Astronomy and Cosmology: Cosmology
Astronomy and Cosmology: Cosmology
Astronomy and Cosmology: Cosmology
Any set of beliefs about the overall nature of the universe is a cosmology. Various societies' cosmologies have been shaped throughout history by cultural, religious, philosophical, political, and other forces. The world “cosmology” comes from the Greek words kosmos, order or universe, and logia, speech or discourse about a subject.
Modern cosmology is the scientific study of the overall properties of the universe, including its origin, age, size, structure, composition, and likely future. According to the modern cosmological picture, the universe is finite in size, growing at an accelerating pace, and about 13.7 billion years old. The modern cosmological picture is supported by many kinds of evidence, although some important questions remain unresolved.
Historical Background and Scientific Foundations
From Myth to Cosmos
The earliest speculations about the origin and nature of the world took the form of religious myths. Almost all ancient cultures developed cosmological stories to explain the basic features of the cosmos: Earth and its inhabitants, sky, sea, sun, moon, and stars. For example, for the Babylonians, the creation of the universe was seen as born from a primeval pair of human-like gods. In early Egyptian cosmology, eclipses were explained as the moon being swallowed temporarily by a sow or as the sun being attacked by a serpent. For the early Hebrews, whose account is preserved in the biblical book of Genesis, a single God created the universe in stages within the relatively recent past. Such pre-scientific cosmologies tended to assume a flat Earth, a finite past, ongoing active interference by deities or spirits in the cosmic order, and stars and planets (visible to the naked eye only as points of light) that were different in nature from Earth.
Philosophical speculation, characterized by curiosity and abstraction, was developed by the Greeks during the sixth century BC. Greek philosophers pondered the nature of things, including the causes of the motions and the substance of the celestial bodies. Using the principle of uniform circular motion—motion in a circle at an unchanging speed, believed by these thinkers to be the most perfect and therefore fundamental kind of motion—they created mathematical models to predict planetary positions in the sky. The Greek cosmology was standard among educated Europeans for about 2,000 years, being replaced by a modern scientific cosmology only since the 1500s. In the last 100 years, cosmological knowledge has expanded greatly, establishing as fact an unforeseen picture of the universe whose only common feature with early myth is that the universe is finite in age.
The Ancient World
The earliest Greek cosmological speculations may be seen as secularized (non-religious) myths based on analogies from everyday experience. Some early Greek philosophers envisioned a single fundamental substance from which the universe was created, and a means by which this creation might have occurred. For Thales (c.625–547 BC), that substance was water. Anaximander (c.610–545 BC) held that it was a featureless mass that gave rise to various features through a swirling process by which heavy matter moved toward the center, while fire and air moved outward and became celestial bodies.
The followers of Pythagoras (c.580–500 BC) speculated on the nature of the planets and their motions. Ecphantus (fl. 350 BC), taught that Earth rotates about it own center, and that the planets move in uniform circles.
By the sixth century BC, Earth and celestial bodies were held to be spherical. The planets were thought to revolve around Earth at different speeds and distance. According to Anaxagoras (c.500–428 BC), celestial and terrestrial matter are the same. The universe is full, without any empty spaces: there can be no such thing as a vacuum. The moon—Earth-like in nature, with hills, rivers, and inhabitants—shines by reflected light, and eclipses are due to its interposition between the sun and Earth. The sun is a red-hot rock much smaller than Earth.
The concept of matter as composed of unbreakable atoms and void (empty space) was put forward by Democritus (460–370 BC), who also proposed an infinite number of worlds with an infinite range of sizes. These, Democritus taught, varied in different ways from our own: some lacked a moon; some had a larger sun or even two suns. Some worlds, he believed, are always being born and others are dying.
Apart from Earth's roundness, which Greeks verified by scientific observation, none of these teachings were based on experiment or measurement: they were speculations. Some turned out to be remarkably foresightful, while many others turned out to have no relationship to fact.
Plato (427–347 BC) advanced a cosmology in the book Timaeus in which matter and space have always existed, but time was created by a divine craftsman—the Demiurge—who formed the components of the universe out of a primary substance. Sun, moon, and planets move at different speeds about a motionless Earth and are divine in nature. The stars, Plato taught, have souls.
