universe, totality of matter and energy in existence. The study of the origin of the universe, or cosmos, is known as cosmogony, and that of its structure and evolution, cosmology. The age of the universe depends on which theory of cosmology one accepts. According to the big bang theory, favored by most scientists, the universe is between 10 and 20 billion years old; most recent calculations place its age at c.13.7 billion years. The steady-state theory holds that the universe has been in existence for all time.

Matter and Energy in the Universe

The matter in the universe is subject to various forces, but the greatest force on the cosmological scale is gravitation. This force pulls matter together to form stars, which either exist alone or are part of binary star or multiple star systems, or brown dwarfs, which are also known as "failed stars." Gravitation also acts to group billions of stars into galaxies and to group galaxies into clusters and superclusters. The main source of energy in the universe is the conversion of the matter of the stars into energy through thermonuclear reactions (see nuclear energy). These reactions continue throughout the different stages of stellar evolution (see also stellar populations) until the star has consumed all its available nuclear fuel.

The Size and Shape of the Universe

The first systematic theory of the size and shape of the universe that attempted to explain observed data was constructed by Ptolemy in the 2d cent. In this theory the solar system was thought to be the entire universe, with the earth at its center and the distant stars located just beyond the farthest planet. This belief was held until the 16th cent., when Copernicus advanced the idea that the sun, rather than the earth, is at the center of the system and that the stars are at very great distances compared to the planets. During the first part of the 20th cent., astronomers discovered that the sun is only one of billions of stars in the Milky Way galaxy and is located far from the galactic center.

Estimates of the size of the universe have been refined as methods of measuring galactic and extragalactic distances have improved. Close stellar distances were at first found by measuring a star's trigonometric parallax. A more powerful contemporary method is to analyze the light reaching the earth from an object by means of a spectroscope; the distance of a very faint object can be estimated by comparing its apparent brightness to those of similar objects at known distances. Another method depends on the fact that the universe as a whole appears to be expanding, as indicated by red shifts (see Doppler effect) in the spectral lines of distant galaxies. Hubble's law makes it possible to estimate their distances from the speed with which they are rushing away from the earth. At present the universe is believed to be at least 10 billion light-years in diameter. One problem with estimating the size of the universe is that space itself (or more properly, space-time) may be curved, as held by the general theory of relativity. This curvature would affect measurements of distance based on the passage of light through space from objects as far away as 5 billion light-years or more.

expanding universe: see universe.

Concept of an expanding universe whose average density remains constant, matter being continuously created throughout it to form new stars and galaxies at the same rate that old ones recede from sight. A steady-state universe has no beginning or end, and its average density and arrangement of galaxies are the same as seen from every point. Galaxies of all ages are intermingled. The theory was first put forward by William Macmillan (1861–1948) in the 1920s and modified by Fred Hoyle to deal with problems that had arisen in connection with the big-bang model. Much evidence obtained since the 1950s contradicts the steady-state theory and supports the big-bang model.

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Current understanding of the state of the universe. It is based on the finding that all galaxies are moving away from each other. Application of general relativity to cosmology, along with the detection of redshifted light coming from galaxies outside the Milky Way Galaxy, led to the realization in the 1920s that all galaxies are receding (see Edwin Hubble). It is unknown whether the universe will expand indefinitely (open universe) or eventually collapse (closed universe) into an extremely dense, congested state, as it began, according to the big-bang model. Seealso Friedmann universe.

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