The Cosmic Microwave Background Radiation is a form of electromagnetic radiation that fills the whole of the universe. It has the characteristics of black body radiation of a temperature of 2.726 kelvin. It has a frequency in the microwave range.
This radiation is regarded as the best available evidence of the Big Bang theory -- it gives a snapshot of the Universe when the temperature dropped enough to allow electrons and protons to form hydrogen atoms, thus making the universe transparent to radiation. When it originated some 300,000 years after the Big Bang -- this point in time is generally known as the "last scattering surface" -- the temperature of the Universe was about 6000 K. Since then it has dropped because of the expansion of the Universe, which cools radiation inversely proportional to the fourth power of the Universe's scale length[?].
One of the microwave background's most salient features is a high degree of isotropy. There are some anisotropies, the most pronounced of which is the dipole anisotropy at a level of about 10-4 at a scale of 180 degrees of arc. It is due to the motion of the observer against the CBR, which is some 700 km/s for the Earth.
Much smaller variations due to external physics also exist; the Sunyaev-Zel'dovic-Effect[?] is one of the major factors here.
Even more interesting are anisotropies at a level of roughly 1/100000 and on a scale of a few arcminutes. Those very small variations correspond to the density fluctuations at the last scattering surface and give valuable information about the seeds for the large scale structures we observe now. These small-scale variations give observational constraints on the properties of universe, and are therefore one important test for cosmological models.
The CBR was predicted by George Gamow, Ralph Alpher, and Robert Hermann in the 1940s and was accidentally discovered in the 1950s by Penzias and Wilson, who received a Nobel Prize for this discovery. Since the cosmic microwave radiation is rather difficult to observe with ground-based instruments, CMB research makes increasing use of air and space-borne experiments. Probably still the most famous of these is the Cosmic Background Explorer (COBE) satellite that was flown in 1989-1996, which made the first detection of anisotropies (other than the dipole). However, the most detailed measurements to date have been made using instruments on high flying unmanned balloons. Unlike COBE, these instruments only observed small sections of the sky. In June 2001, NASA launched a second CBR space mission, MAP, to make detailed measurements of the anisotropies over the full sky. Preliminary results are expected by the end of 2002. A third, space mission, Planck, is to be launched in 2007. Unlike the previous two space missions, Planck is a collaboration between NASA and ESA (the European Space Agency).