Plato's most famous pupil, Aristotle (384–322 BC), articulated the cosmology that would prove most influential in world intellectual history until the advent of modern scientific cosmology. According to Aristotle, the cosmos is spherical, with Earth at its center. Earth is surrounded by a set of nested transparent spheres, one for each planet. Earth, he posited, is composed of four elements: earth, water, air, and fire. All matter is a combination of varying amounts of these elements. Bodies composed mostly of earth or water tend to move toward the center of Earth, which is also the center of the universe, because that is their proper place, and they have an affinity for it. Bodies made mostly of air and fire tend upward to the limit of terrestrial matter, that is, the sphere of the moon.
The heavens, Aristotle believed, are composed of a fifth element, the ether, which tends to move in uniform circular motion around the center of the universe (i.e., Earth's center). The moon, Mercury, Venus, the sun, Mars, Jupiter, Saturn, and the fixed stars are each embedded in their own spheres. Outside all is the primum mobile (“first mover”), which imparts motion to all the spheres it contains. The Aristotelian cosmos is finite in size, eternal in time, and without any void spaces or vacuum.
However, the Greeks did not hew to any single view: philosophers proposed conflicting speculations and disputed them with each other. For example, Aristarchus (c.310–230 BC) proposed that Earth revolves about the sun. He also discovered the angular measurements and geometrical assumptions necessary to estimate Earth's distance from the sun and moon, concluding that the universe was much larger than commonly thought.
The epitome of Greek astronomical discovery working in the cosmological scheme of Aristotle was the work of Claudius Ptolemy (AD c.90–c.168). His He mathematike syntaxis, known in the West from its Arabic title as the Almagest, was essentially a series of mathematical models and techniques for predicting the positions of the celestial bodies.
European Cosmology in the Middle Ages
Early Christians leaned heavily on biblical creation accounts for their cosmology. By the fourth century AD, theologians such as St. Augustine (354–430) were attempting to reconcile apparently contradictory passages, particularly in Genesis. Some asserted that the “days” of biblical creation, for example, were not 24 hours long; some accepted, others rejected, Earth's sphericity. The order and stability of the universe were viewed as evidence of divine creation. After the fall of the Roman Empire, European scholars lost contact with many Greek books and scientific ideas for centuries. They did retain a basically Aristotelian cosmology, with a spherical Earth at the center and vast crystalline spheres carrying the moon, sun, planets, and stars around that central point. (It is not true, as popularly believed, that Christopher Columbus had anything to do with discovering or proving that Earth is spherical: all educated persons had believed for many centuries that Earth is round.)
During the spread of Islam, Muslims came in contact with Greek science and began to make extensive translations of these works from AD 750–850. Scholarly contacts between West Europeans and Muslims began to develop in the eleventh century, and translations were made from Arabic into Latin. As a result, cosmological ideas, as proposed in Plato's Timaeus, began to expand in the West. In the twelfth century Europe reacquired Ptolemy's Almagest, with its star charts and mathematical formulae describing heavenly motions, by translation from Arabic.
During the Middle Ages Aristotle's ideas were blended with Islamic and Christian beliefs. For example, the eternity of the universe was rejected in favor of a definite creation date (as in Genesis). The possibility of multiple worlds, an infinite universe, the motion of Earth, the cause of planetary motion, and the existence of vacuums were all discussed, but the Aristotelian foundation remained unchanged. The cause of planetary motion, originally thought to be caused by angelic spirits, was replaced by “impetus”—a source of motion embedded by God in the creation of the universe.
The Renaissance and the Copernican Challenge
The heliocentric (sun-centered) universe proposed in the revolutionary 1543 De revolutionibus orbium caelestium (On the Revolutions of the Celestial Spheres) of Nicolaus
Copernicus (1473–1543) was inspired largely by neoPlatonism, a philosophy according to which the sun was the proper regent or king of the universe and so should, properly, reside at its center. The Copernican system accounted for the observed motions of the planets by use of spheres centered on the sun rather than on Earth (except for the moon, which still went around Earth). Contrary to a popular myth, the Copernican system was not, at first, significantly simpler or more accurate than the Ptolemaic system it challenged, though it did eventually become so.
