Editing Cosmological constant (section)

=== Sequence of events 1915–1998 ===
* In 1915, Einstein publishes his equations of general relativity, without a cosmological constant {{math|Λ}}.
* In 1917, Einstein adds the parameter {{math|Λ}} to his equations when he realizes that his theory implies a dynamic universe for which space is a function of time. He then gives this constant a value that makes his Universe model remain static and eternal (Einstein static universe).
* In 1922, the Russian physicist [[Alexander Friedmann]] mathematically shows that Einstein's equations (whatever {{math|Λ}}) remain valid in a dynamic universe.
* In 1927, the Belgian astrophysicist [[Georges Lemaître]] shows that the Universe is expanding by combining general relativity with astronomical observations, those of Hubble in particular.
* In 1931, Einstein accepts the theory of an expanding universe and proposes, in 1932 with the Dutch physicist and astronomer [[Willem de Sitter]], a model of a continuously expanding universe with zero cosmological constant (Einstein–de Sitter spacetime).
* In 1998, two teams of astrophysicists, the [[Supernova Cosmology Project]] and the [[High-Z Supernova Search Team]], carried out measurements on distant supernovae which showed that the speed of galaxies' recession in relation to the [[Milky Way]] increases over time. The universe is in accelerated expansion, which requires having a strictly positive {{math|Λ}}. The universe would contain a mysterious dark energy producing a repulsive force that counterbalances the gravitational braking produced by the matter contained in the universe (see ''[[Standard cosmological model]]''). {{pb}} For this work, Perlmutter, Schmidt, and Riess jointly received the [[Nobel Prize in Physics]] in 2011.