1934 - Richard Tolman shows that blackbody radiation in an expanding universe cools but remains thermal
1941 - Andrew McKellar uses the excitation of CN doublet lines to measure that the "effective temperature of space" is about 2.3 K
1948 - George Gamow, Ralph Alpher, and Robert Herman predict that a Big Bang universe will have a blackbody cosmic microwave background with temperature about 5 K
1955 - Tigran Shmaonov finds excess microwave emission with a temperature of roughly 3 K
1964 - A. G. Doroshkevich and Igor Dmitrievich Novikov write an unnoticed paper suggesting microwave searches for the blackbody radiation predicted by Gamow, Alpher, and Herman
1965 - Arno Penzias, Robert Wilson, Bernie Burke, Robert Dicke, and James Peebles discover the cosmic microwave background radiation
1966 - Rainer Sachs and Arthur Wolfe theoretically predict microwave background fluctuation amplitudes created by gravitational potential variations between observers and the last scattering surface (see Integrated Sachs Wolfe effect)
1968 - Martin Rees and Dennis Sciama theoretically predict microwave background fluctuation amplitudes created by photons traversing time-dependent potential wells
1969 - R. A. Sunyaev and Yakov Zel'dovich study the inverse Compton scattering of microwave background photons by hot electrons
1990 - The Cosmic Background Explorer (COBE) satellite shows that the microwave background has a nearly perfect blackbody spectrum and thereby strongly constrains the density of the intergalactic medium
1992 - The COBE satellite discovers anisotropy in the cosmic microwave background
2003 - the WMAP satellite produces a high resolution map of the cosmic microwave background.