The Copernican system was attacked on theological and physical grounds, and the church decided that it could only be advocated as a theoretical construct, not as physical reality, lest it conflict with geocentrism (argued to be required by the Bible).
During the sixteenth century, precision became increasingly important in commerce, crafts, politics, time measurement, and the sciences. Tycho Brahe (1546–1601) actively pursued greater precision in astronomical observation and created the most precise measuring instruments yet known. As a result of his observations, he began to question some of Aristotle's time-honored principles concerning the nature of the heavens. In 1572 a very bright star appeared that would now be called a supernova. A number of astronomers determined that it was farther away than the moon, in a realm where Aristotle had claimed nothing new could occur because all change was forbidden. Clearly, change could and did occur beyond the moon. Five years later a comet appeared, and observations showed that it too was beyond the sphere of the moon. Its passage around the sun appeared, in fact, to be in the same sphere as Venus.
During the latter part of the sixteenth century there were few Copernicans. Among them, however, was one who speculated that the universe might be infinite in size—another departure from Aristotelian cosmology. Thomas Digges (1546–1595) published his Prognostication Everlastinge in 1576, in which he postulated that the stars extended out to infinity. It is one of the stranger twists of scientific fate that the old, Aristotelian belief in a finite universe would prove to be closer to the truth than this apparently more rational, scientific belief—although the true size of the universe would prove to be many billions of times greater than imagined by any thinker in the Aristotelian cosmological tradition.
The Union of Astronomy and Physics
Johannes Kepler (1571–1630) modified the Copernican theory in substantial ways, using Tycho's data. He held that planetary orbits are elliptical, not circular, and that planetary velocities vary by distance from the sun, becoming more rapid when closer to it, with an imaginary line from the sun to the planet sweeping out equal areas in equal times. He also proposed that a quasi-magnetic force centered on the sun caused planetary motion, a theory that would concern astronomers and cosmologists for the rest of the century.
Another challenge to the Aristotelian cosmos was provided by Galileo Galilei (1564–1642), who in the first years of the seventeenth century became the first person to use a telescope for celestial observation. Galileo discovered that the moon had mountains, and that Venus had phases (changing patterns of illumination, like those seen on the Moon, showing that it is a sphere). He also discovered that Earth is not the only planet that had a satellite: Jupiter had several. The sun, once thought to be perfect, had spots, and it rotated. Galileo was banned from teaching Copernican cosmology by the church, but his works remained available and continued to influence European thinkers.
By the middle of the seventeenth century Aristotelian natural philosophy had been widely replaced by what came to be called the mechanical philosophy. One of the century's leading proponents of this philosophy was René Descartes (1596–1650), who believed that God had created an indefinite amount of matter in a swirling universe composed of three kinds of particles moving in an infinite number of vortices or swirls, each of which carried planets around a star. Step by step, with many errors and misconceptions, a view of the universe that we would recognize as physically true was gradually emerging.
English physicist Isaac Newton (1642–1727) transformed concepts of the universe through his laws of
motion and of universal gravitation, published in Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy, 1687), as well as his creation of the calculus. By incorporating the works of Kepler, Galileo, and others, he proved the inadequacy of the Cartesian system and showed how the attractive forces of all bodies, both terrestrial and celestial, were dependent on their masses and inversely proportional to the squares of their distances. Newton's work was continued and advanced by many other scientists.
The Birth of Stellar Astronomy: Stretching Space and Time
To explain why, under universal gravitation, all stars do not collapse into a single body, Thomas Wright (1711–1786) proposed that all stars and planets revolve about a common center, with the resulting centrifugal force keeping them from falling together under the pull of their mutual gravitation. Both Wright and philosopher Immanuel Kant (1724–1804) proposed the continual development of the cosmos and speculated that the universe was finite and disk-like, with the solar system near its middle; there might even be other universes, outside our own. Kant posited that the constituent elements of the universe at the moment of its creation were in a chaotic state, completely filling space. With the existence of universal gravitation, larger particles served as nuclei for the attraction of others. Particles of greater mass grew by accretion. There was a force of repulsion that is the source of circular motions and the flattening of vortices. Vortices or whirls were created around the sun, which was not yet hot. The planets were formed from belts of swirling matter, whose orbits were slightly inclined to one another. Many elements of these ideas have survived into modern cosmology, though greatly modified, expressed in the precise language of mathematics, and tested against observations that literally could not have been imagined in the eighteenth century.
