until the Scientific Revolution of the sixteenth and seventeenth centuries, the Greek astronomer Ptolemys model of an Earth-centered Cosmos composed of concentric crystalline spheres dominated the Western intellectual tradition.

Polish astronomer Nicolaus Copernicuss (14731543) reassertion of the once discarded heliocentric (sun-centered) theory sparked a revival of cosmological thought and work among the astronomers of the time. The advances in empiricism during the early part of the Scientific Revolution, embraced and embodied in the careful observations of Danish astronomer Tycho Brahe (15461601), found full expression in the mathematical genius of the German astronomer Johannes Kepler (15711630) whose laws of planetary motion swept away the need for the errant but practically useful Ptolemaic models. Finally, the patient observations of the Italian astronomer and physicist Galileo, in particular his observations of moons circling Jupiter and of the phases of Venus, empirically laid to rest cosmologies that placed Earth at the center of the Cosmos.

English physicist and mathematician Isaac Newtons (16421727), important Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) quantified the laws of motion and gravity and thereby enabled cosmologists to envision a clockwork-like universe governed by knowable and testable natural laws. Within a century of Newtons Principia, the rise of concept of a mechanistic universe led to the quantification of celestial dynamics, that, in turn, led to a dramatic increase in the observation, cataloging and quantification of celestial phenomena. In accord with the development of natural theology, scientists and philosophers argued conflicting cosmologies that argued the existence and need for a supernatural God who acted as prime mover and guiding force behind a clockwork universe. In particular, French mathematician, Pierre Simon de Laplace (17491827) argued for a completely deterministic universe, without any scientific need for the intervention of God. Most importantly to the development of modern cosmology, Laplace asserted explanations for celestial phenomena as the inevitable result of time and statistical probability.

By the dawn of the twentieth century, advances in mathematics allowed the development of increasingly sophisticated cosmological models. Many advances in mathematics pointed toward a universe not necessarily limited to three dimensions and not necessarily absolute in time. These intriguing ideas found expression in the intricacies of relativity and theory that, for the first time, allowed cosmologists a theoretical framework upon which they could attempt to explain the innermost workings and structure of the universe both on the scale of the subatomic world and on the grandest of galactic scales.

As direct consequence of German-American physicist Albert Einsteins (18791955) relativity theory, cosmologists advanced the concept that space-time was something that arose from the universe itself that matter and energy not only exist in space but defines space. This insight set the stage for the development of modern cosmological theory and provided insight into the evolutionary stages of stars (e.g., neutron stars, pulsars, black holes, etc.) that carried with it an understanding of nucleosynthesis (the formation of elements) that forever linked the physical composition of matter on Earth to the lives of the stars.

Twentieth-century progress in cosmology has been marked by corresponding and mutually beneficial advances in technology and theory. American astronomer Edwin Hubbles (18891953) discovery that the universe was expanding, Arno A. Penzias and Robert W. Wilsons observation of cosmic background radiation, and the detection of the elementary particles that populated the very early universe all proved important confirmations of the big bang theory. The big bang theory asserts that all matter and energy in the Universe, and the four dimensions of time and space were created from the primordial explosion of a singularity of enormous density, temperature, and pressure.

During the 1940s Russian-born American cosmologist and nuclear physicist George Gamow (19041968) developed the modern version of the big bang model based upon earlier concepts advanced by Russian physicist Alexander (Aleksandr Aleksandrovich) Friedmann (also spelled as Fridman, 18881925) and Belgian astrophysicist and cosmologist Abbé Georges Lemaître (18941966). Big bang based models replaced static models of the universe that described a homogeneous universe that was the same in all directions (when averaged over a large span of space) and at all times. Big bang and static cosmological models competed with each other for scientific and philosophical favor. Although many astrophysicists rejected the steady state model because it would violate the law of mass-energy conservation, the model had many eloquent and capable defenders. Moreover, the steady model was interpreted by many to be more compatible with many philosophical, social and religious concepts centered on the concept of an unchanging universe. The discovery of quasars and of a permeating cosmic background radiation eventually tilted the cosmological argument in favor of big bang-based models.

Technology continues to expand the frontiers of cosmology. The Hubble Space Telescope has revealed gas clouds in the cosmic voids and beautiful images of fledgling galaxies formed when the universe was less than a billion years old. Analysis of these pictures and advances in the understanding of the fundamental constituents of nature continue to keep cosmology a dynamic discipline of physics and the ultimate fusion of human scientific knowledge and philosophy.

Evolution of cosmological thought

Using such instruments as the Hubble Space Telescope, modern cosmology is an attempt to describe the large scale structure and order of the universe. To that end, one does not expect cosmology to deal with the detailed structure such as planets, stars, or even galaxies. Rather it attempts to describe the structure of the universe on the largest of scales and to determine its past and future.

One of the earliest constraints on any description of the universe is generally attributed to Heinreich Olbers in 1826. However, more careful study traces the idea back to Thomas Digges in 1576 and it was thoroughly discussed by Edmond Halley at the time of Isaac Newton. The notion, still called Olbers paradox, is concerned with why the night sky is dark. At the time of Newton it was understood that if the universe was finite then Newtons Law of Gravity would require that all the matter in the universe should pull itself together to that point equidistant from the boundary of the universe. Thus, the prevailing wisdom was that the universe was infinite in extent and therefore had no center. However, if this were true, then the extension of any line of sight should sooner or later encounter the surface of a star. The night sky should then appear to have the brightness of the average star. The sun is an average star, thus one would expect the sky to be everywhere as bright as the sun. It is not, so there must be a problem with the initial assumptions.

An alternative explanation, pointed out in 1964 by Ed Harrison, is that the universe had a finite beginning and the light from distant stars had not yet had time to arrive and that is why the night sky is dark.

At the turn of the twentieth century cosmology placed the sun and its solar system of planets near the center of the Milky Way galaxy, which comprised the full extent of the known Universe. However, early in the century information began to be compiled that would change the popular view that the universe was static and primarily made of stars. On April 26, 1920, there was an historic debate between H.D. Curtis and Harlow Shapley concerning the nature of some fuzzy clouds of light, which were called nebulae. Shapley thought they were objects within the galaxy while Curtis believed them to be island universes lying outside the Milky Way.

Although at the time most agreed that Shapley had won the debate, science eventually proved that Curtis was right. Within a few years Edwin Hubble detected a type of star, whose distance could be independently determined, residing within several of these fuzzy clouds. These stars clearly placed the clouds beyond the limits of the Milky Way. While Hubble continued to use the term island universe, more and more extragalactic nebulae were discovered, and they are now simply known as galaxies. In the span of a quarter of a century the scale for the universe had grown dramatically.

The expanding Universe

During the time of Hubbles work, V.M. Slipher at the Lowell Observatory had been acquiring spectra of these fuzzy clouds. By breaking the light of astronomical objects into the various colors or wavelengths that make up that light, astronomers can determine much about the composition, temperature, and pressure of the material that emitted that light. The familiar spectrum of hydrogen so common to so many astronomical objects did not fall at the expected wavelengths, but appeared shifted to longer wavelengths. We now refer to such a change in wavelength as a redshift. Slipher noted that the fainter galaxies seemed to have larger redshifts and he sent his collection of galactic spectra off to Hubble.

Hubble interpreted the redshift as being caused by the Doppler effect, and thus representing motion away from us. The increasing faintness was related to distance so that in 1929 Hubble turned Sliphers redshift brightness relation into a velocitydistance relation and the concept of the expanding universe was born. Hubble found that the velocity of distant galaxies increased in direct proportion to their distance. The constant of proportionality is denoted by the symbol H0 and is known as Hubbles constant.

Although the historical units of Hubbles constant are (km/s/mpc), both kilometers and megaparsecs are lengths so that the actual units are inverse time (i.e., 1/sec). The reciprocal of the constant basically gives a value for the time it took distant galaxies to arrive at their present day positions. It is the same time for all galaxies. Thus, the inverse of Hubbles constant provides an estimate for the age of the expanding universe called the Hubble age.

Due to its importance, the determination of the correct value of the Hubble constant has been a central preoccupation of many astronomers from the time of Hubble to the present. Since the gravitational pull of the matter in the Universe on itself should tend to slow the expansion, values of Hubbles constant determined by hypothetical astronomers billions of years ago would have yielded a somewhat larger number and hence a somewhat younger age. Therefore the Hubble age is an upper limit to the true age of the universe which depends on how much matter is in the Universe.

The notion of a Heavens (i.e., Universe) that was dynamic and changing was revolutionary for the age. Einstein immediately modified the equations of the general theory of relativity, which he applied to the Universe as a whole to deal with a dynamic universe. Willem de Sitter and quite independently Alexandre Friedmann expanded on Einsteins application of general relativity to a dynamically expanding Universe.

The concept of a universe that is changing in time suggests the idea of predicting the state of the universe at earlier times by simply reversing the present dynamics. This is much like simply running a motion picture backwards to find out how the movie began. A Belgian priest by the name of Georges Lemaitre carried this to its logical conclusion by suggesting that at one time the universe must have been a very congested place with matter so squeezed together that it would have behaved as some sort of primeval atom.

The big bang

The analysis of Einstein, de Sitter, Friedmann, Lemaitre, and others showed that the dynamic future of the expanding universe depended on the local density. Simply put, if the density of the universe were sufficiently high, then the gravitational pull of the matter in any given volume on itself would be sufficient to eventually stop the expansion. Within the description given by the General Theory of Relativity, the matter would be said to warp space to such an extent that the space would be called closed. The structure of such a universe would allow the expansion to continue until it filled the interior of a black hole appropriate for the mass of the entire universe at which point it would begin to collapse. A universe with less density would exhibit less space warping and be said to be open and would be able to expand forever.

There exists a value for the density between these extremes where the matter of the universe can just stop the expansion after an infinite time. Such a universe is said to be flat. One of the central questions for observational cosmology continues to be which of these three cases applies to our universe.

George Gamow concerned himself with the early phases of an expanding universe and showed that Lemaitres primeval atom would have been so hot that it would explode. After World War II, a competing cosmology developed by Hermann Bondi, Thomas Gold, and Fred Hoyle was put forth in order to avoid the ultimate problem with the expanding universe, namely, it must have had an origin. The Steady State Cosmology of Bondi, Gold, and Hoyle suggested that the universe has existed indefinitely and that matter is continuously created so as to replace that carried away by the observed expansion.

This rather sophisticated cosmology replaced the origin problem of the expanding universe by spreading the creation problem out over the entire history of the universe and making it a part of its continuing existence. It is somewhat ironic that the current name for the expanding universe cosmology as expressed by Gamow is derived from the somewhat disparaging name, big bang, given to it by Fred Hoyle during a BBC interview.

Gamow and colleagues noted that a very hot primeval atom should radiate like a blackbody (i.e., a perfect thermal radiator), but that radiation should be extremely red-shifted by the expansion of the universe so that it would appear today like a very cold blackbody. That prediction, made in 1948, would have to wait until 1965 for its confirmation. In that year Arno Penzias and Robert Wilson announced the discovery of microwave radiation which uniformly filled the sky and had a blackbody temperature of about 2.7K (454.5°F[270.3°C]).

While Gamows original prediction had been forgotten, the idea had been re-discovered by Robert Dicke and his colleagues at Princeton University. Subsequent observation of this background radiation showed it to fit all the characteristics required by radiation from the early stages of the big bang. Its discovery spelled the end to the elegant Steady State Cosmology, which could not easily accommodate the existence of such radiation.

Implications of the big bang

After the World War II, the science of nuclear physics developed to a point at which it was clear that nuclear reactions would have taken place during the early phases of the big bang. Again scientists ran the motion picture backwards through an era of nuclear physics, attempting to predict what elements should have been produced during the early history of the universe. Their predictions were then compared to the elemental abundances of the oldest stars and the agreement was amazingly good.

The detailed calculations depended critically on whether the calculated model was for an open or closed universe. If the universe were open, then the era of nuclear reactions would not last long enough to produce elements heavier than hydrogen and helium. In such models some deuterium is formed, but the amount is extremely sensitive to the initial density of matter. Since deuterium tends to be destroyed in stars, the current measured value places a lower limit on the initial amount made in the big bang. The best present estimates of primordial deuterium suggest that there is not enough matter in the universe to stop its expansion at any time in the future.

In the event of an open universe, scientists are pretty clear what the future holds. In 1997, researchers from the University of Michigan released a detailed projection of the four phases of the universe, including the ultimate end in the Dark Era, some 10100 years hence. Currently, the universe is in the Stelliferous Era, dominated by high-energy stars and filled with galaxies. Some 1, 000 trillion years from now, the universe will enter the Degenerate Era. Stars will have burned down into degenerate husks that can no longer support hydrogen burning reactions, and will exist as white dwarfs, red dwarfs, brown dwarfs, or neutron stars; some massive stars will have collapsed into black holes, which will consume the other star relics.

Next, the universe will progress into the Black Hole Era, about 100 trillion trillion trillion years from the present. At that time, black holes will have swallowed up the remaining bodies in the universe and will gradually leak radiation themselves, essentially evaporating away over trillions of years. Finally, the universe will reach the Dark Era, in which no matter will exist, only a soup of elementary particles like electrons, positrons, neutrinos, and other exotic particles.

Since the late 1990s, a large body of astronomical evidence has shown the universes expansion is not slowing at all, but accelerating, which would seem to imply an open universe with an infinite lifespan.

