1791    Erasmus Darwin (Charles Darwin's grandfather) provides the first description in western science for a universe which expands and contracts in a cyclical manner.

1905   Albert Einstein proposes his Special Theory of Relativity which provided scientific and mathematical evidence for the relationship between space and time.

1917   Albert Einstein publishes his final revisions of his General Theory of Relativity.  This theory unifies the Special Theory of Relativity with Newton's law of gravitation.  The modern field of cosmology is generally considered to have begun with this publication.

1922   Russian mathematician Friedmann publishes a set of equations (later known as the Friedmann equations) that explain the expansion of the universe from a central point within the context of General Relativity.

1927   The Big Bang Theory is first proposed by Belgian priest Geroges Lamitre.  The theory states that all matter in the universe originated from the explosion of an extremely small, dense body of matter at the center of our universe.

1929   Edwin Hubble discovers that the redshift observed in light originating from a distant galaxy increases in proportion to its distance.  This discovery provided observational evidence for the Big Bang Theory.

1927   Swiss astrophysicist Fritz Zwicky is the first to infer the existence of unseen matter in space, known as dark matter.

1948   George Gamow predicts the existence of cosmic background radiation by considering the behavior of primordial radiation in an expanding universe.

1964   Astronomers Robert Wilson and Arno Penzias discover cosmic background microwave radiation using a radio telescope.

1966   James Peebles shows that the hot Big Bang predicts the correct level of helium abundance in the universe.

1981   Alan Guth proposes the Theory of Cosmic Inflation.

1998   Observations of Type Ia supernovae by astronomers at the Lawrence Berkeley National Laboratory provide evidence that the expansion of the universe is speeding up.

2003   NASA's Wilkinson Microwave Anisotropy Probe (WMAP) measures the temperature of the radiant heat left over from the Big Bang.  Measurements predict an age of 13.7 years for our universe.