William Herschel (1738–1822) was another influential figure in the birth of stellar astronomy and the expansion of post-Aristotelian cosmological ideas. In the course of his effort to map the heavens in 1781, he noted that stars were often found in clusters, some denser than others. This led him to the idea of “island universes”—what we now call galaxies.
Herschel also worked to produce bigger and better telescopes. About 2,000 stars are visible with the naked eye; a 4-inch (10.16-cm) refractor can reveal a million stars. Herschel's telescopes showed vast numbers of astronomical objects, and they were not all alike. By 1781 over 300 nebulae—smeared-out, gaseous-looking objects (nebula is Latin for “mist”)—had been discovered. The first catalogue of them by Charles Messier (1730–1817) was published in that year. By the early nineteenth century, several thousand nebulae and star clusters had been located.
After Herschel's death, his work was continued by his son John (1792–1871), who spent four years at the Cape of Good Hope at the southern tip of Africa, classifying double stars and nebulae in the southern hemisphere. The younger Herschel concluded that the sun was embedded in a system of stars distributed with some regularity, shaped like a convex lens, and that the sun is displaced from the center of this lens-shaped cloud of stars. We would now say that John Herschel had correctly identified Earth's position in the Milky Way galaxy.
English astronomer Edmond Halley (1656–1742), proposed in 1811 that spiral nebulae may be Milky Way-type “island universes.” In 1814 he speculated that the Milky Way would eventually be resolved into globular clusters and raised the possibility of devising a celestial time scale. He also concluded that the universe was finite.
Astrochemistry, Astrophysics, and New Instruments
In the seventeenth century, there were only two well established observatories—in Paris, France, and Greenwich, England. By the end of the nineteenth century, there were many more throughout Europe and at the Cape of Good Hope, with larger and better telescopes, including instruments that could measure angles and times more precisely. William Parsons, third earl of Rosse (1800–1867), a wealthy Irish nobleman, spent 18 years and a substantial sum building the “Leviathan,” a telescope with a 6-foot (1.8-m) mirror, and a magnification of 650. By 1850 he had seen 14 spiral nebulae, with branching arms from their nuclei. Like Halley, he speculated that they were outside our universe and similar to it. In this, of course, the two men were basically correct, although it would be many more years before their conjecture was proved.
There was a lag in picking up on Herschel's work on cosmic structure. Cosmology was thought to be in the realm of speculation; there was not enough hard data to go on. Stellar astronomy was confined to the study of the proper stellar motions and distances by observable parallax. However, as Europe industrialized in the nineteenth century, instruments that significantly improved astronomical precision were built. This led to the first observation of the long-sought stellar parallax. Parallax is the shift in the apparent relative positions of two or more objects at different distances from an observer as the observer changes position: astronomers had long reasoned that as Earth swings around the sun, the apparent relative positions of the stars should shift slightly, showing parallax. From this parallax, the distances of stars could be calculated. However, the stars are so far away that the parallax effects even of such a large change in position as is given by Earth's orbit were hard to detect. Parallax was detected in 1838 by German mathematician and astronomer Friedrich Bessel (1784–1846) for 61 Cygni. Parallaxes for other stars were found shortly thereafter. The measurement of parallaxes yielded shockingly large distances. Alpha Centauri was 272,000 times farther away than the distance between Earth and the sun (1 astronomical unit or AU, about 93 million miles = 150 million kilometers) (AU); 61 Cygnus was at 691,000 AU.