Cosmologists still battle over the exact nature of the universe. Most scientists agree that the age of the universe ranges between 13 and 15 billion years. The exact age is a matter of great controversy between rival research teams at Carnegie Observatories and the Space Telescope Science Institute. When researchers recently used an analysis of polarized light to show that the universe is not isotropic, i.e., not the same in all directions, their findings were disputed almost as soon as they were published.

Trouble in paradise

In the last quarter of the twentieth century some problems with the standard picture of the big bang emerged. The extreme uniformity of the cosmic background radiation, which seemed initially reasonable, leads to a subtle problem. Consider the age of elements of the cosmic background radiation originating from two widely separated places in the sky. The distance between them is so great that light could not travel between them in an amount of time less than their age. Thus the two regions could never have been in contact during their existence. Why then, should they show the same temperature? How was their current status coordinated? This is known as the horizon problem.

The second problem has to do with the remarkable balance between the energy of expansion of the universe and the energy associated with the gravitational forces of the matter opposing that expansion. By simply counting the amount of matter we see in the universe, we can account for about 1% of the matter required to stop the expansion and close the universe. Because the expansion causes both the expansion energy and the energy opposing the expansion to tend to zero, the ratio of their difference to either one tends to get larger with time. So one can ask how good the agreement between the two was, say, when the cosmic background radiation was formed.

The answer is that the agreement must have been good to about one part in a million. If one extends the logic back to the nuclear era where our physical understanding is still quite secure, then the agreement must be good to about thirty digits. The slight departure between these two fundamental properties of the universe necessary to produce what we currently observe is called the Flatness Problem. There is a strong belief among many cosmologists that agreement to thirty digits suggests perfect agreement and there must be more matter in the universe than we can see.

This matter is usually lumped under the name dark matter since it escapes direct visible detection. It has become increasingly clear that there is indeed more matter in the universe than is presently visible. Its gravitational effect on the rotation of galaxies and their motion within clusters of galaxies suggests that

KEY TERMS

Grand Unified Theory Any theory which brings the description of the forces of electromagnetism, weak and strong nuclear interactions under a single representation.

Hubble constant The constant of proportionality in Hubbles Law which relates the recessional velocity and distance of remote objects in the universe whose motion is determined by the general expansion of the universe.

Inflation cosmology A modification to the early moments of the big bang cosmology which solves both the flatness problem and the horizon problem.

Megaparsec A unit of distance used in describing the distances to remote objects in the universe. One megaparsec (i.e., a million parsecs) is approximately equal to 3.26 million light years or approximately ten trillion trillion centimeters.

Olbers paradox A statement that the dark night sky suggests that the universe is finite in either space or time.

Planck time An extremely short interval of time (i.e., 1043sec) when the conventional laws of physics no longer apply.

Primeval atom The description of the very early expanding universe devised by Abbe Lemaitre.

Spectra The representation of the light emitted by an object broken into its constituent colors or wavelengths.

Steady state cosmology A popular cosmology of the mid twentieth century which supposed that the universe was unchanging in space and time.

we see perhaps only a tenth of the matter that is really there. However, while this amount is still compatible with the abundance of deuterium, it is not enough to close the universe and solve the flatness problem.

Any attempt to run the motion picture further backwards before the nuclear era requires physics which, while less secure, is plausible. This led to a modification of the big bang by Alan Guth called inflation. Inflation describes an era of very rapid expansion where the space containing the matter-energy that would eventually become galaxies spread apart faster than the speed of light for a short period of time.

Inflation solves the horizon problem in that it allowed all matter in the universe to be in contact with all other matter at the beginning of the inflation era. It also requires the exact balance between expansion energy and energy opposed to the expansion, thereby solving the flatness problem. This exact balance requires that there be an additional component to the dark matter that did not take part in the nuclear reactions that determined the initial composition of the universe. The search for such matter is currently the source of considerable effort.

Finally, one wonders how far back one can reasonably expect to run the movie. In the earliest microseconds of the universes existence the conditions would have been so extreme that the very forces of nature would have merged together. Physical theories that attempt to describe the merger of the strong nuclear force with the electro-weak force are called Grand Unified Theories, or GUTs for short. There is currently much effort being devoted to testing those theories. At sufficiently early times even the force of gravity should become tied to the other forces of nature. The conditions which lead to the merging of the forces of nature are far beyond anything achievable on Earth so that the physicist must rely on predictions from the early universe to test these theories.

Ultimately quantum mechanics suggests that there comes a time in the early history of the universe where all theoretical descriptions of the universe must fail. Before a time known as the Planck Time, the very notions of time and space become poorly defined and one should not press the movie further. Beyond this time science becomes ineffective in determining the structure of the universe and one must search elsewhere for its origin.

See also Relativity, general; Relativity, special; String theory; Symmetry.

Resources

BOOKS

Dinwiddie, Robert, et al. Universe. London: DK Adult, 2005.

Ferreira, Pedro. The State of the Universe: A Primer in Modern Cosmology. London: Cassell, 2006.

Hawkings, Stephen and Mlodinow, Leonard. A Briefer History of Time. New York: Bantam, 2005.

PERIODICALS

Bennett, Charles L. Cosmology from Start to Finish. Nature. 440 (2006): 1126-1131.

Brumfiel, Geoff. Our Universe: Outrageous Fortune. Nature. 439 (2006): 10-12.

Cho, Adrian. Long-Awaited Data Sharpen Picture of Universes Birth. Science. 311 (2006): 1689.

Guth, Alan H. and David I. Kaiser. Inflationary Cosmology: Exploring the Universe from the Smallest to the Largest Scales. Science. 307 (2005): 884-890.

K. Lee Lerner

George W. Collins, II

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Cosmology

Cosmology

CENTERING THE UNIVERSE

Sources

The Cosmos. Despite the persistent myth that scholars in Christopher Columbus's day feared he would sail off the edge of a flat earth, there is ample evidence in texts from the early Middle Ages suggesting a basic understanding of Earth's sphericity, its location at the center of the spherical cosmos, and its immobility. These ideas survived the economic collapse of the Western Roman Empire. The seven liberal arts that well-born Roman citizens studied included the study of astronomy. This curricular model became the foundation for the revival of education in the Carolingian period and was a presumed basis for advanced study in the high medieval university; hence, rudimentary knowledge of cosmology never wholly disappeared from learned culture. However, a working knowledge of astronomy, sufficient to cast a medical horoscope, was a rarity in the Latin West before Arabic treatises were sought out and translated during the first great wave of the recovery of classical knowledge in the eleventh and twelfth centuries. At that time Europeans learned to use the astrolabe, a sighting instrument perfected in the Moslem East for calculating celestial coordinates. Also in the twelfth century the treatises of Aristotle were translated. By the thirteenth century these texts became the basis for all branches of philosophy taught at the university. Aristotle had disregarded observational astronomy, which he considered as a subdiscipline of mathematics, and instead elaborated a model of the physical structure of the cosmos, the study of which is called cosmology.

Two Distinct Realms. Aristotelian cosmology taught that the terrestrial world comprises four concentric elemental spheres, into which the four basic material elements— earth, water, air, and fire—generally settle. Earth, being the heaviest, seeks the center of the cosmos, which is why earth is central. Water settles in around earth, then air, then fire. The outer reaches of the sphere of fire extended almost to the invisible sphere that allegedly carried the Moon around Earth. The four elements were in constant motion, mixing with one another and penetrating each other's native region, which explains moisture in the air, fire within wood, and so on. In stark contrast, the large region that began with the sphere of the Moon and extended out to the celestial sphere, which was the physical limit of the cosmos and held all of the visible stars, was regarded as eternal and unchanging and composed of a fifth element, quintessence. All motions in this region, which encompassed the orbs of Mercury, Venus, the Sun, Mars, Jupiter, and Saturn, were obliged to be circular and of constant rotational speed, because only uniform spherical (circular) motion was at once moving and unchanged, inasmuch as all parts of a sphere remain at a constant distance from its center and cyclically and regularly return to the same locations. The divinely inspired motive power for the cosmos originated in the outermost sphere and propagated inward toward the center, turning each planet and eventually stirring up the terrestrial elements. Thus, Aristotle divided the cosmos into two distinct realms—terrestrial and celestial— each with its own intrinsic laws and nature. In the celestial world nothing changed and all beings naturally moved in circles, and there was perfect order. In the terrestial world, birth, growth, death, and decay, which entailed the combination and recombinations of the elements, ensured a constant state of change and disorder.

Accepted Theory. This cosmos was the one Dante described in The Divine Comedy (circa 1308–1321) and was, with a few modifications, warmly embraced by Christian theologians, philosophers, and the learned laity. The order of Aristotle's cosmos reflected the wisdom, order, and benevolence of the Creator, who placed his special creature, man, in the center. This view made perfect sense, since Earth seems at rest, heavy objects seem to fall and light ones rise, and the heavens seem to go through their cycles of conjunctions with unchanging regularity, shaping the seasons and their effects. The gradient of perfection, beginning with the most perfect, outermost heaven, where the souls of the departed joined Christ and awaited resurrection, passed inward through the celestial orbs, where the angelic intelligences dwelled, and then through the changeable fiery, aerial, and watery spheres, ending up in the interior elemental earth, where base matter seethed and spontaneously generated snakes, insects, and other “infernal,” or lower, creatures.

CENTERING THE UNIVERSE

In tliis passage feorn book one. of his “On the Revolutions of the .Heavenly. Spheres” (1543), Nicholas Copernicus describes the physical makeup of the cosmos as he sees it, with the Earth having been replaced by the Sun as the center. Chief among the reasons he .expresses here such, a radical shift in cosmology are spatial symmetry, mathematical ordering, and a feeling that the Sun, Which is a visible emblem of life-giving divinity, should hold a central place in the system. It is crucial to Copernicus's. argument that he convince readers that all the apparent .movements of the stars and planets can be accounted for by the relative {notions of the earth and planets and by the great distance between, the Sun and the stars.

The first and highest of all is the sphere, of the fixed Stars, which contains itself and all things, and is therefore motionless. It is the location of the universe, to which the motion and position of all the remaining stars is referred. For though some consider that it also changes in some respect, we. shall assign, another cause for .its, appearing to do so in our deduction of the Earth's motion. There follows: Saturn, the first of .the wandering stars, which completes its circuit in thirty years. After it comes Jupiter which moves ia a twelve-yearlong revolution. Next is Mars, which goes:round biennially. An annual revolution holds the fourth place, in which as we have said is contained the Earth. aldng with the lunar sphere which; is like an epicycle. In fifth place Venus returns every nine .months. Lastly, Mercury holds the sixth place, making a circuit in the space of eighty days. In the middle of all is the seat of the Sun. For who in this most beautiful of temples would put this lamp in any other or better place; than the one from which it can ffluminate everything at the same time? Aptly indeed is he, named, by, some- the lantern of the universe, by others the mind, by others the ruler. Trismegistus. called him the visible God, Sophocles' Electra, the watcher over all things. Thus indeed, the Bun as if seated on a royal throne governs his household of Stars as they circle round him. Earth also is by no means cheated of the Moon's attendance, but as Aristotle says in his book On Animals the Moon has the closest affinity with the Earth. Meanwhile the Earth conceives from the Sun,, and is made pregnant with annual offspring. We find, then, in this arrangement the marvellous symmetry of the universe, and a sure linking together in harmony of the motion and size of the spheres, such as could be perceived in no other way. For here one may understand, by attentive observation, why Jupiter appears to have a larger progression and retrogression than Saturn, and smaller than Mars, and again why Venus has larger ones than Mercury, why such a doubling back appears more frequently in Saturn than in Jupiter, and stifl more rarely in Mars and Venus than in Mercury; and furthermore, why Saturn, Jupiter and Mars are nearer to the Earth when in opposition than in the region of their occulta-tion by the Sun and re-appearance. Indeed Mars in particular at the time when it is visible throughout the night seems to equal Jupiter in size, though marked out by its reddish colour; yet it is scarcely distinguishable among stars of the second magnitude, though recognised by those who track it with careful attention. All these phenomena proceed from the same cause, which lies in the motion of the Earth. But the fact that none of these phenomena appears in the fixed stars shows their immense elevation, which makes even the circle of their annual motion, or apparent motion, vanish from our eyes. For every visible object has some limit of distance, beyond which it is no longer seen, as is shown in the Optics. That there is still a very great distance from Saturn, the highest of the wandering stars, to the sphere of the fixed stars is shown by the scintillation of their lights. It is by this mark that the planets are particularly distinguished, for there had to be a particular point of difference between the moving and unmoved stars. Such truly is the size of this structure of the Almighty's.

Source: Nicholas Copernicus, Copernicus: On the Revolutions of the Heavenly Spheres, translated by A. M. TJuncan (Newton Abbot, ILK.: David & Charles, 1976; New York: Barnes & Noble, 1976), pp. 49–51.

Mirror of Society. Dante's Aristotelian image of the physical world mirrored the social and political structure of medieval Europe, where the unquestioned perfection and authority of the kings and popes trickled down through the feudal and ecclesiastical hierarchies to touch the basest bondsmen, whose world also was characterized by constant change, imperfection, birth, corruption, and death. To paraphrase the Tabula smaragdina (Emerald Tablet of Hermes)—a compilation of ancient alchemical, occultic, and theological works reputedly by the Hellinistic author Hermes Trismegistos—what is below reflects and is governed by that which is above; every being in the cosmos owes its livelihood to its superiors, and its condition mirrors their wisdom and benevolence. By the late Middle Ages, Aristotelian cosmology was well integrated into the political, religious, and moral order of Europe, which is why its eventual demise has been considered by historians as a “scientific revolution” concurrent with the painful social, religious, and economic revolutions that marked the transition from medieval feudalism to the modern nation-state.

