# Timeline of cosmology

**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-Philippe de Cheseaux puts forth an early form of Olbers' paradox
- 1826 - Heinrich Wilhelm 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, 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 - Howard Percy 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 Lemaître 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 inflationaryary Big Bang universe as a possible solution to the horizon and flatness problems
- 2001 - Evidence that the fine structure constant does vary over the lifetime of the universe becomes conclusive.
- 2003 - NASA's WMAP takes first detailed "baby picture" of the universe by means of the Cosmic microwave background radiation. The image reveals the universe is 13.7 billion years old (within one percent error) and that the inflationaryary theory is correct.