The Twentieth Century's Cosmological Revolution
In 1900 the Milky Way, the galaxy in which our solar system resides, was the only one known, although there had been speculation that spiral nebulae were similar “island universes.” In 1920, Heber Doust Curtis (1872–1942) and Harlow Shapley (1885–1972) engaged in what became known as the “Great Debate,” with Shapley arguing that this galaxy comprises the entire universe and Curtis countering that the universe contains many such galaxies. The argument was resolved in 1923, when American astronomer Edwin P. Hubble (1889–1953) used spectral analysis (dissection of the mixed types of light coming from a star or other body) to show that the stars in the Andromeda Nebula—the most easily observed spiral nebula—are 25 times farther away than the most remote inside the Milky Way. He did this by using stars called Cepheid variables, a type of star whose brightness changes regularly over time.
It had already been shown that the absolute brightness of a Cepheid variable is closely related to the frequency with which it dims and brightens: measuring a Cepheid's period (rate of dimming and brightening) thus reveals how bright the star is, which can be compared to its apparent brightness (which diminishes with distance) to show the star's true distance. Such objects are called standard candles by astronomers.
Hubble, analyzing Cepheids in the M33 galaxy, calculated them to be 700,000 light years away. (A light year is the distance traveled by light in 1 year, about 6 billion miles or 9.5 billion kilometers.) Soon distances of a million light years had been observed. In 1925, images from the Mt. Wilson observatory in California showed galaxies stretching outward without end. The true vastness of the universe was becoming apparent through direct observation.
Relativity and Cosmology
A profound transformation of modern concepts of space, time, and mechanics—and thus of the very nature of the universe—occurred when Albert Einstein (1879–1955) proposed his theories of special and general relativity. Equally significant was the development of quantum theory by Niels H.D. Bohr (1885–1962) and others in the early twentieth century, though it would be decades before the close connection of quantum physics (the physics of the very small) to cosmology (the physics of the very large) would become apparent.
The general theories of relativity (published in 1915) posited that space itself is curved and that the universe is finite, though unbounded, with no edges and no center. It was assumed that the universe could be neither expanding nor contracting, since either of these conditions would imply a finite lifespan for the universe, which was assumed to be eternal. However, in a static (unmoving), finite universe, mutual gravitational attraction, if unopposed by any other force, would result in contraction. Einstein therefore added a “cosmic constant” to his equations of gravity to provide a cosmic repulsion or antigravity that would counterbalance universal gravitation, allowing a non-expanding, non-contracting universe to be stable.
Alexander A. Friedmann (1888–1925) suggested eliminating the cosmological constant in 1922; it was unnecessary if one assumed that the universe is either expanding or contracting. The cosmological constant may be plus, minus, or constant. Meanwhile, evidence for expansion was accumulating as astronomers measured spectral (color) shifts in light coming from distant nebulae. When an object moves away from an observer, its light is reddened in proportion to its velocity. By 1917 the analysis of 15 nebulae showed recession up to 400 miles per second (644 km/s). By 1922, 42 nebulae were shown to be receding up to 1,100 miles per second (1,770 km/s).
Hubble's law, formulated in 1929, stated that galaxies recede with velocities proportional to distance: v= Hr (distance); where H is Hubble's constant, 14 miles per second per million light years, and r is the distance of the galaxy from us. That is, the farther away a galaxy is from us, the faster it is moving away from us. Measuring the red shift in a galaxy's spectrum indicates its velocity of recession—as long as one knows the value of Hubble's constant, H. This, however, proved difficult to measure exactly.
It had become clear that the universe is not only big but getting bigger—expanding, as Friedmann had urged was possible. In an expanding universe, all galaxies are flying away from each other, although individual galaxies do not expand. This expansion, imagined backward in time, implies a time when all the galaxies were together at a single point. In 1927, Belgian priest and astronomer Georges-Henri Lemaître (1894–1966) proposed the big bang theory of the origin of the universe. Beginning with Einstein's cosmological constant and Hubble's law, Lemaître posited that before the creation of the universe, all matter was condensed into a tightly packed “primeval atom,” a region about one astronomical unit across. The primeval atom, Lemaître proposed, contained all space, time, and matter. Its average density increased according to Hubble's law. At the moment of creation—cause unknown, but not necessarily miraculous—it exploded. Gravitation began to slow down the expansion. The universe might eventually collapse again under its own weight, or continue to expand forever, as the stars burned out and eternal darkness closed in.
