Cosmic background radiation
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Cosmic background radiation is electromagnetic radiation from the sky with no discernable source. The origin of this radiation depends on the region of the spectrum that we are observing. One component is the cosmic microwave background radiation. This component is redshifted photons that have freely streamed from an epoch when the Universe became transparent for the first time to radiation. Its discovery and detail observations of its properties are considered one of the major confirmations of the Big Bang.
The Sunyaev–Zel'dovich effect shows the phenomena of radiant cosmic background radiation interacting with "electron" clouds distorting the spectrum of the radiation.
There is also background radiation in the infrared, x-rays, etc., with different causes, and they can sometimes be resolved into an individual source. See cosmic infrared background and X-ray background. See also cosmic neutrino background and extragalactic background light.
History of significant events 
1931: The term microwave first appears in print: ""When trials with wavelengths as low as 18 cm were made known, there was undisguised surprise that the problem of the micro-wave had been solved so soon." Telegraph & Telephone Journal XVII. 179/1"
1946: Robert Dicke predicts a microwave background radiation temperature of 20 K (ref: Helge Kragh)
1946: Robert Dicke predicts a microwave background radiation temperature of "less that 20 K" but later revised to 45 K (ref: Stephen G. Brush)
1946: George Gamow estimates a temperature of 50 K
1949: Ralph Alpher and Robert Herman re-re-estimate Gamow's estimate at 28 K.
1960s: Robert Dicke re-estimates a MBR (microwave background radiation) temperature of 40 K (ref: Helge Kragh)
1960s: Arno Penzias and Robert Woodrow Wilson measure the temperature to be approximately 3 K. Robert Dicke, P. J. E. Peebles, P. G. Roll and D. T. Wilkinson interpret this radiation as a signature of the Big Bang.