Einstein tweaked his own equations to stop the universe from moving
In 1917 Einstein added a cosmological constant to general relativity for one reason: to keep the universe static, as he believed it was. Within about a decade a Belgian priest proposed a primeval atom and Hubble measured galaxies racing away. The universe was expanding, and modern cosmology was born out of the argument.
Physical cosmology uses the laws of physics to model the universe as a whole: its origin, structure, evolution and fate. Its deep roots lie in the Copernican idea that celestial objects obey the same laws as earthly ones, and in Newton's mechanics. The modern field began with general relativity, which treats gravity as the geometry of space and time. Einstein's original equations would not allow a static cosmos, since matter pulls on matter, so he inserted a constant to balance gravity. The resulting model turned out to be unstable, and Alexander Friedmann showed in the early 1920s that relativity also permits universes that expand or contract.
Observation caught up quickly. In the 1910s Vesto Slipher read the redshifts of spiral nebulae as a sign they were receding, but nobody yet knew how far away they were. In 1927 Georges Lemaître, a Roman Catholic priest, proposed that the universe began from a primeval atom, the idea later called the Big Bang. In 1929 Edwin Hubble used Cepheid variable stars to show the nebulae were distant galaxies and found that the farther away they were, the faster they receded, although his numerical factor was off by about ten.
Two explanations competed for years: the Big Bang, championed by George Gamow, and Fred Hoyle's steady state model, in which new matter appears as galaxies separate. The discovery of the cosmic microwave background in 1965 tipped the balance, and after the Cosmic Background Explorer measured it precisely in the early 1990s, few cosmologists seriously backed alternatives. The Big Bang forged hydrogen and helium; stars later built heavier nuclei up to iron and nickel.
Today's standard picture, the Lambda-CDM model, dates the universe to around 13.8 billion years and fits a wide range of data, from distant supernovae to galaxy surveys. Yet it leans on dark matter and dark energy, neither of which is well understood; one hypothesis treats dark energy as the energy of empty space. Even energy conservation is contested, since light stretched by cosmic expansion seems to lose energy that goes nowhere.
Source: Physical cosmology