The Almagest. Ironically, the effort to perfect the Aristotelian cosmology is what eventually destroyed it and laid the basis for a more modern view of the universe as a huge, largely empty space, in which the Sun is but one of many and Earth just another planet. The need for a more accurate calendar and a refined astrology, along with a general humanist zeal to recover and understand the wisdom of the ancient Greeks, Egyptians, Hebrews, and Chaldeans, encouraged scholars to seek out Greek astronomical and mathematical manuscripts and improve on their science. In the fifteenth century Georg Puerbach and Johannes Regiomontanus searched out new manuscripts of Ptolemy’s Almagest (second century C.E.), the greatest ancient treatment of astronomy, and set about adapting his methods to the three-dimensional, physical cosmology of the medieval Aristotelians. Regiomontanus wrote an epitome, or digested form, of the Almagest, which helped explain the details of Ptolemy’s mathematical approach, and a humanist Greek edition of the text was printed in the sixteenth century. However, by then astronomers were beginning to realize that refinement in Ptolemy’s methods was not enough to correct the errors that kept throwing off the calendar and would also, logically, reduce the accuracy of astrological prediction.

Copernicus. In the next generation, a young Pole from the Baltic north named Nicholas Copernicus determined that a true restoration of ancient astronomy required rejection of the geocentric model that formed the basis of Ptolemy’s astronomy and Aristotle’s cosmology. Instead, he went back to the earlier astronomy of Aristarchus of Samos, who was reported to have taught that the Sun is the center of the cosmos and Earth revolves about it, as a planet. Copernicus had studied both astronomy and cosmology at the University of Cracow in Poland before heading to Italy to study law and medicine. After his return to his homeland, he set down his ideas in a short manuscript called the Commentariolus, or “Little Commentary” (1514), which was copied and circulated among astronomers.

Wandering Earth Copernicus’s hypothesis was that Earth rotated on its axis, rather than lying stationary, while the huge celestial sphere turned round once every twenty-four hours, giving the illusion that the planets and the stars rise in the east, stretch across the vault of the sky, and set to the west. He also thought that the Sun, as symbol of God and source of life-giving warmth, was the true center of the cosmos, and that the Earth revolved about it, traveling at a considerable speed. In short, he argued that Earth was a planet or “wanderer” (planetoi), like Mars or any other. The idea was generally greeted as idle speculation, contrary to both common sense and intellectual tradition, but it eventually sparked the curiosity of a Lutheran philosopher at Wittenberg named Georg Joachim Rheticus, who traveled to Poland to learn more from Copernicus and persuade him to publish the details of his new astronomy. The result was the Narratio prima (First Narration), which Rheticus published in 1540, followed by Copernicus’s fully articulated mathematical astronomy, De revolutionibus orbium coelestium (On the Revolutions of the Celestial Orbs) in 1543.

Heliocentrism. From the beginning, the religious implications of the Sun-centered cosmology (heliocentrism) were of concern, since the Bible taught that Earth was stationary and central and that the Sun rose and set, moving about it. However, the absurdity of a moving Earth, both from the standpoint of Aristotelian cosmology and the common-sense view that the Earth did not seem to be moving and that stones fell “downward” toward the center, prevented the new system from being seriously considered as reality before the arguments of Galileo and the mathematical refinements of Kepler began to win converts to heliocentrism in the seventeenth century. In the intervening half century or more, astronomers who taught the Copernican system and used it to calculate planetary movements did so because it was mathematically convenient, not because they regarded it as a step toward grasping the cosmological truth. It was in fact not astronomers who first defended the Copernican model as a plausible physical system, but rather natural philosophers, who rejected elements of Aristotelian philosophy for other reasons and were drawn to heliocentrism as an alternative. The most flamboyant of these was the renegade Dominican monk and philosopher, Giordano Bruno.

Dissenter. Bruno ran afoul of his Dominican brothers in southern Italy for holding religious and cosmological views that were at variance with Catholicism. He abandoned his monastery and headed north. After a brief stay in Paris, he traveled to England, where he espoused his doctrine of the infinity of the universe, in which Earth was not central. His argument was more theologically than scientifically motivated, since he reasoned that the cosmos must reflect God and be infinitely large, with no particular center—or rather that its center was present everywhere, just as God was fully present everywhere. Nevertheless, Bruno’s vision of a radically new world order attracted the attention of his contemporaries, who were just beginning to undo the Aristotelian cosmology. In the hindsight of nineteenth-century historians, Bruno became something of an emblem of the struggle of scientific truth against the

intransigence of tradition and the totalitarian authority of the Church, which ordered Bruno to be burned alive at the stake in Rome in 1600. His crime, however, was not Copernicanism, but espousing heretical ideas about Christ. The Catholic Church did not yet have an official, legal policy about the Copernican hypothesis.

Ellipses. Bruno was not alone in questioning traditional cosmology in the late sixteenth century. The English mathematician Thomas Digges believed that there was no celestial sphere, but that the stars filled an extended (perhaps boundless) space, and this notion supported Copernicus’s claim that the cosmos was larger than the ancients had supposed. However, he was not ready to put Earth in motion. His countryman, William Gilbert, likewise could not abandon the idea of a central Earth, but did argue that it rotated on its axis every day. Medieval scholars had entertained this notion in the early fourteenth century, arguing that it would be more economical for the Earth to rotate once every twenty-four hours than to have the entire mass of the cosmos—all the stars, planets, Sun, Moon, and their various spheres—revolve about the fixed Earth. However, where the fourteenth-century scholastic philosophers considered this idea as a logical possibility and dismissed it for want of philosophical merit, Gilbert attributed Earth’s motion to its intrinsic magnetic soul, the existence of which he demonstrated through careful experimentation and reported in his De magnete (On the Magnet, 1600). Gilbert’s proposal planted the seed of an idea in the head of Johannes Kepler, who argued that all the celestial bodies have intrinsic moving souls, which account for the motions of the Moon about Earth and Earth and other planets about the Sun. Kepler argued that a magnetic soul, such as the kind postulated by Gilbert for Earth, accounted for why the planets did not orbit in perfect circles but were alternately attracted and then repelled, bending the circles into ellipses.

Royal Favorite. The extent to which Renaissance theorists resisted discarding the belief that Earth was at rest in the center of the cosmos is well illustrated by the work of its greatest astronomer, Tycho Brahe, whose publications were authoritative at the close of the sixteenth century. Brahe’s astronomy was shaped by his conviction that the real problem with the science in his day, which was obviously not capable of accurate predictions, was not a failure of theory but an absence of dependable observational facts. His determination to systematically scrutinize and measure the heavens and reform astronomy was stimulated by his 1572 observation of a nova Stella (new star) in the constellation Cassiopeia. Brahe’s observation was but one of many in Europe, but it was more detailed and accurate than others and was published at a time when it attracted the attention of the king of Denmark, who was anxious to reward Brahe and also foster natural philosophy in his kingdom. The result was that Brahe, already a wealthy member of Denmark’s high nobility, was granted state funding to build Europe’s first scientific research institution on the island of Hven. There he built an observatory and alchemical laboratory, both of which surpassed their contemporaries.

Alternative. The key to the significance of the “new star,” which appeared out of nowhere, went through a sequence of color changes, and then vanished, was that it occurred in the perfect and unchanging celestial region and was therefore in violation of Aristotelian theory. Subsequent observations by Brahe and his assistants revealed that comets also were celestial but passed through the supposed celestial orbs that moved the planets, descending toward Earth and then ascending again to the stars. This view, too, ran counter to Aristotelian cosmology, according to which comets were meteorological phenomena and occurred in the terrestrial atmosphere, but Brahe’s measurements proved decisively that comets were extraterrestrial. Despite his readiness to break down the terrestrial-celestial distinction in medieval cosmology, Brahe was unwilling to abandon the stationary central Earth. None of his observations supported Copernicus’s claim that Earth revolved about the Sun, and Brahe found no reason to oppose the religious consensus that God, speaking through the Bible, taught that the Earth was at rest. Instead, he devised a compromise model, in which the Sun and the Moon revolve about the Earth, while the planets revolve around the Sun. This model was mathematically equivalent to the Copernican, heliocentric system, but did not require Earth to be in motion. It was sufficiently successful to be adopted by Catholic educators in the seventeenth century. Since the church rejected Galileo’s Copernican model on the grounds that it violated Scriptural authority, which required a central Earth, Brahe’s system was a logical alternative to the ancient Aristotelian/Ptolemaic model of the cosmos, which Galileo’s telescopic observations of the phases of Venus had disproved.

Staying Power. As the sixteenth century closed, the classical cosmology was in serious disrepair but still the dominant view. It would eventually fall during the course of the seventeenth century, partly put to rest by the successful mathematical astronomy of Isaac Newton, which brought the observations and arguments of Galileo, Kepler, Brahe, and others into a coherent explanation of celestial motions that was based solely on matter, inertial motion, and gravitational attraction. In 1600, however, little of this development was in evidence. Indeed, arguments about cosmology then seemed to occupy not only a philosophical but also a theological space. Bruno’s imaginative universe was postulated on his view of the creative and sustaining divinity that powered it. His philosophical writings assume a divine, Platonic world soul that unifies everything. Brahe’s assistant, Kort Aslakssøn, argued that the processes at work in the heavens were also those that work on Earth and in the alchemical laboratory, and that these were essentially spiritual. Quite likely his views reflected those of his mentor, and he carried them with him into a theological career at the University of Copenhagen. Even Kepler’s early arguments about the structure and functioning of the planetary system were founded on ideas of divinity, soul, and a kind of intelligence that was part of created matter, accounting for how remote objects could influence one another—how the things above could affect those below, as the Emerald Tablet of Hermes taught. Modernity had not arrived.

Sources

Marie Boas, The Scientific Renaissance 1450-1630 (New York: Harper, 1962).

James M. Lattis, Between Copernicus and Galleo: Christoph Clavius and the Collapse of Ptolemaic Cosmology (Chicago: University of Chicago Press, 1994).

C. S. Lewis, The Discarded Image: An Introduction to Medieval and Renaissance Literature (Cambridge: Cambridge University Press, 1964).

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Cosmology

COSMOLOGY

Although astronomers and natural philosophers have always been interested in cosmology, it is only in the twentieth century that a science of the universe has become a reality. In particular, from about 1950 cosmological theory began to interact ever more strongly with nuclear and particle physics. This development has accelerated in the later part of the 20th century with the result that parts of cosmology (especially the early big bang theory) have become thoroughly integrated with new theories of elementary particles and fundamental interactions. Not only have theories of microphysics greatly advanced cosmological knowledge, but the early universe has also proved a valuable testing ground for fundamental particle physics.

Early Cosmology

Modern cosmology dates essentially from 1917, when Albert Einstein suggested a cosmological model based on his new theory of gravitation, the general theory of relativity. Einstein's universe was static, spatially finite, and filled with dilute matter. The same year, 1917, Willem de Sitter showed that the theory of relativity allowed another cosmological solution, corresponding to an empty universe. Both models made use of the cosmological constant that Einstein had introduced in his field equations as a parameter of a hypothetical antigravity force.

Nonstatic or evolutionary models were first investigated in 1922, by the Russian physicist Alexander Friedmann, but his work was ignored. Five years later, in 1927, the Belgian Georges Lemaître rediscovered the expanding universe and suggested a model in which the universe expands steadily from an Einstein world. The theories of Friedmann and Lemaître only became known in 1930, after Edwin Hubble had found observational evidence for the expanding universe. From that time onward, most physicists and astronomers accepted that the universe is in a state of expansion. At the same time, the cosmological constant fell in discredit and was often seen as an unnecessary complication.

A further step was taken by Lemaître in 1931, when he suggested for the first time what would later be called a Big Bang model, that is, a universe of finite age that has originated from an ultradense state. Lemaître called this state the primeval atom and suggested that its explosion was a quantum process, like in radioactive decay. The Einstein–de Sitter model of 1932, an early Big Bang model, assumed that there is just enough matter to keep the universe spatially flat. This matter density, which depends on the value of the Hubble constant, is called the critical density.

When Lemaître suggested his primeval atom hypothesis, the proton and the electron were the only known (massive) elementary particles. With the emergence of nuclear physics in 1932, new possibilities were offered for astrophysics and a more physical approach to cosmology. The entry of nuclear and particle physics into cosmology was largely a result of astrophysicists' attempts to understand the energy production of stars and the formation of heavier elements out of protons and neutrons. In a 1938 paper, Carl Friedrich von Weizsäcker sought to infer the state of the early universe by assuming that the chemical elements had been formed under conditions that would reproduce their present individual abundance. Von Weizsäcker was led to a kind of Big Bang universe, starting in an extremely hot and dense state, but he did not develop his ideas further. Such development was left to George Gamow, the Russian-American nuclear physicist and pioneer of cosmology.

Gamow's Physical Cosmology

Von Weizsäcker's methodology—to use the cosmic distribution of chemical elements as evidence for the physical state of the early universe—is sometimes known as nuclear archaeology. It was this method that guided Gamow's development of physical Big Bang cosmology in the years between 1946 and 1956. Together with his collaborators Ralph Alpher and Robert Herman, he was led to consider the primordial universe as a hot, dense gas of neutronic matter. Right after the Big Bang, neutrons would decay into protons and electrons, and some of the protons would combine with neutrons to form deuterons. The essential process in the building-up of higher nuclei was believed to be the capture of neutrons. Between 1948 and 1950 it was realized that this picture was too primitive and that the assumption of a matter-dominated early universe was un-tenable. Gamow and Alpher argued that during the first phase of the expansion, the energy content of the universe would be governed almost entirely by electromagnetic radiation (that is, photons). Only at a later stage, when the universe had become colder, would matter begin to dominate over radiation.

