In 1917 Albert Einstein proposed a model of the universe based on his new theory of general relativity. He considered time as a fourth dimension and he showed that gravitation was equivalent to a curvature of the resulting four-dimensional space-time. 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" into his equations, resulting in a static universe. He therefore missed a chance to predict the expansion of the universe. 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 "primaeval 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 .
The fate of the Friedmann universe is determined by the average density of
matter in the universe. If there is relatively little matter, 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.
"Cosmology", Microsoft® Encarta® Encyclopedia 2001. © 1993-2000 Microsoft Corporation. All rights reserved.