In 1946, American astronomer George Gamow (1904–1968) proposed the prevailing evolutionary theory. In the first few minutes after the big bang, the lightest few elements were built up by collisions between originally free neutrons and protons; all heavier elements (including those of which Earth and our bodies are made) were forged later, in the hearts of exploding stars, a process called nucleosynthesis. The present distribution of the elements agrees well with the predictions of this theory.
The steady-state theory, proposed by Thomas Gold (1920–2004), Hermann Bondi (1919–2005), and Fred Hoyle (1915–2001), derived from a philosophical distaste for an expanding universe that led to eternal dispersion and darkness. In the steady-state model, there is continuous creation of hydrogen atoms throughout space to replace matter being dispersed by cosmic expansion. Created out of nothing in space, these atoms begin to coalesce into groups and subgroups by gravitation. Compression creates high pressures, nuclear processes begin, and stars form in a continuous process.
According to this view, the universe is eternal in a steady state. There is no cosmogony or moment of cosmic beginning, but systems continue to evolve. Stars age and disappear over the edge of observation. The steady-state theory, however, disagrees with a number of astronomical observations: for example, it predicts that the velocity of recession slows with distance, but it does not. Indeed, in the late 1990s observation showed that the universe's expansion is accelerating, and that something similar to Einstein's long-abandoned cosmological constant was in fact real.
Cosmic background microwave radiation, strong evidence for the big bang theory, was discovered by Arno Penzias (1933–) and Robert Wilson (1936–). This is weak, low-temperature radio radiation that shines from every part of the sky and was predicted by the big bang theory as an afterglow of the primordial explosion.
Modern Cultural Connections
The present state of scientific cosmology can be summarized as follows: The universe originated in a big bang about 13.7 million years ago. The big bang commenced with a period of rapid inflation, during which the universe increased in size so rapidly that subatomic-scale variations in the quantum field were amplified to universe-spanning dimensions. Eventually the material of the big bang cooled and dissipated until atoms could form, then, millions of years later, galaxies and stars. Atoms of heavy elements were forged in the cores of exploding early-generation stars and blown into space by supernova explosions. These materials later re-formed as new stars and planets, including our own. The observable part of the universe contains about 100 billion galaxies, each containing anywhere from a few million to many billions of stars. These galaxies are all receding from each other as the universe expands at an ever-accelerating rate due to a still-mysterious force that astronomers have dubbed dark energy.
The equations of quantum physics seem to indicate that our whole universe may be one of an infinite number of universes continually sprouting from divergent quantum events—an infinite number of big bangs, each possibly resulting in a universe with different physical laws from our own. A number of such theories, termed multiverse theories, are debated today among cosmologists. No physical evidence is yet known for the existence of other universes, however, and in the twenty-first century the possible existence and nature of a multiverse is debated among cosmologists. There is no consensus or majority view on the topic of whether the observable universe is unique.
The ultimate fate of our own universe—which may or may not be the entire or only universe—is also debated. Some earlier theories proposed an eternal cycle of explosion, eventual collapse, and re-explosion, but this seems to have been ruled out by recent proof that the expansion of the universe is accelerating. Several otherpossibilities are discussed among cosmologists. Under some conditions, the universe may simply burn out and continue to expand through infinite time, dark and uneventful. If dark energy increases in density with time, it may eventually disintegrate all physical structures and end in a supreme, all-embracing singularity similar to a black hole. Or, quantum fluctuations at the horizon of observation might someday yield a new big bang. In some multiverse theories, various universes will experience different fates, some having finite duration, some infinite duration. No experimental evidence yet exists that could sift these possibilities. Indeed, it is possible that such evidence may be impossible to acquire. Cosmologists' arguments on these points are highly esoteric and mathematical: there is no consensus position at this time on the ultimate fate of the universe.