Gamow's program of Big Bang cosmology culminated in a 1953 paper, which can be considered the first example of particle cosmology. Written by Ralph Alpher, Robert Herman and James Follin, the paper provided a detailed analysis of the early universe, now thought to consist mainly of photons, neutrinos, and electrons (including positrons), but also with small amounts of nucleons and muons. The Alpher-Herman-Follin theory made full use of the most recent progress in particle physics, including the theory of weak interactions as applied to beta decay and processes involving electrons, neutrinos, and muons. The three physicists found the present abundance of helium to be about 30 percent and also calculated the neutron-to-proton ratio at the time nucleosynthesis started. However, the wider aim of the Gamow program—to account for the formation of all chemical elements—failed. Gamow and his collaborators were unable to build up elements heavier than helium, and this was widely considered a failure of the Big Bang theory itself.

Only in 1965 did Big Bang cosmology experience a renaissance, this time irreversibly. The discovery of the cosmic microwave background radiation of temperature about 3K provided an important parameter for new and improved calculations of the synthesis of the lightest elements. In 1966, James Peebles calculated from the Big Bang theory a helium abundance between 26 and 28 percent, in excellent agreement with observations. Subsequent calculations of the light elements (including deuterium and lithium) only improved the fit between theory and experimental data. The standard cosmological model accounts satisfactorily for the primordial abundance not only of helium-4, but also of deuterium, helium-3, and lithium-7. (The heavier elements are produced in stars and novae, not cosmologically.) The success of the predictions strongly suggests that the hot Big Bang theory is accurate all the way back to 1 second after the universe came into origin. Moreover, theories of nucleosynthesis give a good estimate of one of cosmology's most important parameters, the density of baryons (essentially protons and neutrons). Calculations imply that the density of baryons is somewhere between 1 and 10 percent of the critical density, a result that has important implications for cosmology.

Antimatter and Baryogenesis

Antiparticles, predicted by Paul Dirac in 1931, have played an important role in the development of cosmology. As early as 1933, Dirac suggested the existence of entire antistars (made up of positrons and antiprotons), and in 1956 Maurice Goldhaber even speculated about an "anticosmos" symmetric to the cosmos in which we live. Neither these nor other speculations about abundant masses of antimatter have received observational support.

On the contrary, observations strongly indicate that there is only very little antimatter in the universe. If the universe were initially symmetric, annihilation between matter and antimatter would have resulted in a present state almost completely dominated by photons and with only trace amounts of baryons and antibaryons. This annihilation catastrophe obviously has not occurred, which means that the early universe must have possessed a slight excess of matter over antimatter. Until about 1970 the only explanation was to postulate a charge asymmetry in the very early universe, a slight predominance of quarks over antiquarks. Why would there be, for every 300 million quarks, just 299 million antiquarks? This question relates critically to the possibility of creation of baryons—baryogenesis—out of processes in which the number of baryons is not conserved.

The earliest attempt to explain how a baryon excess could be generated was published by Andrei Sakharov in a 1967 paper, but his suggestion did not attract much interest at the time. To explain the baryon asymmetry, Sakharov assumed violation of charge conjugation and parity (CP) conservation, a process which in 1964 had been detected in the decay of neutral kaons. More speculatively, he postulated an interaction that violated baryon conservation. With the first versions of Grand Unified Theories (GUTs), the idea of baryogenesis received theoretical support, and Sakharov's speculations were reconsidered. According to the early GUTs, as developed by Howard Georgi, Sheldon Glashow, Steven Weinberg, and others in 1974–1975, transitions between quarks and leptons are possible, that is, the baryon number is not precisely conserved. In 1978, Motohiko Yoshimura used the new GUTs to predict a baryon-antibaryon asymmetry caused by primordial vector bosons, and since then baryogenesis has attracted great interest. Within the GUT framework there is no need to postulate a baryon excess, for the observed baryon number could have been created by baryon number nonconserving processes.

During the years 1978–1980, Yoshimura, Weinberg, Frank Wilczek, and others used GUT to solve another cosmological problem. The number of photons in the observable part of the universe is around 1088 and that of baryons is around 1079. The photon-to-baryon ratio is considered a fundamental quantity, because theory prescribes it to be constant in time. Why are there one billion times as many photons than baryons? The answer, according to the GUT theorists, is that this has not always been the case but was the result of the slight asymmetry in quark-antiquark annihilation processes in the early universe. Although the GUT-based theory of baryo-genesis has still no direct experimental support, it is considered compelling by many particle physicists who see it as support for an intimate relationship between cosmology and particle physics.

Nucleosynthesis and baryogenesis are examples of particle physics applied to cosmology. Conversely, cosmology has also been used as a probe of fundamental physics, to gain knowledge of physics at very high energies. For example, in 1977 Gary Steigman, David Schramm, and James Gunn showed that the number of neutrino types could not be larger than four if the hot Big Bang theory were correct. Further refinements led to a lower limit of three types or families of neutrinos. At that time, only two neutrino types were known (the electron and muon neutrinos), but in 1993 evidence for a third (tau) neutrino was produced in accelerator experiments, which was seen as a brilliant confirmation of the cosmological prediction. Moreover, calculations showed that the primordial nucleosynthesis required the mass of the tau neutrino to be less than 0.5 MeV. This constraint agreed with, but was finer than, the one obtained experimentally and thus afforded another test of the big bang scenario. No wonder that Schramm concluded that "the marriage between particle physics and cosmology had indeed been consummated" (Schramm 1996, p. xvii).

Inflation Models

Grand unified theories also led to the first inflation models, introduced in the early 1980s by Alan Guth, Andrei Linde, and others. According to the inflation model, the very early universe underwent an extreme phase transition and approached a state known as a false vacuum. In Guth's original model, the energy density of the false vacuum was attributed to a mechanism derived from GUT, spontaneous symmetry breaking. The mechanism of inflation can be imagined as the vacuum energy effectively acting as a cosmological constant that boosts the expansion of the universe exponentially until the vacuum energy is converted into heat and the universe enters its epoch of slow expansion. However, most later versions of the inflation model do not depend critically on GUT or relate to the spontaneous symmetry breaking of particle physics. The essential feature is the false vacuum. Guth's paper of 1981 inspired a massive influx of particle physicists into cosmology and a burst of theoretical activity. By 1997, more than 3,000 papers had been published on inflation theory, most by particle physicists.

Among the many attractive features of the inflation model is that it offers an explanation of how the energy of the universe came into existence, namely, when the huge energy stored in the inflated false vacuum was released at the end of the brief inflation era. Moreover, it avoids the problem of the magnetic monopoles, particles that have never been detected. Most GUTs predict an abundance of primordial monopoles, many of which should still exist, but inflation takes care of the problem.

The confidence that many particle physicists and cosmologists have in the inflation model (in one of its several versions) is related to one of cosmology's most exciting problems, the problem of dark matter. According to the inflation model, the mass (or energy) density of the universe should be critical. However, observations show that there is far from enough ordinary (baryonic) matter to produce a critical density. It is known that most of the matter in the universe must be in an exotic, dark form. Particle physics suggests a number of dark matter candidates, from massive neutrinos to hypothetical particles predicted by certain fundamental theories of the GUT type. However, none of these particles has been detected, and the problem of dark matter is thus still unsolved. Nonetheless, there is a growing consensus that dark matter particles are "cold," that is, slowly moving. Particles called axions and neutralinos are examples of such cold dark matter, but, even if they exist, they may not be the most abundant form of energy in the universe.

An Accelerating Universe

Observations of supernovae in 1998 indicated that the expansion rate of the universe is much greater than hitherto assumed. Subsequent observations have substantiated the result, and it is now generally accepted that the universe is in a state of acceleration rather than deceleration. According to the standard big bang theory, an accelerated universe contains less matter than given by the critical density, but things look different if a form of energy with negative pressure is admitted. Such strange forms of energy were studied prior to the inflation model, and in 1974 Linde argued that a vacuum energy with negative pressure acts as an effective cosmological constant. (The same insight can be found as early as 1934, in a paper by Lemaître.) However, Linde did not realize that this effective cosmological constant may greatly influence the initial stage of the evolution of the universe. This was an insight of the inflation theory in which the cosmological constant has a natural interpretation.

Today, it is widely assumed that the vacuum energy of the accelerated universe must be attributed to a positive cosmological constant which is responsible for as much as two-thirds of the total energy content of the universe. The symbol for the cosmological constant is Λ, and cold dark matter is abbreviated CDM. For this reason, physicists sometimes speak of the ΛCDM scenario or, because of the connection to inflation theory, the inflation +CDM scenario. Now, in the beginning of the twenty-first century, many cosmologists explore both this and alternative theories of the early universe.

See also:Astrophysics; Big Bang; Big Bang Nucleosynthesis; Cosmic Strings, Domain Walls; Cosmological Constant and Dark Matter; Einstein, Albert; Hubble Constant; Inflation; Universe

Bibliography

Gribben, J. Companion to the Cosmos (Phoenix Giant, London, 1997).

Guth, A. The Inflationary Universe (Addison-Wesley, Reading, MA, 1997).

Kolb, E. W., and Turner, M. S. The Early Universe (Addison-Wesley, Reading, MA, 1993).

Kragh, H. Cosmology and Controversy: The Historical Development of Two Theories of the Universe (Princeton University Press, Princeton, NJ, 1996).

Lightman, A., and Brawer, R. Origins: The Lives and Worlds of Modern Cosmology (Harvard University Press, Cambridge, MA, 1990).

Schramm, D. N. The Big Bang and Other Explosions in Nuclear and Particle Astrophysics (World Scientific, Singapore, 1996).

Turok, N., ed. Critical Dialogues in Cosmology (World Scientific, Singapore, 1997).

Helge Kragh

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Cosmology

COSMOLOGY

Although the earliest Buddhist texts of the Mainstream Buddhist schools—the nikāyas or āgamas (fourth to third century b.c.e.)—do not set out a systematic cosmology, many of the ideas and details of the developed cosmology of the later traditions are, in fact, present in these texts. Some of these have been borrowed and adapted from the common pool of early Indian cosmological notions indicated in, for example, the Vedic texts (1500 to 500 b.c.e.). The early ideas and details are elaborated in the later texts of systematic Buddhist thought, the abhidharma (third to second century b.c.e.), and presented as a coherent and consistent whole, with some variation, in the exegetical abhidharma commentaries and manuals that date from the early centuries c.e. Three principal abhidharma traditions are known to contemporary Buddhism and scholarship, those of the TheravĀda, the Sarvāstivāda, and the Yogācāra. The Theravāda or "southern" tradition has shaped the outlook of Buddhism in Sri Lanka and Southeast Asia. The Sarvāstivāda or "northern" tradition fed into the abhidharma system of the MahĀyĀna school of thought known as "yoga practice" (yogācāra) or "ideas only" (vijñaptimātra), and their perspective on many points has passed into the traditions of East Asian and Tibetan Buddhism. The elaborate cosmology presented by these abhidharma systems is substantially the same, differing only on points of detail. This traditional cosmology remains of relevance to the worldview of ordinary Buddhists in traditional Buddhist societies.

Along with many of the details, the four basic principles of the developed abhidharma Buddhist cosmology are assumed by the nikāya and āgama texts:

  1. The universe has no specific creator; the sufficient cause for its existence is to be found in the Buddhist cycle of causal conditioning known as pratĪtyasamutpĀda (dependent origination).
  2. There is no definite limit to the universe, either spatially or temporally.
  3. The universe comprises various realms of existence that constitute a hierarchy.
  4. All beings are continually reborn in the various realms in accordance with their past karma (action); the only escape from this endless round of rebirth, known as saṃsĀra, is the knowledge that constitutes the attainment of nirvĀṆa.

Levels of existence

The abhidharma systems agree that saṃsāra embraces thirty-one principal levels of existence, although they record slight variations in the lists of these levels. Any being may be born into any one of these levels. In fact, during the course of their wandering through saṃsāra it is perhaps likely that all beings have at some time or another been born in most of these levels of existence. The most basic division of the thirty-one levels is threefold: the realm of sensuality (kāmadhātu, -loka) at the bottom of the hierarchy; the realm of pure form or subtle materiality (rūpadhātu, -loka) in the middle; and the formless realm (arūpadhātu, -loka) at the top.

The realm of sensuality is inhabited by beings endowed with the five physical senses and with minds that are in one way or another generally occupied with the objects of the senses. The sensual realm is further divided into unhappy destinies and happy destinies. Unhappy destinies comprise various unpleasant forms of existence consisting of a number of hells, hungry ghosts (preta), animals, and jealous gods (asura, which are, according to some, a separate level, but to others, a class of being subsumed under the category of either hungry ghosts or gods). Rebirth in these realms is as a result of unwholesome (akuśala) actions of body, speech, and thought (e.g., killing, taking what is not given, sexual misconduct, idle chatter, covetousness, ill will, wrong view, and untrue, harsh, or divisive speech). The happy destinies of the sensual realm comprise various increasingly pleasant forms of existence consisting of human existence and existence as a divinity or god (deva) in one of the six heavens of the sense world. Rebirth in these realms is a result of wholesome (kuśala) actions of body, speech, and thought, which are opposed to unwholesome kinds of action.

