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### Timeline of cosmology

* 1576 - Thomas Digges modifies the Copernican system by removing its outer edge and replacing the edge with a star-filled unbounded space * 1610 - Johannes Kepler uses the dark night sky to argue for a finite universe * 1720 - Edmund Halley puts forth an early form of Olbers' paradox * 1744 - Jean-Phillipe de Cheseaux puts forth an early form of Olbers' paradox * 1826 - Heinrich Olbers puts forth Olbers' paradox * 1917 - Willem de Sitter derives an isotropic static cosmology with a cosmological constant as well as an empty expanding cosmology with a cosmological constant * 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 Lema”tre 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 * 1933 - Edward Milne names and formalizes the cosmological principle * 1934 - Georges Lema”tre interprets the cosmological constant as due to a vacuum energy with an unusual perfect fluid equation of state * 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 * 1948 - Hermann Bondi, Thomas Gold, and Fred Hoyle propose steady state cosmologies based on the perfect cosmological principle * 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 * 1966 - Stephen Hawking and George Ellis show that any plausible general relativistic cosmology is singular * 1966 - Jim Peebles shows that the hot Big Bang predicts the correct helium abundance * 1967 - Andrei Sakharov presents the requirements for a baryon-antibaryon asymmetry in the universe * 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 * 1973 - Edward Tryon proposes that the universe may be a large scale quantum mechanical vacuum fluctuation where positive mass-energy is balanced by negative gravitational potential energy * 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 inflationary 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 inflationary theory is correct.