During 1917, Albert Einstein added a positive
cosmological constant to his
equations of
general relativity to counteract the attractive effects of
gravity on ordinary matter, which would otherwise cause a static, spatially finite universe to either
collapse or
expand forever. This model of the universe became known as the Einstein World or
Einstein's static universe. This motivation ended after the proposal by the astrophysicist and Roman Catholic priest
Georges Lemaître that the universe seems to be not static, but expanding.
Edwin Hubble had researched data from the observations made by astronomer
Vesto Slipher to confirm a relationship between
redshift and distance, which forms the basis for the modern
expansion paradigm that was introduced by Lemaître. According to
George Gamow this caused Einstein to declare this cosmological model, and especially the introduction of the cosmological constant, his "biggest blunder". Einstein's static universe is
closed (i.e. has hyperspherical topology and positive spatial curvature), and contains uniform dust and a positive
cosmological constant with value precisely \Lambda_E = 4\pi G\rho/c^2, where G is Newtonian gravitational constant, \rho is the energy density of the matter in the universe and c is the
speed of light. The
radius of curvature of space of the Einstein universe is equal to :R_E = \Lambda_E^{-1/2} = {c \over \sqrt{4\pi G\rho}}. The Einstein universe is one of
Friedmann's solutions to Einstein's field equation for dust with density \rho, cosmological constant \Lambda_E, and radius of curvature R_E. 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==