One of the strongest pieces of evidence in support of the Standard Hot Big Bang Model is the 2.7 degree Kelvin radiation seen in all directions, discovered by Arno Penzias and Robert Wilson in 1965.
Using a special "horn" radio antenna, Penzias and Wilson found an unidentified source of radio emission that seemed to be the same in all directions. When they measured its spectrum, they discovered that it perfectly fit the Planck curve (the variation in brightness with wavelength) from a thermal blackbody source with a temperature of 2.7 degrees Kelvin.
At about the same time, Robert Dicke was proposing to search for the thermal remnant of the Big Bang predicted by George Gamow to resemble a blackbody of less than 10 degrees Kelvin. When Penzias and Wilson learned of Dicke's proposition, they realized that they had already made the discovery.
The 1978 Nobel Prize in Physics was partly awarded to Penzias and Wilson for their discovery.Check out some links to further discussion:
The Cosmic Background Explorer is a satellite that measured the spectrum of the CMB very precisely and provided important clues that on some scales the background radiation is not perfectly isotropic during its mapping mission in the 1990s. It was specifically designed to look for variations in the primordial background radiation.
The Wilkinson Microwave Anisotropy Probe WMAP is the current NASA mission designed to study the CMB. It was launched in 2001 and has achieved many exiting results, measuring the values of the cosmological parameters with great precision.
Wayne Hu maintains a detailed CBM page.
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