The Nobel Prize in Physics 2006 has been awarded to John C. Mather and George F. Smoot, "for their discovery of the blackbody form and anisotropy of the cosmic microwave background radiation".
The cosmic microwave background radiation was famously found by accident. In attempting to calibrate a 20-foot horn antenna at radio frequencies, Arno Penzias and Robert Wilson1 stumbled across three kelvins' worth of excess noise for which they could find no source. It was the signature of the Big Bang.
At this point in the early 1960s, there were competing theories for the nature of the Universe: had it begun in a hot Big Bang, or did it exist in a more benign steady state? The signal found by Penzias and Wilson (for which they won the Nobel Prize in 1978) was firm evidence in favour of cataclysm: about 380,000 years after the Big Bang, photons would have eventually cooled sufficiently to 'decouple' from matter and move unimpeded; as the Universe further cooled and expanded, those photons would exist today as an all-pervasive black-body radiation, with a temperature of 2.7 K.
Penzias and Wilson had shown that the relic radiation existed, but did it have the expected black-body spectrum? Furthermore, was it anisotropic? Certainly, given the length of time since its creation, the cosmic microwave background (CMB) would be reasonably smooth across the sky. But the large-scale structure in the Universe – the clumping into galaxies and clusters of galaxies – suggests there should have been at least some small-scale fluctuations in the density of the early Universe. Could their imprint be found in the CMB?
The answers came with the launch of the COBE satellite in 1989. Championed by principal investigator John Mather, following a 1974 proposal to NASA, COBE carried three instruments for detailed study of the CMB, including the Differential Microwave Radiometer, for which George Smoot was the principal investigator. Almost immediately, data from COBE proved that the CMB has a black-body spectrum2, with a temperature of 2.725 K. And in 1992, the existence of anisotropies was also revealed3: the temperature of the CMB varies across the sky, but at the level of only 10-5 K.
The COBE results have since been confirmed by the balloon-borne experiments, MAXIMA and BOOMERANG, and by data4 from WMAP, the Wilkinson Microwave Anisotropy Probe.
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Nature Chemical Biology (2006)