1915: Albert Einstein proposes the theory of general relativity, which equates gravity to the curvature of space-time.

1929: Edwin Hubble makes the first measurement leading to the conclusion that the universe is expanding.

1932: Richard Tolman works out the thermodynamics of an oscillating cyclic universe.

1949: Fred Hoyle coins the term "big bang."

1965: Robert Dicke, James Peebles, P.G. Roll and D.T. Wilkinson provide interpretation for the newly observed cosmic background radiation.

1969: George Ellis, Stephen Hawking and Roger Penrose derive the singularity theorems for the big bang, proving that general relativity must break down.

1981: Alan Guth proposes an early version of the inflationary universe.

1982: Andre Linde, Andreas Albrecht and Paul Steinhardt put inflation on firmer footing.

1983: Andrei Linde proposes a model for a chaotic inflationary universe.

1992: John Mather and George Smoot identify fluctuations in cosmic background radiation with COBE satellite.

1993: Gabriele Veneziano and Maurizio Gasperini introduce their "pre-big bang" cosmology based on ideas in string theory.

1996: Petr Horava and Ed Witten construct an 11-dimensional string theory.

1998: Saul Perlmutter and colleagues make supernovae observations indicating that the expansion of the universe is accelerating.

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