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Cosmology, study of the universe as a whole, including theories about its origin, evolution, large-scale structure, and future. The more specific study of the origin of the universe and of astronomical systems, such as the solar system, is often called cosmogony.
 

The earliest cosmological theories known-from about 4000 BC-are from the Mesopotamians, who believed that the earth is the center of the universe and that the other heavenly bodies move around it. The nightly motion of stars across the sky was explained by some ancients, such as Aristotle and the Alexandrian astronomer Ptolemy, as the result of stars being fixed on rotating crystalline spheres. The Greek astronomer Aristarchus of Samos maintained, about 270 BC, that the earth revolves around the sun. Mainly because of Aristotle's authority, however, the concept of the earth as the center of the universe remained generally unchallenged until 1543, when the Polish astronomer Nicolaus Copernicus published his theories in De Revolutionibus Orbium Coelestium (On the Revolutions of the Celestial Spheres). Copernicus proposed a system in which the planets revolve in circular orbits around the sun, which he defined as the center of the universe. He attributed the rising and setting of the stars to the rotation of the earth on its axis. The German astronomer Johannes Kepler adopted the Copernican system and discovered that the planets move in elliptical orbits at varying speeds, according to three well-defined laws (since called Kepler's laws). Galileo, who first observed planets with a telescope, also rejected Aristotle's idea of the earth as the center of the universe and became a champion of the Copernican world view. The English mathematician and physicist Sir Isaac Newton showed that Kepler's laws of planetary motion could be derived from the general laws of motion and gravitation that Newton had discovered, thus indicating that these physical laws were valid in the heavens as well as on the earth.

An idea of the scale of the distances between stars was given in the early 19th century by the German astronomer Friedrich Wilhelm Bessel. He found that the nearby star 61 Cygni was at a distance of about 3 parsecs, or about 600,000 times the distance from the earth to the sun. In 1917 the American astronomer Harlow Shapley estimated that the earth's galaxy, the Milky Way, was about 100,000 parsecs in diameter, thus providing the first indication of the Milky Way's size. Unfortunately, Shapley neglected to consider the absorption of light from distant stars by dust particles in the Milky Way, which makes objects appear dimmer, and hence farther away than they really are. The modern value for the size of the earth's visible galaxy is roughly 30,000 parsecs (100,000 light-years) in diameter. The Dutch astronomer Jan Hendrik Oort found that the sun takes approximately 250 million years to travel once around the center of our galaxy, and he thus was able to calculate that the mass of the Milky Way is roughly 100 billion times the mass of the sun.

Until the beginning of the 20th century, astronomers were still uncertain about the nature of the spiral and elliptical nebulas. In particular, they could not determine whether such nebulas were inside or outside the galaxy. In 1924 the American astronomer Edwin Hubble succeeded in resolving individual stars in several nearby nebulas, including the Andromeda nebula. Several of these stars were pulsating stars called Cepheid variables. By measuring their period of pulsation, astronomers can determine the intrinsic brightness of these stars. By comparing the apparent brightnesses of these Cepheids with the known brightnesses of nearby Cepheids, Hubble proved that these nebulas lie far outside the galaxy. This meant that the thousands of spiral and elliptical nebulas were galaxies in their own right-external to the Milky Way Galaxy with each containing hundreds of billions of stars. Hubble estimated that the distance to the Andromeda Galaxy was 900,000 light-years, a figure later corrected to 2.2 million light-years when astronomers discovered that the Cepheids were more distant than was first thought.

The American astronomer Vesto M. Slipher, who studied the spectra of galaxies, had already noticed in 1912 that, except for a few nearby systems such as the Andromeda galaxy, the spectral lines were shifted toward longer (red) wavelengths. This shift in wavelength, caused by the Doppler effect, showed that most galaxies were receding from the Milky Way at several hundred kilometers per second.

In 1929 Hubble compared the distances he had estimated for various galaxies with the red shifts determined by Slipher for the same galaxies. He found that the more remote the galaxy, the higher was its recession velocity. This important relationship has become known as the law of the red shifts, or Hubble's law; it states that the recession velocity of a galaxy is proportional to its distance. The ratio of the recession velocity of a galaxy to its distance (the Hubble constant) is now estimated to be between 50 and 100 km/sec per megaparsec (1 megaparsec equals 1 million parsecs).

Because galaxies in all directions seem to recede from the Milky Way, it might appear that the Milky Way is at the center of the universe. This is not the case, however. One can imagine a balloon with evenly spaced dots painted on it. As the balloon is blown up, an observer on each spot would see all the other spots expanding away from it, just as observers see all the galaxies receding from the Milky Way. The analogy also provides a simple explanation for Hubble's law; the universe is expanding like a balloon.

