COSMOLOGY | 260 BC | Greek astronomer Aristarchus of Samos (c.320–230 BC) proposes a sun-centred universe. |
| c.150 AD | Greek-Egyptian astronomer Ptolemy (2nd century AD) proposes an earth-centred universe. |
| 1543 | Copernicus publishes his sun-centred theory of the universe (solar system). |
| 1576 | English mathematician Thomas Digges (c.1546–95) proposes that the universe is infinite (because stars are at varying distances). |
| 1584 | Italian philosopher Giordano Bruno (1548–1600) states that the universe is infinite. |
| 1633 | Galileo champions Copernicus's sun-centred universe, but is forced by the Roman Catholic Inquisition to recant. |
| 1854 | Helmholtz predicts the heat death of the universe, based on thermo-dynamics. |
| 1917 | Einstein proposes a static universe theory. |
| 1922 | Russian astronomer Alexander Friedmann proposes the expanding universe theory. |
| 1927 | George Lemaître proposes the big-bang theory of the universe. |
| 1929 | Edwin Hubble demonstrates the expansion of the universe. |
| 1948 | US physicists George Gamow (1904–68), Ralph Alpher (1921– ), and Hans Bethe (1906– ) develop the big-bang theory, and the α-ß-γ theory of the origin of the elements; Alpher also predicts that the big bang would have produced a microwave background. British astronomers Herman Bondi (1919– ), Thomas Gold (1920– ), and Fred Hoyle propose the steady-state theory of the universe. |
| 1965 | US astrophysicists Arno Penzias (1933– ) and Robert Wilson (1936– ) discover the microwave background radiation. |
| 1980 | US physicist Allan Guth (1947– ) proposes the inflationary theory of the universe. |
| 1992 | US COBE astronomical satellite detects ripples in residual cosmic radiation (cited as evidence of the big bang). |