Above the relatively gross world of the senses is the subtler world of "pure form." This consists of further heavenly realms (reckoned as sixteen, seventeen, or eighteen in number) occupied by higher gods called brahmās, who have consciousness but only two senses—sight and hearing. Beings are reborn in these realms as a result of mastering increasingly subtle meditative states known as the four dhyĀna (trance state). These are attained by stilling the mind until it becomes completely concentrated and absorbed in an object of meditation, temporarily recovering its natural brightness and purity. The five highest realms of the form world are known as the pure abodes, and these are occupied by divinities who are all either nonreturners (spiritually advanced beings of great wisdom who are in their last birth and who will reach enlightenment before they die) or beings who have already gained enlightenment. All the beings of the pure abodes are thus in their last life before their final liberation from the round of rebirth through the attainment of nirvĀṆa.

The subtlest and most refined levels of the universe are the four that comprise "the formless realm." At this level of the universe the body with its senses is completely absent, and existence is characterized by pure and rarified forms of consciousness, once again corresponding to higher meditative attainments.

World systems

The lower levels of the universe, that is, the realms of sensuality, arrange themselves into various distinct world discs (cakravāḍa). At the center of a cakravāḍa is the great world mountain, Sumeru or Meru. This is surrounded by seven concentric rings of mountains and seas. Beyond these mountains and seas, in the four cardinal directions, are four great continents lying in the great ocean. The southern continent, Jambudvīpa (the continent of the rose-apple tree), is inhabited by ordinary human beings; the southern part, below the towering range of mountains called the abode of snows (himālaya), is effectively India, the known world and the land where buddhas arise. At the outer rim of this world disc is a ring of iron mountains holding in the ocean. In the spaces between world discs and below are various hells; in some sources these are given as eight hot hells and eight cold hells. An early text describes how in the hell of Hot Embers, for example, beings are made to climb up and down trees bristling with long, red hot thorns, never dying until at last their bad karma is exhausted (Majjhimanikāya iii, 185).

On the slopes of Mount Sumeru itself and rising above its peak are the six heavens inhabited by the gods of the sense world. The lowest of these is that of the Gods of the Four Kings of Heaven, who guard the four directions. On the peak of Mount Sumeru is the heaven of the Thirty-Three Gods, which is ruled by its king, Indra or Śakra (Pāli, Sakka), while in the shadow of Mount Sumeru dwell the jealous gods called asuras, who were expelled from the heaven of the Thirty-Three by Indra. Above the peak is the Heaven of the Contented Gods or TuṢita, where buddhas-to-be, like the future Maitreya, are reborn and await the time to take birth. The highest of the six heavens of the sense world is that of the Gods who have Power over the Creations of Others, and it is in a remote part of this heaven that MĀra, the Evil One, lives, wielding his considerable resources in order to prevent the sensual world from losing its hold on its beings. The six heavens of the sense world are inhabited by gods and goddesses who, like human beings, reproduce through sexual union, though some say that in the higher heavens this union takes the form of an embrace, the holding of hands, a smile, or a mere look. The young gods and goddesses are not born from the womb, but arise instantly in the form of a five-year-old child in the lap of the gods (Abhidharmakośa iii, 69–70).

Above these sense-world heavens is the Brahmā World, a world of subtle and refined mind and body. Strictly, brahmās are neither male nor female, although it seems that in appearance they resemble men. The fourteenth-century Thai Buddhist cosmology, the Three Worlds According to King Ruang, describes how their faces are smooth and very beautiful, a thousand times brighter than the moon and sun, and with only one hand they can illuminate ten thousand world systems (Reynolds and Reynolds, p. 251). A Great Brahmā of even the lower brahmā heavens may rule over a thousand world systems, while brahmās of the higher levels are said to rule over a hundred thousand. Yet it would be wrong to conclude that there is any one or final overarching Great Brahmā—God the Creator. It may be that beings come to take a particular Great Brahmā as creator of the world, and a Great Brahmā may himself even form the idea that he is creator, but this is just the result of delusion on the part of both parties. In fact the universe recedes upwards with one class of Great Brahmā being surpassed by a further, higher class of Great Brahmā. Thus the world comprises "its gods, its Māra and Brahmā, this generation with its ascetics and brahmins, with its princes and peoples" (Dīghanikāya i, 62).

The overall number of world systems that constitute the universe in its entirety cannot be specified. The nikāya/āgama texts sometimes talk in terms of the thousandfold world system, the twice-thousandfold world system, and the thrice-thousandfold world system or trichilicosm. According to north Indian traditions, the last of these embraces a total of one billion world systems, while the southern traditions say a trillion. But even such a vast number cannot define the full extent of the universe; there is no spatial limit to the extent of world systems.

Cycles of time

The temporal limits of the universe are equally elusive. World systems as a whole are not static; they themselves go through vast cycles of expansion and contraction across vast eons of time. World systems contract in great clusters of a billion at a time. Most frequently this contraction is brought about by the destructive force of fire, but periodically it is brought about by water and wind. This fire starts in the lower realms of the sense-sphere and, having burnt up these, it invades the form realms; but having burnt up the realms corresponding to the first dhyāna, it stops. The realms corresponding to the second, third, and fourth dhyānas and the four formless realms are thus spared destruction. But when the destruction is wreaked by water, the three realms corresponding to the second dhyāna are included in the general destruction. The destruction by wind invades and destroys even the realms corresponding to the third dhyāna. Only the subtle realms corresponding to the fourth dhyāna and the four formless meditations are never subject to this universal destruction.

The length of time it takes for the universe to complete one full cycle of expansion and contraction is known as a mahākalpa (great eon). A mahākalpa is made up of four intermediate eons consisting of the period of contraction, the period when the world remains contracted, the period of expansion, and the period when the world remains expanded. The length of a great eon is not specified in human years but only by reference to similes:

Suppose there was a great mountain of rock, seven miles across and seven miles high, a solid mass without any cracks. At the end of every hundred years a man might brush it just once with a fine Benares cloth. That great mountain of rock would decay and come to an end sooner than even the eon. So long is an eon. And of eons of this length not just one has passed, not just a hundred, not just a thousand, not just a hundred thousand. (Saṃyuttanikāya ii, 181–182)

The Buddha is said to have declared that saṃsāra's—that is, our—beginning was inconceivable and that its starting point could not be indicated; the mother's milk drunk by each of us in the course of our long journey through saṃsāra is greater by far than the water in the four great oceans (Saṃyuttanikāya ii, 180–181).

Within this shifting and unstable world of time and space that is saṃsāra, beings try to make themselves at ease. The life spans of beings vary. In general, beings who inhabit the lower realms of existence live shorter, more

precarious, lives, while the gods live longer; at the highest realms, gods live vast expanses of time—up to eightyfour thousand eons. Yet the happiness that beings find or achieve cannot be true happiness, not permanently lasting, but merely a relatively longer or shorter temporary respite. Beings in the lowest hell realms experience virtually continuous pain and suffering until the results of the actions that brought them there are exhausted. In contrast, beings in the higher brahmā worlds experience an existence entirely free of all overt suffering; but while their lives may endure for inconceivable lengths of time in human terms, they must eventually come to an end once again when the results of the actions that brought them there are exhausted.

Cosmology and psychology

An important principle of the Buddhist cosmological vision lies in the equivalence of cosmology and psychology, the way in which the various realms of existence relate rather closely to certain commonly (and not so commonly) experienced states of mind. Buddhist cosmology is at once a map of different realms of existence and a description of all possible experiences. This can be appreciated by considering more fully the Buddhist understanding of the nature of karma. Essentially the world we live in is our own creation: We have created it by our own karma, by our deeds, words, and thoughts motivated either by greed, hatred, and delusion or by nonattachment, friendliness, and wisdom. The cosmos is thus a reflection of our actions, which are in turn the products of our hearts and minds. For in this fathom-long body with its mind and consciousness, said the Buddha, lies the world, its arising, its ceasing, and the way leading to its ceasing (Saṃyuttanikāya i, 62).

Essentially the states of mind that give rise to unwholesome actions—strong greed, hatred, and delusion—lead to rebirth in the unhappy destinies or realms of misfortune. A life dominated by the mean spiritedness of greed leads to rebirth as a hungry ghost, a class of being tormented by unsatisfied hunger; a life dominated by the mental hell of hatred and anger leads to rebirth in one of the hell realms where one suffers terrible pain; while a life dominated by willful ignorance of the consequences of one's behavior leads to rebirth as an animal, a brute existence ruled by the need to eat and reproduce. On the other hand, the generous, friendly, and wise impulses that give rise to wholesome actions lead to rebirth in the happy realms as a human being or in one of the six realms of the gods immediately above the human realm, where beings enjoy increasingly happy and carefree lives. By developing states of deep peace and contentment through the practice of calm meditation, and by developing profound wisdom through insight meditation, one is reborn as a brahmā in a realm of pure form or form-lessness, which is a reflection of those meditations.

In short, if one lives like an animal, one is liable to reborn as an animal; if one lives like a human being, one will be reborn as a human being; if one lives like a god, one will be reborn as a god. But just as in dayto-day experience one fails to find any physical or mental condition that is reliable and unchangeable, that can give permanent satisfaction and happiness, so, even if one is reborn in the condition of a brahmā living eighty-four thousand eons, the calm and peaceful condition of one's existence is not ultimately lasting or secure. Just as ordinary happiness is in this sense duḤkha (suffering) or unsatisfactory, so too are the lives of the brahmās, even though they experience no physical or mental pain.

Nirvāṇa and buddhas

The only escape from this endless round is the direct understanding of the four noble truths—suffering, its cause, its cessation, and the path leading to its cessation—and the attainment of nirvāṇa. Significantly nirvāṇa is not included in the thirty-one realms of rebirth, since these define the conditioned world of space and time, and nirvāṇa is precisely not a place where one can be reborn and where one can exist for a period of time. Nirvana is the unconditioned, the deathless, beyond space and time, known directly at the moment of enlightenment. Some beings may find the path to nirvāṇa by their own efforts and become a pratyekabuddha (solitary buddha), but most must await the appearance in the world of a samyaksaṃbuddha (perfectly and fully awakened one), like Gautama, the buddha of the current age. Such buddhas tread the ancient path of all buddhas, and can show others the way to release. Yet they appear in the world only rarely, though views on precisely how rarely vary. According to the Theravāda, some eons like our present are auspicious (bhadda) with a total of five buddhas, of whom Gautama (Pāli, Gotama) was the fourth and Maitreya (Pāli, Metteyya) will be the fifth. Other eons may have no buddhas at all.

A buddha's sphere of influence is known as his buddha-field (buddhakṢetra) and is not confined to the particular world system into which he is born. The Theravāda sources (e.g., Visuddhimagga xiii, 31) distinguish his (1) field of birth, which extends to the ten thousand world systems that tremble when he is conceived, born, gains enlightenment, teaches, and attains final nirvāṇa; (2) field of authority, which extends to the hundred billion world systems throughout which the utterance of the great protective discourses (mahāparitta) is efficacious; and (3) field of experience, which potentially extends to infinite numbers of world systems.

Mahāyāna perspectives

The basic cosmology outlined above with some variation is assumed by the Mahāyāna sūtras, as well as the authors of the systematic treatises of Indian Mahāyāna Buddhist thought. However, the Mahāyāna cosmological vision increasingly expands its attention beyond "our" world system and our buddha to include other buddhas and their spheres of influence. Early Buddhist writings and the non-Mahāyāna schools such as the Theravāda and Sarvāstivāda emphasize the impossibility of the appearance in the world of two buddhas at the same time (for how could there be two "bests"?). But this raises the question of what precisely constitutes the world. Mahāyāna writings tend to respond by suggesting that while it is true that there can be only one buddha at a time in a single trichilicosm (set of a billion world systems), since there are innumerable trichilicosms, there can in fact be innumerable buddhas at the same time in these different trichilicosms. Thus Mahāyāna writings tend to focus on the universe as made up of innumerable clusters of world systems, and each of these sets of world systems has its own series of buddhas. Since these sets of world systems are not absolutely closed off from each other, we even now in our part of the universe—called the Saha world—have access to the living buddhas of other parts. A cluster of vast numbers of world systems constitutes in effect the buddha-field or potential sphere of influence of a buddha. It is this buddha-field that a bodhisattva seeks to purify through his wisdom and compassion on the long road to buddhahood. The notion of a purified buddha-field is related in the development of Mahāyāna thought to the notion of a buddha's pure land, such as Sukhāvatī—the Realm of Bliss of the buddha AmitĀbha/Amitāyus, where the conditions for attaining enlightenment are particularly propitious if one can but be reborn there. But the question persists whether such pure lands are to be found in some far flung part of the cosmos or are here now, if we had but the heart to know it.

The Mahāyāna notion of buddha-fields with their buddhas and bodhisattvas finds expression in the Huayan jing in a wondrous cosmic vision of a universe constituted by innumerable world systems, each with its buddha, floating in the countless oceans of a cosmic lotus, of which again the numbers are countless. This vision ends in the conception of a multiverse of worlds within worlds where the buddha, or buddhas, are immanent.

See also:Divinities; Realms of Existence

Bibliography

Boyd, James W. Satan and Māra: Christian and Buddhist Symbols of Evil. Leiden, Netherlands: Brill, 1975.

Cleary, T., trans. The Flower Ornament Scripture: A Translation of the Avatamsaka Sutra, 3 vols. Boston: Shambala, 1984–1987.

