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 many scientists, the universe is between 10 and 20 billion years old. 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. Bibliography See A. S. Eddington, The Expanding Universe (1933); J. H. Jeans, The Universe Around Us (4th ed. 1953); G. Gamow, The Creation of the Universe (1961); F. Hoyle, Man in the Universe (1966); L. B. Young, ed., Exploring the Universe (2d ed. 1971). The Columbia Encyclopedia, Sixth Edition Copyright© 2004, Columbia University Press. Licensed from Lernout & Hauspie Speech Products N.V. All rights reserved. | 