In 1917 Albert Einstein proposed a model of the universe based on his new theory of general relativity. By considering time as a fourth dimension, he showed that gravitation was equivalent to a curvature of this four-dimensional space. His solution indicated that the universe was not static but must be expanding or contracting. The expansion of the universe had not yet been discovered, so Einstein postulated the existence of a force of repulsion between galaxies that counterbalanced the gravitational force of attraction. This introduced a "cosmological constant" in his equations, resulting in a static universe. He therefore missed a chance to predict the expansion of the universe by introducing an arbitrary constant. Einstein later called this the "biggest mistake of my life."
Nonstatic models of the universe were developed in 1917 by the Dutch astronomer Willem de Sitter, in 1922 by the Russian mathematician Alexander Friedmann, and in 1927 by the Belgian abbé, Georges Lemaître. The de Sitter universe solved Einstein's relativistic equations for an empty universe, so that gravitational forces were not important. Friedmann's solution depended directly on the density of matter in the universe and is the currently accepted model of the universe. Lemaître also worked out a solution to Einstein's equation, but he is better known for having introduced the idea of the "primeval atom." He stated that galaxies are fragments that have been ejected by the explosion of this atom, resulting in the expansion of the universe. This was the beginning of the big bang theory of the origin of the universe (see below).

The fate of the Friedmann universe is determined by the average density of matter in the universe. If there is relatively little matter in the universe, the mutual gravitational attraction among the galaxies will slow the recessional velocities only slightly, and the universe will expand forever. This would result in a so-called open universe that is infinite in extent. If, however, the density of matter is above a critical value, now estimated at 5 × 10-30 g/cu cm, the expansion will slow to a halt and reverse to contraction, ending in the total gravitational collapse of the entire universe. This would be a "closed" universe that is finite in extent. The fate of this collapsed universe is uncertain, but one theory is that it would explode again, producing a new expanding universe, which would again collapse, and so on ad infinitum. This model is called the pulsating, or oscillating, universe.

If the present rate of expansion of the universe is known, its age can be estimated by determining the length of time required for the universe to reach its present size. This will actually be an upper limit, as the present expansion has already been slowed by the mutual gravitational attraction among the galaxies. The first calculations of the age of the universe yielded a value of only 2 billion years. This age was considerably less than the 5-billion-year age of the earth that had been derived from the abundances of certain radioactive isotopes and their decay products in rocks . Subsequent corrections in the distance scale have removed this discrepancy. It was found, for example, that there are two types of Cepheid variables, each with a different intrinsic brightness. This confusion had caused Hubble to underestimate the distance to the Andromeda galaxy. At the present time estimates of the age of the universe range between 7 and 20 billion years, and thus they do not conflict with the age of the earth. Lower estimates in this range, however, seem to conflict with the age of the oldest stars, which are believed to be about 16 billion years old.

In 1948 the British astronomers Hermann Bondi, Thomas Gold, and Sir Fred Hoyle presented an entirely different model of the universe called the steady-state theory. They found the idea of a sudden beginning to the universe philosophically unsatisfactory. Their model was derived from an extension of the "cosmological principle" that had been used for previous theories such as Friedmann's model. It stated that the universe appeared the same from any location, but not necessarily for all times. They proposed that the decrease in the density of the universe caused by its expansion is exactly balanced by the continuous creation of matter condensing into galaxies that take the place of the galaxies that have receded from the Milky Way, thereby maintaining forever the present appearance of the universe. The steady-state theory is no longer accepted by most cosmologists, particularly after the incompatible discovery of cosmic background radiation in 1965.

The discovery of quasars also provided evidence contradicting the steady-state theory. Quasars are very small but brilliantly luminous extragalactic systems, found only at great distances. Their light has taken several billion years to reach the earth. Quasars are therefore objects from the remote past, which indicates that a few billion years ago the constitution of the universe was very different than it is today.

In 1948 the Russian-American physicist George Gamow modified Lemaître's theory of the primeval atom into the big bang theory of the origin of the universe. Gamow proposed that the universe was created in a gigantic explosion and that the various elements observed today were produced within the first few minutes after the big bang, when the extremely high temperature and density of the universe would fuse subatomic particles into the chemical elements. More recent calculations indicate that hydrogen and helium would have been the primary products of the big bang, with heavier elements being produced later within stars. This theory, however, provided a basis for understanding the earliest stages of the universe and its subsequent evolution. The extremely high density within the primeval atom would cause the universe to expand rapidly. As it expanded, the hydrogen and helium would cool and condense into stars and galaxies. This explains the expansion of the universe and the physical basis of Hubble's law.

According to Gamow's theory, as the universe expanded, the residual radiation from the big bang would continue to cool. By today it should be a temperature of about 3 K (about -270° C/-454° F). This relic radiation was detected by radio astronomy in 1965, thereby providing what most astronomers consider to be confirmation of the big bang theory.

Contributed by:
Bradley C. Whitmore

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