Gethin, Rupert. "Meditation and Cosmology: From the Aggañña Sutta to the Mahāyāna." History of Religions 36 (1997): 183–219.

Gombrich, Richard F. "Ancient Indian Cosmology." In Ancient Cosmologies, ed. Carmen Blacker and Michael Loewe. London: Allen and Unwin, 1975.

Gombrich, Richard F. Precept and Practice: Traditional Buddhism in the Rural Highlands of Ceylon. Oxford: Oxford University Press, 1971. Second edition: Buddhist Precept and Practice: Traditional Buddhism in the Rural Highlands of Ceylon. Delhi: Motilal Banarsidass, 1991.

Kirfel, Willibald. Die Kosmographie der Inder. Bonn and Leipzig, Germany: Schroeder, 1920.

Kloetzli, Randy. Buddhist Cosmology: From Single World System to Pure Land. Delhi: Motilal Banarsidass, 1983.

Kongtrul, Jamgön Lodrö Tayé. Myriad Worlds: Buddhist Cosmology in Abhidharma, Kalacakra, and Dzog-chen. Ithaca, NY: Snow Lion, 1995.

Ling, Trevor O. Buddhism and the Mythology of Evil: A Study in Theravāda Buddhism. London: Allen and Unwin, 1962. Reprint, Oxford: Oneworld, 1997.

Mārasinghe, M. M. J. Gods in Early Buddhism: A Study in Their Social and Mythological Milieu as Depicted in the Nikāyas of the Pāli Canon. Vidyalankara: University of Sri Lanka, 1974.

Masson, Joseph. La Religion populaire dans le canon bouddhique pali. Louvain, Belgium: Bureaux du Muséon, 1942.

Reynolds, Frank E., and Reynolds, Mani B., trans. Three Worlds According to King Ruang: A Thai Buddhist Cosmology. Berkeley: University of California, 1982.

Sadakata, Akira. Buddhist Cosmology: Philosophy and Origin. Tokyo: Kosei, 1997.

Tambiah, Stanley J. Buddhism and the Spirit Cults in North-East Thailand. Cambridge, UK: Cambridge University Press, 1970.

Rupert Gethin

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Cosmology

Cosmology

Cosmology is the study of the origin, structure and evolution of the universe.

The origins of cosmology predate the human written record. The earliest civilizations constructed elaborate myths and folk tales to explain the wanderings of the Sun , Moon , and stars through the heavens. Ancient Egyptians tied their religious beliefs to celestial objects and Ancient Greek and Roman philosophers debated the composition and shape of Earth and the Cosmos. For more than 13 centuries, until the Scientific Revolution of the sixteenth and seventeenth centuries, the Greek astronomer Ptolemy's model of an Earth-centered Cosmos composed of concentric crystalline spheres dominated the Western intellectual tradition.

Polish astronomer Nicolaus Copernicus's (1473–1543) reassertion of the once discarded heliocentric (Sun-centered) theory sparked a revival of cosmological thought and work among the astronomers of the time. The advances in empiricism during the early part of the Scientific Revolution, embraced and embodied in the careful observations of Danish astronomer Tycho Brahe (1546–1601), found full expression in the mathematical genius of the German astronomer Johannes Kepler (1571–1630) whose laws of planetary motion swept away the need for the errant but practically useful Ptolemaic models. Finally, the patient observations of the Italian astronomer and physicist Galileo, in particular his observations of moons circling Jupiter and of the phases of Venus , empirically laid to rest cosmologies that placed Earth at the center of the Cosmos.

English physicist and mathematician Sir Isaac New ton's (1642–1727), important Philosophiae Naturalis Principia Mathematica (Mathematical principles of natural philosophy) quantified the laws of motion and gravity and thereby enabled cosmologists to envision a clockwork-like universe governed by knowable and testable natural laws. Within a century of Newton's Principia, the rise of concept of a mechanistic universe led to the quantification of celestial dynamics, that, in turn, led to a dramatic increase in the observation, cataloging and quantification of celestial phenomena. In accord with the development of natural theology, scientists and philosophers argued conflicting cosmologies that argued the existence and need for a supernatural God who acted as "prime mover" and guiding force behind a clockwork universe. In particular, French mathematician, Pierre Simon de Laplace (1749–1827) argued for a completely deterministic universe, without a need for the intervention of God. Most importantly to the development of modern cosmology, Laplace asserted explanations for celestial phenomena as the inevitable result of time and statistical probability.

By the dawn of the twentieth century, advances in mathematics allowed the development of increasingly sophisticated cosmological models. Many of advances in mathematics pointed toward a universe not necessarily limited to three dimensions and not necessarily absolute in time. These intriguing ideas found expression in the intricacies of relativity and theory that, for the first time, allowed cosmologists a theoretical framework upon which they could attempt to explain the innermost workings and structure of the universe both on the scale of the subatomic world and on the grandest of galactic scales.

As direct consequence of German-American physicist Albert Einstein's (1879–1955) relativity theory, cosmologists advanced the concept that space-time was a creation of the universe itself. This insight set the stage for the development of modern cosmological theory and provided insight into the evolutionary stages of stars (e.g., neutron stars, pulsars, black holes, etc.) that carried with it an understanding of nucleosythesis (the formation of elements) that forever linked the physical composition of matter on Earth to the lives of the stars.

Twentieth-century progress in cosmology has been marked by corresponding and mutually beneficial advances in technology and theory. American astronomer Edwin Hubble's (1889–1953) discovery that the universe was expanding, Arno A. Penzias and Robert W. Wilson's observation of cosmic background radiation , and the detection of the elementary particles that populated the very early universe all proved important confirmations of the Big Bang theory . The Big Bang theory asserts that all matter and energy in the Universe, and the four dimensions of time and space were created from the primordial explosion of a singularity of enormous density , temperature , and pressure .

During the 1940s Russian-born American cosmologist and nuclear physicist George Gamow (1904–1968) developed the modern version of the big bang model based upon earlier concepts advanced by Russian physicist Alexander (Aleksandr Aleksandrovich) Friedmann (also spelled as Fridman, 1888–1925) and Belgian astrophysicist and cosmologist Abbé Georges Lemaître (1894–1966). Big bang based models replaced static models of the universe that described a homogeneous universe that was the same in all directions (when averaged over a large span of space) and at all times. Big bang and static cosmological models competed with each other for scientific and philosophical favor. Although many astrophysicists rejected the steady state model because it would violate the law of mass-energy conservation, the model had many eloquent and capable defenders. Moreover, the steady model was interpreted by many to be more compatible with many philosophical, social and religious concepts centered on the concept of an unchanging universe. The discovery quasars and of a permeating cosmic background radiation eventually tilted the cosmological argument in favor of big bang-based models.

Technology continues to expand the frontiers of cosmology. The Hubble Space Telescope has revealed gas clouds in the cosmic voids and beautiful images of fledgling galaxies formed when the universe was less than a billion years old. Analysis of these pictures and advances in the understanding of the fundamental constituents of nature continue to keep cosmology a dynamic discipline of physics and the ultimate fusion of human scientific knowledge and philosophy.


Evolution of cosmological thought

Using such instruments as the Hubble Space Telescope, modern cosmology is an attempt to describe the large scale structure and order of the universe. To that end, one does not expect cosmology to deal with the detailed structure such as planets, stars, or even galaxies. Rather it attempts to describe the structure of the universe on the largest of scales and to determine its past and future.

One of the earliest constraints on any description of the universe is generally attributed to Heinreich Olbers in 1826. However, more careful study traces the idea back to Thomas Digges in 1576 and it was thoroughly discussed by Edmond Halley at the time of Isaac Newton. The notion, still called Olbers' paradox, is concerned with why the night sky is dark. At the time of Newton it was understood that if the universe was finite then Newton's Law of Gravity would require that all the matter in the universe should pull itself together to that point equidistant from the boundary of the universe. Thus, the prevailing wisdom was that the universe was infinite in extent and therefore had no center. However, if this were true, then the extension of any line of sight should sooner or later encounter the surface of a star . The night sky should then appear to have the brightness of the average star. The sun is an average star, thus one would expect the sky to be everywhere as bright as the sun. It is not, so there must be a problem with the initial assumptions.

An alternative explanation, pointed out in 1964 by Ed Harrison, is that the universe had a finite beginning and the light from distant stars had not yet had time to arrive and that is why the night sky is dark.

At the turn of the twentieth century cosmology placed the sun and its solar system of planets near the center of the Milky Way Galaxy which comprised the full extent of the known Universe. However, early in the century information began to be compiled that would change the popular view that the universe was static and made of primarily stars. On April 26, 1920, there was a historic debate between H. D. Curtis and Harlow Shapley concerning the nature of some fuzzy clouds of light which were called nebulae. Shapley thought they were objects within the galaxy while Curtis believed them to be "island universes" lying outside the Milky Way.

Although at the time most agreed that Shapley had won the debate, science eventually proved that Curtis was right. Within a few years Edwin Hubble detected a type of star, whose distance could be independently determined, residing within several of these fuzzy clouds. These stars clearly placed the "clouds" beyond the limits of the Milky Way. While Hubble continued to use the term "island universe," more and more extragalactic nebulae were discovered, and they are now simply known as galaxies. In the span of a quarter of a century the scale for the universe had been grown dramatically.


The expanding universe

During the time of Hubble's work, V. M. Slipher at the Lowell Observatory had been acquiring spectra of these fuzzy clouds. By breaking the light of astronomical objects into the various "colors" or wavelengths which make up that light, astronomers can determine much about the composition, temperature, and pressure of the material that emitted that light. The familiar spectrum of hydrogen so common to so many astronomical objects did not fall at the expected wavelengths, but appeared shifted to longer wavelengths. We now refer to such a change in wavelength as a redshift . Slipher noted that the fainter galaxies seemed to have larger redshifts and he sent his collection of galactic spectra off to Hubble.

Hubble interpreted the redshift as being caused by the Doppler effect , and thus representing motion away from us. The increasing faintness was related to distance so that in 1929 Hubble turned Slipher's redshift-brightness relation into a velocity-distance relation and the concept of the expanding universe was born. Hubble found that the velocity of distant galaxies increased in direct proportion to their distance. The constant of proportionality is denoted by the symbol H0 and is known as Hubble's constant.

Although the historical units of Hubble's constant are (km/s/mpc), both kilometers and megaparsecs are lengths so that the actual units are inverse time (i.e. 1/sec). The reciprocal of the constant basically gives a value for the time it took distant galaxies to arrive at their present day positions. It is the same time for all galaxies. Thus, the inverse of Hubble's constant provides an estimate for the age of the expanding Universe called the Hubble age.

Due to its importance, the determination of the correct value of the Hubble constant has been a central pre-occupation of many astronomers from the time of Hubble to the present. Since the gravitational pull of the matter in the universe on itself should tend to slow the expansion, values of Hubble's constant determined by hypothetical astronomers billions of years ago would have yielded a somewhat larger number and hence a somewhat younger age. Therefore the Hubble age is an upper limit to the true age of the Universe which depends on how much matter is in the Universe.

The notion of a Heavens (i.e. universe) that was dynamic and changing was revolutionary for the age. Einstein immediately modified the equations of the General Theory of Relativity which he applied to the universe as a whole to deal with a dynamic universe. Willem de Sitter and quite independently Alexandre Friedmann expanded on Einstein's application of General Relativity to a dynamically expanding universe.

The concept of a universe that is changing in time suggests the idea of predicting the state of the universe at earlier times by simply reversing the present dynamics. This is much like simply running a motion picture backwards to find out how the movie began. A Belgian priest by the name of Georges Lemaitre carried this to its logical conclusion by suggesting that at one time the universe must have been a very congested place with matter so squeezed together that it would have behaved as some sort of primeval atom.


The big bang

The analysis of Einstein, de Sitter, Friedmann, Lemaitre, and others showed that the dynamic future of the expanding universe depended on the local density. Simply put, if the density of the universe were sufficiently high, then the gravitational pull of the matter in any given volume on itself would be sufficient to eventually stop the expansion. Within the description given by the General Theory of Relativity, the matter would be said to warp space to such an extent that the space would be called closed. The structure of such a universe would allow the expansion to continue until it filled the interior of a black hole appropriate for the mass of the entire universe at which point it would begin to collapse. A universe with less density would exhibit less space warping and be said to be open and would be able to expand forever.

There exists a value for the density between these extremes where the matter of the universe can just stop the expansion after an infinite time. Such a universe is said to be flat. One of the central questions for observational cosmology continues to be which of these three cases applies to our universe.

George Gamow concerned himself with the early phases of an expanding universe and showed that Lemaitre's primeval atom would have been so hot that it would explode. After World War II, a competing cosmology developed by Hermann Bondi, Thomas Gold, and Fred Hoyle was put forth in order to avoid the ultimate problem with the expanding universe, namely, it must have had an origin. The Steady State Cosmology of Bondi, Gold, and Hoyle suggested that the universe has existed indefinitely and that matter is continuously created so as to replace that carried away by the observed expansion.

This rather sophisticated cosmology replaced the origin problem of the expanding universe by spreading the creation problem out over the entire history of the universe and making it a part of its continuing existence. It is somewhat ironic that the current name for the expanding universe cosmology as expressed by Gamow is derived from the somewhat disparaging name, big bang, given to it by Fred Hoyle during a BBC interview.

Gamow and colleagues noted that a very hot primeval atom should radiate like a blackbody (i.e., a perfect thermal radiator), but that radiation should be extremely red-shifted by the expansion of the universe so that it would appear today like a very cold blackbody. That prediction, made in 1948, would have to wait until 1965 for its confirmation. In that year Arno Penzias and Robert Wilson announced the discovery of microwave radiation which uniformly filled the sky and had a blackbody temperature of about 2.7K (-454.5°F [-270.3°C]).

