1917 - Willem de Sitter derives an isotropic static cosmology with a cosmological constant as well as an empty expanding cosmology with a cosmological constant, termed a de Sitter universe
1922 - Vesto Slipher summarizes his findings on the spiral nebulae's systematic redshifts
1922 - Alexander Friedmann finds a solution to the Einstein field equations which suggests a general expansion of space
1927 - Georges Lemaître discusses the creation event of an expanding universe governed by the Einstein field equations
1928 - Harold Robertson briefly mentions that Vesto Slipher's redshift measurements combined with brightness measurements of the same galaxies indicate a redshift-distance relation
1929 - Edwin Hubble demonstrates the linear redshift-distance relation and thus shows the expansion of the universe
1938 - Paul Dirac presents a cosmological theory where the gravitational constant decreases slowly so that the age of the universe divided by the atomic light-crossing time always equals the ratio of the electric force to the gravitational force between a proton and electron
1948 - Ralph Alpher, Hans Bethe("in absentia"), and George Gamow examine element synthesis in a rapidly expanding and cooling universe and suggest that the elements were produced by rapid neutron capture
1951 - William McCrea shows that the steady state C-field can be accommodated within general relativity by interpreting it as a contribution to the energy-momentum tensor with an unusual equation of state
1961 - Robert Dicke argues that carbon-based life can only arise when the Dirac large numbers hypothesis is true because this is when burning stars exist; first use of the weak anthropic principle
1963 - Fred Hoyle and Jayant Narlikar show that the steady state theory can explain the isotropy of the universe because deviations from isotropy and homogeneity exponentially decay in time
1964 - Fred Hoyle and Roger Tayler point out that the primordial helium abundance depends on the number of neutrinos
1965 - Martin Rees and Dennis Sciama analyze quasar source count data and discover that the quasar density increases with redshift
1965 - Edward Harrison resolves Olbers' paradox by noting the finite lifetime of stars
1967 - John Bahcall, Wal Sargent, and Maarten Schmidt measure the fine-structure splitting of spectral lines in 3C191 and thereby show that the fine-structure constant does not vary significantly with time
1968 - Brandon Carter speculates that perhaps the fundamental constants of nature must lie within a restricted range to allow the emergence of life; first use of the strong anthropic principle
1969 - Charles Misner formally presents the Big Bang horizon problem
1969 - Robert Dicke formally presents the Big Bang flatness problem
1974 - Robert Wagoner, William Fowler, and Fred Hoyle show that the hot Big Bang predicts the correct deuterium and lithium abundances
1976 - A.I. Shlyakhter uses samarium ratios from the prehistoric natural fission reactor in Gabon to show that some laws of physics have remained unchanged for over two billion years
1977 - Gary Steigman, David Schramm, and James Gunn examine the relation between the primordial helium abundance and number of neutrinos and claim that at most five lepton families can exist
1980 - Alan Guth proposes the inflationaryary Big Bang universe as a possible solution to the horizon and flatness problems
2003 - NASA's WMAP takes first detailed "baby picture" of the universe. The image reveals the universe is 13.7 billion years old (within one percent error) and that the inflationaryary theory is correct.
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