Einstein's static universe
Einstein's static universe, also known as the Einstein universe or the Einstein world, is a relativistic model of the universe proposed by Albert Einstein in 1917. Shortly after completing the general theory of relativity, Einstein applied his new theory of gravity to the universe as a whole. Assuming a universe that was static in time, and possessed of a uniform distribution of matter on the largest scales, Einstein was led to a finite, static universe of spherical spatial curvature.
In order to achieve a consistent solution to the Einstein field equations for the case of a static universe with a non-zero density of matter, Einstein found it necessary to introduce a new term to the field equations, the cosmological constant. In the resulting model, the radius R and density of matter ρ of the universe were related to the cosmological constant λ according to λ = 1/R2 = κρ/2 where κ is the Einstein constant.
Following the discovery by Edwin Hubble of a linear relation between the redshifts of the galaxies and their distance in 1929, Einstein abandoned his static model of the universe and proposed expanding models such as the Friedmann-Einstein universe and the Einstein-de Sitter universe. In both cases, he set the cosmological constant to zero, declaring it "no longer necessary ... and theoretically unsatisfactory".
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