While Gamow's original prediction had been forgotten, the idea had been re-discovered by Robert Dicke and his colleagues at Princeton University. Subsequent observation of this background radiation showed it to fit all the characteristics required by radiation from the early stages of the big bang. Its discovery spelled the end to the elegant Steady State Cosmology which could not easily accommodate the existence of such radiation.

Implications of the big bang

After the Second World War, the science of nuclear physics developed to a point at which it was clear that nuclear reactions would have taken place during the early phases of the big bang. Again scientists ran the motion picture backwards through an era of nuclear physics, attempting to predict what elements should have been produced during the early history of the universe. Their predictions were then compared to the elemental abundances of the oldest stars and the agreement was amazingly good.

The detailed calculations depended critically on whether the calculated model was for an open or closed universe. If the universe were open, then the era of nuclear reactions would not last long enough to produce elements heavier than hydrogen and helium. In such models some deuterium is formed, but the amount is extremely sensitive to the initial density of matter. Since deuterium tends to be destroyed in stars, the current measured value places a lower limit on the initial amount made in the big bang. The best present estimates of primordial deuterium suggest that there is not enough matter in the universe to stop its expansion at any time in the future.

In the event of an open universe, scientists are pretty clear what the future holds. In 1997, researchers from the University of Michigan released a detailed projection of the four phases of the universe, including the ultimate end in the Dark Era, some 10100 years hence. Currently, the universe is in the Stelliferous Era, dominated by high-energy stars and filled with galaxies. Some 1000 trillion years from now, the universe will enter the Degenerate Era. Stars will have burned down into degenerate husks that can no longer support hydrogen burning reactions, and will exist as white dwarfs, red dwarfs, brown dwarfs, or neutron stars; some massive stars will have collapsed into black holes, which will consume the other star relics.

Next, the universe will progress into the Black Hole Era, about 100 trillion trillion trillion years from the present. At that time, black holes will have swallowed up the remaining bodies in the universe and will gradually leak radiation themselves, essentially evaporating away over trillions of years. Finally, the universe will reach the Dark Era, in which no matter will exist, only a soup of elementary particles like electrons, positrons, neutrinos, and other exotic particles.

In 1998, astronomers studying a certain group of supernovas discovered that the older objects were receding at a speed about the same as the younger objects. According to the theory of a closed universe, the expansion of the universe should slow down as it ages, and older supernovas should be receding more rapidly than the younger supernovas. The fact that observations have shown the opposite has led scientists to believe that the universe is either open or flat.

Cosmologists still battle over the exact nature of the universe. Most scientists agree that the age of the universe ranges between 13 and 15 billion years. The exact age, for instance, is a matter of great controversy between rival research teams at Carnegie Observatories and the Space Telescope Science Institute. When researchers recently used an analysis of polarized light to show that the universe is not isotropic, i.e., not the same in all directions, their findings were disputed almost as soon as they were published.


Trouble in paradise

In the last quarter of the twentieth century some problems with the standard picture of the big bang emerged. The extreme uniformity of the cosmic background radiation, which seemed initially reasonable, leads to a subtle problem. Consider the age of elements of the cosmic background radiation originating from two widely separated places in the sky. The distance between them is so great that light could not travel between them in an amount of time less than their age. Thus the two regions could never have been in contact during their existence. Why then, should they show the same temperature? How was their current status coordinated? This is known as the horizon problem.

The second problem has to do with the remarkable balance between the energy of expansion of the universe and the energy associated with the gravitational forces of the matter opposing that expansion. By simply counting the amount of matter we see in the universe, we can account for about 1% of the matter required to stop the expansion and close the universe. Because the expansion causes both the expansion energy and the energy opposing the expansion to tend to zero , the ratio of their difference to either one tends to get larger with time. So one can ask how good the agreement between the two was, say, when the cosmic background radiation was formed.

The answer is that the agreement must have been good to about 1 part in a million. If one extends the logic back to the nuclear era where our physical understanding is still quite secure, then the agreement must be good to about thirty digits. The slight departure between these two fundamental properties of the universe necessary to produce what we currently observe is called the "Flatness Problem." There is a strong belief among many cosmologists that agreement to 30 digits suggests perfect agreement and there must be more matter in the Universe than we can see.

This matter is usually lumped under the name dark matter since it escapes direct visible detection. It has become increasingly clear that there is indeed more matter in the universe than is presently visible. Its gravitational effect on the rotation of galaxies and their motion within clusters of galaxies suggests that we see perhaps only a tenth of the matter that is really there. However, while this amount is still compatible with the abundance of deuterium, it is not enough to close the Universe and solve the flatness problem.

Any attempt to run the motion picture further backwards before the nuclear era requires physics which, while less secure, is plausible. This led to a modification of the big bang by Alan Guth called inflation. Inflation describes an era of very rapid expansion where the space containing the matter-energy that would eventually become galaxies spread apart faster than the speed of light for a short period of time.

Inflation solves the horizon problem in that it allowed all matter in the universe to be in contact with all other matter at the beginning of the inflation era. It also requires the exact balance between expansion energy and energy opposed to the expansion, thereby solving the flatness problem. This exact balance requires that there be an additional component to the dark matter that did not take part in the nuclear reactions that determined the initial composition of the Universe. The search for such matter is currently the source of considerable effort.

Finally, one wonders how far back one can reasonably expect to run the movie. In the earliest microseconds of the universe's existence the conditions would have been so extreme that the very forces of nature would have merged together. Physical theories that attempt to describe the merger of the strong nuclear force with the electro-weak force are called Grand Unified Theories, or GUTs for short. There is currently much effort being devoted to testing those theories. At sufficiently early times even the force of gravity should become tied to the other forces of nature. The conditions which lead to the merging of the forces of nature are far beyond anything achievable on Earth so that the physicist must rely on predictions from the early Universe to test these theories.

Ultimately quantum mechanics suggests that there comes a time in the early history of the universe where all theoretical descriptions of the universe must fail. Before a time known as the Planck Time, the very notions of time and space become poorly defined and one should not press the movie further. Beyond this time science becomes ineffective in determining the structure of the universe and one must search elsewhere for its origin.

See also Relativity, general; Relativity, special; String theory; Symmetry.


Resources

books

Harrison, E.R. Cosmology: The Science of the Universe. Cambridge, England: Cambridge University Press, 1981.

Hawking, Stephen. W. The Illustrated A Brief History of Time. 2nd ed. New York: Bantam Books, 2001.

Kirshner, Robert P. The Extravagant Universe: ExplodingStars, Dark Energy, and the Accelerating Cosmos. Princeton, NJ: Princeton University Press, 2002.

Weinberg, S. The First Three Minutes. New York: Basic Books, 1977.

periodicals

Glanz, James. "Evidence Points to Black Hole At Center of the Milky Way." New York Times. October 17, 2002.

Guth, A.H., and P.J. Steinhardt. "The Inflationary Universe," Scientific American 250, no. 5 (1984): 116–28.

other

Cambridge University. "Cambridge Cosmology." <http://www. damtp.cam.ac.uk/user/gr/public/cos_home.html> (cited February 14, 2003).

National Air and Space Administration. "Cosmology: The Study of the Universe." <http://map.gsfc.nasa.gov/m_uni.html> (cited February 5, 2003).


K. Lee Lerner

George W. Collins, II

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Grand Unified Theory

—Any theory which brings the description of the forces of electromagnetism, weak and strong nuclear interactions under a single representation.

Hubble constant

—The constant of proportionality in Hubble's Law which relates the recessional velocity and distance of remote objects in the universe whose motion is determined by the general expansion of the universe.

Inflation cosmology

—A modification to the early moments of the big bang Cosmology which solves both the flatness problem and the horizon problem.

Megaparsec

—A unit of distance used in describing the distances to remote objects in the universe. One megaparsec (i.e., a million parsecs) is approximately equal to 3.26 million light years or approximately ten trillion trillion centimeters.

Olbers' paradox

—A statement that the dark night sky suggests that the universe is finite in either space or time.

Planck time

—An extremely short interval of time (i.e., 1043 sec) when the conventional laws of physics no longer apply.

Primeval atom

—The description of the very early expanding universe devised by Abbe Lemaitre.

Spectra

—The representation of the light emitted by an object broken into its constituent colors or wavelengths.

Steady state cosmology

—A popular cosmology of the mid twentieth century which supposed that the universe was unchanging in space and time.

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Cosmology

COSMOLOGY

The night sky is a primal wonder whose infinite nature spurs a longing to understand human existence. The realization that they are beneath a vastness and majesty beyond their personal experience impels people to attempt to know themselves and their place in all that there is. This is a religious impulse and is also the impulse behind cosmology.


From Astronomy to Cosmology

Cosmology is, however, a uniquely modern science of the history, structure, and dynamics of the universe. Although astronomy is a transliteration from the Greek, the word cosmology is a seventeenth-century coinage from an imaginary Greek term. It thus denotes a new, uniquely scientific way to deal with primal wonder about the night sky that was designed to replace the myths that represented primordial efforts to respond to that wonder.

The myths on which traditional societies were built were inspired by and speak to the origins of humankind and its place in the universe. Because the nature of the firmament is unknowable by the direct senses, until recently those myths were untestable and therefore perennial. The birth of technology changed that situation. Tools that take advantage of natural laws and allow humankind to manipulate those laws changed what was knowable. Systematic observations of the motion of the planets that were motivated by Tycho Brahe's (1546–1601) desire to find God's perfection in the sky led Johannes Kepler (1571–1630) to devise a model of the solar system with the sun at its center. Timepieces and levers set the stage for Isaac Newton's (1643–1727) grasp of gravity and its implications for the cosmos. Newton's calculus, a kind of conceptual technology, captured physical law with a generality and precision of unprecedented scope.

Today fossil light from the beginning of time is collected by immense machines both on the earth and in space and analyzed electronically to reveal the most intimate details of the universe and its beginnings. Modern cosmology weaves a creation story that passes the tests of science. The same methodology that has laid out physical truth and made possible the ability to control nature has allowed humankind to know the extent and origin of all that there is. In the process the inevitable imperial nature of science has taken over, displacing the old myths with cold certainty and weakening the ground beneath religions, belief systems, and structures of morality. As science replaces older foundational beliefs, it becomes complicit in the moral confusion of the modern age.

Can heaven survive the heat death of the universe? Will the cherished views of earlier cultures on the origin and meaning of human existence be another casualty of modern science? As astronomers divine the mysteries of the origin and evolution of the universe, are they culpable for the elimination of worldviews that may have had legitimate purposes but did not stand up to the scrutiny of scientific methodology?


The Emergence of the Big Bang Theory

In 1929 the astronomer Edwin Hubble (1889–1953) announced that the recessional velocities of galaxies are proportional to how far away they are. The farthest galaxies were said to be receding the fastest, as measured by the Doppler shifts of their emitted light. The Doppler shift is the stretching of light waves from objects that are receding from the earth at high velocity. Hence, distant galaxies appear redder. The constant of proportionality (between distance and recession velocity) became known as the Hubble constant. The implications of this relationship are profound. The simplest explanation of it is that at some time in the very distant past all the galaxies were packed together. The reciprocal of the Hubble constant is approximately the age of the universe: about 14 billion years.

How far back in time is it possible to see? What immense, sophisticated, and expensive instruments are required to see something as esoteric as the first light of the universe? In fact, one can see the radiation from the explosion of the Big Bang in almost every living room in the United States and almost any household in the world. All that it is necessary to do is to unplug the cable from a television set and set it to a channel where there is no broadcast. Part of that chaotic, somewhat disturbing pattern known as snow is the microwave echo of the Big Bang, which was released when the universe became transparent 200,000 years after it was born.

In 1965 Arnio Penzias (b. 1933) and Robert Wilson (b. 1936) of Bell Laboratories were working on a state-of-the-art antenna for the emerging technology of satellite telecommunications. Wherever they pointed their antenna in the sky, they heard a constant hum. In one of the most serendipitous discoveries in the history science, the cosmic microwave background (CMB) radiation had been found, and at a frequency exactly in agreement with the theory of the Big Bang (Sciama 1973). (Penzias and Wilson won the 1978 Nobel Prize for their discovery.) Since the Big Bang space has been cooling as it expands. If one runs the movie of the evolution of the universe backward to the point where all the galaxies coalesce, one finds that the "primeval egg" began expanding at nearly the speed of light 14 billion years ago. From the inferno of creation to the present the science of thermodynamics predicts that space should have cooled to 2.7 degrees Celsius above absolute zero. The frequencies Penzias and Wilson heard in the CMB correspond exactly to that temperature.

Cosmology and nuclear physics began to merge when scientists started to consider the first three minutes of the universe, a point made clear in Steven Weinberg's The First Three Minutes: A Modern View of the Origin of the Universe (Weinberg 1977). During that time all the fundamental particles—the neutrons, protons, and electrons that make up atoms and the rest of the fundamental particle zoo—were formed. As the universe expanded and cooled, mostly hydrogen nuclei were formed, but a fraction of them teamed with neutrons to make helium, deuterium, and lithium. According to nuclear physics, the relative amounts of each of these elements are quite sensitive to the conditions of the early universe. From that period of nucleosynthesis right after the Big Bang nuclear physics predicted that the universe should have been formed with about 76 percent hydrogen, 24 percent helium, and less than 1 percent heavier elements. In an affirmation of the Big Bang theory spectroscopists have shown that wherever one looks in the universe those ratios prevail.

