Editing Albert Einstein (section)

==== Physical cosmology ====
{{Main|Physical cosmology}}
[[File:MillikanLemaitreEinstein.jpg|thumb|right|[[Robert Andrews Millikan|Robert A. Millikan]], [[Georges Lemaître]] and Einstein at the [[California Institute of Technology]] in January 1933]]
In 1917, Einstein applied the general theory of relativity to the structure of the universe as a whole.<ref>Einstein (1917a).</ref> He discovered that the general field equations predicted a universe that was dynamic, either contracting or expanding. As observational evidence for a dynamic universe was lacking at the time, Einstein introduced a new term, the [[cosmological constant]], into the field equations, in order to allow the theory to predict a static universe. The modified field equations predicted a static universe of closed curvature, in accordance with Einstein's understanding of [[Mach's principle]] in these years. This model became known as the Einstein World or [[Einstein's static universe]].{{Sfnp|Pais|1994|pp=285–286}}<ref name="iJwuX"/>

Following the discovery of the recession of the galaxies by [[Edwin Hubble]] in 1929, Einstein abandoned his static model of the universe, and proposed two dynamic models of the cosmos, the [[Friedmann–Einstein universe]] of 1931<ref name=E1931>Einstein (1931).</ref><ref name="cor-2013"/> and the [[Einstein–de Sitter universe]] of 1932.<ref>Einstein & de Sitter (1932).</ref><ref name="J9Tqu"/> In each of these models, Einstein discarded the cosmological constant, claiming that it was "in any case theoretically unsatisfactory".<ref name=E1931/><ref name="cor-2013"/><ref name="sxfvo"/>

In many Einstein biographies, it is claimed that Einstein referred to the cosmological constant in later years as his "biggest blunder", based on a letter [[George Gamow]] claimed to have received from him. The astrophysicist [[Mario Livio]] has cast doubt on this claim.<ref name="qmmVf"/>

In late 2013, a team led by the Irish physicist [[Cormac O'Raifeartaigh]] discovered evidence that, shortly after learning of Hubble's observations of the recession of the galaxies, Einstein considered a [[steady-state model]] of the universe.<ref name="Tq53z"/><ref name="8pfEk"/> In a hitherto overlooked manuscript, apparently written in early 1931, Einstein explored a model of the expanding universe in which the density of matter remains constant due to a continuous creation of matter, a process that he associated with the cosmological constant.<ref name="cor-steady-state" /><ref name="Einstein's aborted model"/> As he stated in the paper, {{qi|In what follows, I would like to draw attention to a solution to equation (1) that can account for Hubbel's {{sic}} facts, and in which the density is constant over time [...] If one considers a physically bounded volume, particles of matter will be continually leaving it. For the density to remain constant, new particles of matter must be continually formed in the volume from space.}}

It thus appears that Einstein considered a [[steady-state model]] of the expanding universe many years before Hoyle, Bondi and Gold.<ref name="ILjYQ"/><ref name="ThZb0"/> However, Einstein's steady-state model contained a fundamental flaw and he quickly abandoned the idea.<ref name="cor-steady-state"/><ref name="Einstein's aborted model"/><ref name="7ShC9"/>