Social and Cultural Implications
The social and cultural implications of modern cosmology have been mostly negative. That is, the loss of the earlier, Aristotelian-Christian cosmology, which featured a spherical Earth placed meaningfully at the cosmic center, has entailed for some persons a sense of cosmic alienation or meaninglessness. This view has been well-expressed by American physicist Steven Weinberg (1933–), who wrote in The First Three Minutes (1977) that human life is “just a more-or-less farcical outcome of a chain of accidents reaching back to the first three minutes.” The universe, Weinberg argued, is revealed by modern science to be “overwhelmingly hostile,” evolved from “unspeakably unfamiliar early conditions.” “The more the universe seems comprehensible,” he wrote, “the more it also seems pointless.” Almost the only bright spot in such a world is, in fact, Weinberg's own profession, physics: “The effort to understand the universe is one of the very few things that lifts human life a little above the level of farce, and gives it some of the grace of tragedy.”
But such emotional responses are not dictated by science, which is value-neutral. Religious believers, in contrast to thinkers such as Weinberg, tend to see the scale and grandeur of modern cosmology as affirming their own belief in a transcendent Creator. Even multiverse theories have been embraced by some theologians, recalling early Christian theological resistance to the possibility of a finite universe, which was felt to be unworthy of an infinite God. Exposure to cosmological knowledge does not seem to reliably cause abandonment of religious beliefs or adoption of heroic nihilism like Weinberg's. Cosmology impacts individual life mostly at the level of religious (or anti-religious) convictions about the nature of the cosmos, but in this realm science is notoriously unable to dictate conclusions.
For some religious believers, however, the claims of modern cosmology are unacceptable, no matter how much astronomical evidence is mustered to support them. Millions of creationists in the United States and elsewhere continue to vigorously defend a pre-scientific worldview based on a literal reading of the biblical book of Genesis. (There are also non-Judeo-Christian forms of creationism appealing to other religious traditions.) According to biblical creationists, the universe was created out of nothing a mere 10,000 or so years ago. This class of believers reject the scientific account not only of biological evolution, but of the big bang and the age of the universe.
For many persons, questions about the structure, origin, and ultimate fate of the universe are fascinating in their own right. Some people, however, even in industrialized societies, are unaware of even the most basic elements of cosmological thought. A 1988 Gallup poll found that over 18% of Americans believed that the sun orbits Earth, rather than the other way around, while a further 8% percent responded that they had no opinion on the subject.
Primary Source Connection
The following article was written by Peter N. Spotts, the science and technology writer for The Christian Science Monitor. Founded in 1908, The Christian Science Monitor is an international newspaper based in Boston, Massachusetts. The article describes a cyclical model of the universe put forth in 2002 by scientists Paul Steinhardt and Neil Turok, in which the universe continually cycles through periods of expansion and contraction.
THE BIG BANG (ONE MORE TIME)
Just when it SEEMS the broad outline of the universe's origin is as safely in hand as money in the bank, along come two physicists who could prove to be the Butch Cassidy and Sundance Kid of cosmology.
For 20 years, Paul Steinhardt has played a key role in helping to write and refine the inflationary “big bang” origin of the universe.
But over the past few years, the Princeton University physicist and some of his colleagues have struggled with a vexing question. “Even if our story seems to describe what we see, how do we know it's the right story?” Dr. Steinhardt asks.
He decided to see if he could come up with a plausible alternative to the prevailing notion. The inflationary big-bang model posits that the universe began as a random fluctuation in empty space, grew with extraordinary speed through an “inflationary” period, then slowed, cooled, and formed all the matter and energy astronomers see and infer today. Steinhardt wanted to be able to describe the universe with as much precision as the inflation theory does but without some ofits “baggage,” including the need for an inflationary period itself. The result: He and Cambridge University physicist Neil Turok have unveiled a model in which the universe has no beginning or end, but replenishes itself in a cycle of expansion and contraction. Each expansion is triggered by its own big bang.
Steinhardt adds that, among its other selling points, the new model naturally accounts for recent observations that the universe is entering an epoch of accelerated expansion, a feature he says is not directly predicted by the inflationary big bang.
The model, published late last month in the journal Science, is a work in progress. Some researchers, such as Princeton astrophysicist Jeremiah Ostriker, have hailed it as “extraordinarily exciting and… the first new big idea in cosmology in over two decades.” Others, such as Stanford University's Andrei Linde, another inflation pioneer, are intensely skeptical.