With the evidence provided by Hubble's observation that the universe is expanding, the measurement of the CMB, and the correct prediction of nucleosynthesis during the first three minutes of the universe the Big Bang has been accepted as the real story of the universe. However, adjustments have been made to it.



The Structure of the Universe

A map of the universe as it is currently understood is shown in Figure 1. The bottom of the chart shows the center of the earth, and the top represents the farthest that can be seen: the CMB. The scale is logarithmic so that any quarter inch on the chart represents ten times the distance of the quarter inch below it. Two populations of artificial satellites populate space immediately above the earth: low orbit satellites at about 200 miles and geostationary satellites at 23,000 miles. The planets, asteroid belt, and Kuiper belt can be seen in the bottom half of the chart. The Kuiper belt is a vast ring of large comets that orbit the sun outside Pluto. Midway on the chart is the Oort cloud, a much larger spherical shell of comets that are bound loosely to the sun. Nearby stars, galactic stars, and the center and edge of the galaxy follow as one moves outward. The Milky Way is part of the local group, a loose collection of about two dozen galaxies that are gravitationally bound. Beyond that is the large-scale structure of the universe. Galaxies fill the heavens in these vast reaches, but they are not randomly placed. Not only do they form clusters, there are coherent structures that are significant fractions of the size of the universe. The Great Wall is one such structure: a long filament of galaxies that is 300 million light-years from the earth.

In fact, the large-scale structure of the universe is foamy and filamentary, as shown in Figure 2 (Gott et al. 2004). In this figure each point represents a galaxy: The foamy nature of the universe can be seen out to 2.7 billion light-years in this diagram. The foam seems to become less dense farther from the earth or, equivalently, farther back in time. In fact, it extends as far back as can be seen. The blank wedge-shaped regions are places in the sky where it is impossible to see out of this galaxy. This is the plane of the Milky Way.

The foamy structure of the universe must be indicative of the small, quantum asymmetries that were imparted during the Big Bang. One can imagine that a perfectly spherical explosion would result in a smooth, uniform universe with no structure. However, somehow small asymmetries must have been present and were amplified by the force of gravity as the universe evolved and expanded. The structure that is seen is not consistent with the amount of matter and energy observed in the universe. There does not appear to be enough gravity to hold it all together, and this is where dark matter comes in.



Dark Matter

The direct evidence for dark matter is simple. Galaxies usually exist in gravitationally bound clusters of a few to several dozen. The motion of the galaxies around their common center, a matter of Newtonian physics, is completely inconsistent with the amount of matter that is seen. The motion of individual galaxies within a cluster can be explained only by the existence of an additional strong gravitational field. In fact, every galaxy or cluster must have a spherical halo of matter around it that is undetectable with electromagnetic radiation but is five times more abundant than the matter in the galaxies themselves. Little else is known about this mysterious cold dark matter, but its existence is generally accepted and there is an ongoing effort to detect it directly.



The Cosmic Microwave Background and Dark Energy

The microwave background also has structure. If the universe began as a microscopic primeval egg, it must have undergone vigorous quantum fluctuations in energy, shape, and even dimensionality. The imprint of those quantum fluctuations is seen in the spatial structure of the microwave background. To an incredible degree, however (about one part in a million), the microwave background is uniform. This implies that at one time the universe was small enough that it could come to thermal equilibrium but then grew rapidly, freezing in both the large-scale isotropy and the quantum fluctuations. This freezing in would have happened during an inflationary period when the universe accelerated outward at an exponential rate.

This is a decidedly nonintuitive move for a universe to make. What caused the universe to accelerate in the first place? In the old standard model of the Big Bang, without inflation, a prime mover is required, but only at the instant of creation. The explosion casts matter and energy outward, expanding under this initial, unimaginable force but eventually slowing down as gravity pulls everything back to the center. The central question in cosmology at the start of the twenty-first century has been, What is the density of the universe? If the density is too low, gravity will never win and the universe will expand forever. If the density is high, beyond a critical point, the universe eventually will slow to a stop and begin to fall in on itself. The end is the Big Crunch, perhaps followed by reincarnation as the cycle begins all over again.


Neither of these scenarios appears to be the likely fate of the universe, however, based on the smooth nature of the microwave background radiation. Instead, the universe appears to exist in a state in between these scenarios, like a penny that has landed on its edge. It seems that the universe is flat, a spacetime geometry that means that the universe will continue to expand forever, although more and more slowly, approaching a stop at t equals infinity. The problem is that when one adds up all the mass and energy and dark matter, the universe is shy of the total amount required for a flat geometry by a factor of two.

This is where two problems are solved at once by the inflationary theory. There are quantum mechanical reasons to suspect that the vacuum itself has energy. That is, there is some underlying fabric that wildly undulates, popping fundamental particles into existence from nothing and swiftly returning them to the weave. Those particles have been observed, although the nature of the fabric and the energy it imparts to the vacuum remain mysterious. At one time the physicist Albert Einstein (1879–1955) postulated that energy, which he inserted into his equations as a cosmological constant. His goal was to produce a model of a steady-state universe, infinite and isotropic in time and space, largely because he felt that that was more aesthetically reasonable than a universe that began with a Big Bang. Although Alexander Friedman (1888–1925) showed that the Big Bang was a valid solution to Einstein's equations, Einstein abhorred that theory. However, he abhorred the ad hoc adjustment to his equations even more, and when the empirical evidence for a Big Bang could not be ignored, he declared the cosmological constant his biggest mistake. On new empirical grounds it must be included again, although a fundamental theory of its origins probably will require the achievement of a grand unified theory, a theory of everything, that string theory seems to promise for the future (Greene 2003).

This quantum vacuum energy is called the dark energy, and there is twice as much of it as there is of everything else that can be seen and measured. The dark energy has been implicated in the inflationary era of the universe and may have been the driving force for it. Still, aside from problems with identifying the quantum vacuum energy with the missing energy of the universe, the invention of the dark energy seems contrived.

There has, however, been an important recent discovery whose status has increased steadily. By very carefully measuring the red shifts, and hence the recessional velocities of galaxies deep into the universe, cosmologists have been able to map the evolution of the expansion rate of the universe. They have found that although the universe slowed down steadily after inflation, as a result of gravity, about 5 billion years ago it began to speed up again (Greene 2003). Today not only is the universe expanding, its expansion rate is increasing. The universe is accelerating, and something must be causing that. The culprit is the dark energy that permeates the vacuum.

The Story of The Creation

The newest creation story is surely not the final answer. A final theory will emerge only when there is a full understanding of how gravity is related to the other three forces and when the theories of gravitation and quantum mechanics are united. Enormous conceptual progress has been made with the development of string theory and its big brother, M (membrane) theory. String theory envisions particles as one-dimensional strings that vibrate not only in the known universe but also within six other hidden dimensions that are curled too small to be seen but that exist at every point in space (Greene 2003). A majority of cosmologists and theoretical physicists consider string theory the most promising and testable avenue for developing a true "theory of everything."

In the beginning there was an incredibly hot multidimensional nugget that was about one Planck scale (1033 centimeters) in length. According to string theory, this Planckian egg is the smallest that anything can be. Squeezing it tighter makes it bigger and cooler. String theory avoids the singularity of the conventional Big Bang theory by considering the behavior of matter and energy at the very finest scales. It cannot say, however, what may have existed before this state, although this is an area of ongoing research.

The nugget had the entire mass of the universe in it, and it underwent transitions in its topography rapidly and randomly. Between 1036 and 1034 seconds after the start of time three dimensions suddenly broke free of their confining strings and inflated ferociously in a violent, exponential expansion. Alan Guth (b. 1947) of the Massachusetts Institute of Technology first showed that inflationary expansion of the universe represents a particular solution to Einstein's equations and can explain a deeply perplexing aspect of the CMB: its overall isotropy. The remaining dimensions stayed curled together, fundamentally influencing the nature of the particles and forces that became manifest in the three macroscopic dimensions. At one-hundred-thousandth of a second quarks began to clump into protons and neutrons.

Meanwhile, as the universe cooled, something strange was happening to the force within it. It was born with only one force, but as it cooled, it underwent phase transitions by which new forces were cleaved from the original one. Ultimately, for reasons that are not understood, the universe ended up with four forces: gravitation, electromagnetism, and the weak and strong nuclear forces. From a hundredth of a second to three minutes after the Big Bang the elements were formed.

At 200,000 years the universe had cooled enough for stable atoms to form. In other words, the universe cooled from a plasma to a gas and became transparent. The photons streaming outward at that time are the blips seen on television sets.

Perhaps a billion years after the Big Bang galaxies began to form. The universe continued to expand at close to the speed of light, but the relentless action of gravity caused its expansion to slow. However, 9 billion years after the origin of the universe its expansion began to accelerate, most likely as a result of the repulsive force of the quantum vacuum energy. If this trend continues, the acceleration of the universe will cause galaxies to fly ever more rapidly away from one another. Some day even the closest galaxy will be too far away to see; the galaxies will be beyond the light horizon. Some day all the fuel for the stars will be used up, first hydrogen and then helium, carbon, and oxygen, until the last sun flickers and the universe is plunged into eternal darkness.


The Ethical and Political Dimensions of Cosmology

For many scientific disciplines the cause-and-effect relationship between scientific outcomes and the well-being of people is of great importance: Scientific results and their technological progeny are the dominant forces shaping the future of the world. The role science will play in determining the quality of life for every human being on the planet is of course determined by the elite that funds science. In this way all scientific enterprise is embedded in the greater moral problem of how individuals and groups should conduct themselves. Is it better for the powerful to channel their efforts solely for competitive self-benefit or to distribute knowledge and technology among all people? What are the consequences of pushing technologies on societies that may not want them? In some fields these issues spring directly from contemplation of the promise and implications of their projects. If it is possible to choose the human qualities of a person through genetic engineering, who will decide what those qualities will be, and to whose progeny will they go? Other subjects may be further afield, but the conceptual shift forced on science by the quantum nature of the infinitesimal in the 1920s has led to the most transforming technology in history: electronics.

Cosmology evokes a sense of the most benign and pure of sciences. The fascination of contemplating what is out there, combined with the fact that humankind cannot do anything to it, lends the study of space its alluring innocence. That of course is the old view. Cosmology is coming dangerously close to asking God rather direct questions.

To some degree scientific disciplines can be categorized by how influential ethics is thought to be in a particular field. Indeed, the ethical weight of astronomy, compared with that of genetics, lends it a kind of lightness and purity that is perceived by the people who fund it. Virtually everyone on the planet has gazed up and rested briefly in that human space where one wonders what it all is and what it all means. The pursuit of these wonders feels ennobling, partly because of the human space it comes from and partly because it is difficult to imagine how contemplation of the stars could alter the fate of humankind.

The modern science of cosmology is perhaps as far removed from the day-to-day concerns of humanity as any human endeavor can be. Futurists may conjure colorful uses for the discoveries of scientific research on the nature and origin of the universe, but this is not a matter of dealing with transistors or life-extending drugs. No one argues that cosmology is studied because of its economic impact. However, this does not mean that the study of the universe lacks an economic impact. The latest discoveries in astronomy have always depended on progress in computer, space, and detector technology (Tegmark 2002). Synergism between the astronomical sciences and industrial and military concerns is strong and growing, and both enterprises benefit.



Philosophical Issues

As self-aware beings people share a special, emergent property of the universe: consciousness. Is the quality of this aspect of nature in some way different from, say, the way space is curved as a result of the distribution of mass in the universe? What is special about the way living, replicating systems employ available resources to thrive, evolve, and produce beings that are capable of studying the deepest questions about their existence? Is mind a statistically unlikely property to have emerged from a universe with 1,000,000,000,000,000,000,000 solar systems? Or is the quality of mind ubiquitous and unifying like gravitation or other universal physical laws? Science is engaged in exploring the origin and nature of the universe as it never has before, along with the role of life and consciousness within it.

Every culture has a cosmology. Science has become the sine qua non of truth, and its revelations are taken as gospel. The insights of science into the nature of the universe therefore are assumed to or allowed to subsume all prior knowledge. It is incumbent on all scientists to ask whether their work leads to living together in harmony or interferes with that harmony. Where is the role of heart or spirit in the exploration of the cosmos or, for that matter, in any scientific endeavor? The scientific study of the origin and structure of the universe is a journey that has begun to yield answers to questions that once were the purview of religion and myth. What is done with this knowledge and what its ultimate meaning may be should be an essential component of the science of cosmology.

MARK A. BULLOCK

SEE ALSO Astronomy.

BIBLIOGRAPHY

Greene, Brian. (2003). The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory, 2nd edition. New York: Vintage. A superb popular treatment of the physics of the ultra-small.

Gott, J. Richard; Mario Juric; David Schlegel; et al. (2004). A Map of the Universe. Princeton, NJ: Princeton University Press. Available from http://arxiv.org/abs/astro-ph/?0310571. An innovative approach to mapping the universe at all scales.

Sciama, D. W. (1973). Modern Cosmology. Cambridge, UK: Cambridge University Press. A classic text in cosmology.

Tegmark, Max. (2002). "Measuring Spacetime: From the Big Bang to Black Holes." Science 296: 1427–1433. A technical article by one of the leaders in cosmology.

Weinberg, Steven. (1977). The First Three Minutes: A Modern View of the Origin of the Universe. New York: Basic Books. A widely-read popular account of the role of nuclear physics in the origin of the universe.

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