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 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 Isaac Newton’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 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 Einstein’s (1879–1955) 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 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 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 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 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 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 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 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 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 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 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 universe’s 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 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., 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 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

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 Universe’s 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