field equation for dust with density , cosmological constant , and radius of curvature . It is the only non-trivial static solution to Friedmann's equations. Because the Einstein universe soon was recognized to be inherently unstable, it was presently abandoned as a viable model for the universe. It is unstable in the sense that any slight change in either the value of the cosmological constant, the matter density, or the spatial curvature will result in a universe that either expands and accelerates forever or re-collapses to a singularity. After Einstein renounced his cosmological constant, and embraced the Friedmann-LeMaitre model of an expanding universe, most physicists of the twentieth century assumed that the cosmological constant is zero. If so (absent some other form of dark energy), the expansion of the universe would be decelerating. However, after Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess introduced the theory of an accelerating universe during 1998, a positive cosmological constant has been revived as a simple explanation for dark energy. In 1976 Irving Segal revived the static universe in his chronometric cosmology. Similar to Zwicky, he ascribed the red shift of distant galaxies to curvature in the cosmos. Though he claimed vindication in astronomic data, others find the results to be inconclusive. Requirements of a static infinite model In order for a static infinite universe model to be viable, it must explain three things: First, it must explain the intergalactic redshift. Second, it must explain the cosmic microwave background radiation. Third, it must have a mechanism to re-create matter (particularly hydrogen atoms) from radiation or other sources in order to avoid a gradual 'running down' of the universe due to the conversion of matter into energy in stellar processes. With the absence of such a mechanism, the universe would consist of dead objects such as black holes and black dwarfs. See also Milne model Steady State theory Plasma cosmology References In George Gamow's autobiography, My World Line (1970), he says of Einstein: "Much later, when I was discussing cosmological problems with Einstein, he remarked that the introduction of the cosmological term was the biggest blunder of his life." Physical cosmology Exact solutions in general relativity Universe Obsolete theories in physics