The duo's rebellion against the reigning explanation for the universe's origin may seem out of character for what many people view as the staid world of science. “People have the idea that scientists and cosmologists believe something like it was dogma,” says Rocky Kolb, a cosmologist at the Fermi National Accelerator Laboratory in Batavia, Ill. “We seem like a conservative lot, but … everybody wants to turn everything upside down.”
But new ideas have to pass “sniff tests.” And for some researchers, ideas posed in prose, with no math for backup, are sure-fire candidates for the circular file.
“Really beautiful ideas have experimental and observational consequences,” although decades may pass before predicted results emerge, Kolb says.
He also points out that “a lot of ideas come from people in different fields or people you may not know. In 1980, not many cosmologists had heard of Alan Guth, who came up with inflation.”
Drawing on quantum physics and Einstein's theories of relativity, the inflationary big bang begins 11 billion to 15 billion years ago, when a random change in an astonishingly small bit of all-encompassing vacuum grew very rapidly.
For a tiny fraction of a second, this burgeoning fluctuation in the vacuum expanded faster than light can travel. When the burst of inflation abruptly ended, the proto-universe reached enormous energies, temperatures, and pressures. As this roiling universe continued to expand at a more sedate rate and cooled, its matter, energy, and structure emerged.
The inflationary model has fended off all comers; it matches many observations and has described features of the early universe that astronomers searched for and found, Steinhardt says.
For Dr. Guth, a physics professor at the Massachusetts Institute of Technology, inflation was a solution to a particle-physics conundrum. The problem emerged from efforts in the late 1970s to develop grand unified theories to describe the emergence of three of the four forces of nature and the associated subatomic particles from the big bang.
At the time, Guth and collaborator Henry Tye determined that grand unified theories predicted—and sometimes required—formation of magnetic monopoles. These “outrageously heavy” particles “had always been consistent with the laws of physics, but no one had ever seen one and there was no real reason to believe they existed,” Guth says.
Prodded by Dr. Tye, Guth says he reluctantly calculated the number of these particles that would have been created during the big bang. They discovered that monopoles would have been as ubiquitous as protons—leading to a universe that would look much different from the one we inhabit.
Guth's calculations led him to conclude that a brief period of inflation stifled monopole formation. As a bonus, inflation also appeared to solve problems cosmologists had in squaring conventional big-bang thinking with astronomers' observations of the universe.
Particle-physics theorists were the most receptive to this view, Guth says. By contrast, he says, astronomers and cosmologists “took a wait-and-see attitude.”
This same interplay between the world of the very tiny and the world of the very large weaves its way through Steinhardt's and Dr. Turok's cyclical universe. They hold that much of their model works well in a four-dimensional universe of height, depth, width, and space-time. They add that it finds its true home in the nine to 10 dimensions of string theory, which tries to explain how the four forces of nature emerged from one unified force early on.
One variation, known as M theory, holds that the universe consists of two parallel sheets, or membranes. The two membranes are separated by a “fifth” dimension a tiny fraction of a centimeter wide.
Steinhardt and Turok's calculations describe the membranes meeting in a slap, triggering the big bang. On the membrane humans inhabit, the bang yields the particles, energy, and forces familiar to scientists. The other contains “we know not what,” Steinhardt says. The duo posits the second one may be home to “dark matter.”
Over trillions of years, the membranes expand, growing darker, colder, and less dense, until the logic of Steinhardt's equations brings them back together in another cosmic slap. The membranes resume expanding, even as they drift apart, only to repeat the cycle.
Steinhardt says this model yields all the features of the inflationary model, without inflation. What his calculations don't show is what happens when membranes “bounce.”
As always, nature will provide the ultimate reality check on his model. “There are many beautiful, insightful ideas that turn out not to be the way nature works. The cyclical universe may be one of them,” Kolb says. “I don't think this is the way nature works. Maybe there'll be an application for it someday. Or some of the ideas may be used in some other way.”
Steinhardt hasn't tossed in the towel on inflation, either: “I'm just hedging my bets.”
Peter N. Spotts
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