Cosmic Background (CMB) Radiation Arno Penzias and Robert Wilson discovered in 1965 that the Universe is full of microwave radiation, that has, to a good approximation, a black-body spectrum with an effective temperature of about 2.7 degrees above absolute zero (2.7 Kelvin). The existence of such radiation had been predicted by George Gamow on the basis of the Big Bang theory of cosmology. It is explained as relic from when the Universe had a temperature of thousands of degrees, as a remnant from the combination of nuclei and electrons to form atoms in the early Universe, when it was a few hundred thousand years old. Small fluctuations The cosmic background radiation is almost isotropic, i.e., the same in all directions. However, there is one direction in the sky where it appears to be a few thousandths of a degree hotter, and correspondingly cooler in the opposite direction. This difference is due to the motion of the Earth relative to this radiation bath, as it orbits the Sun, as the Sun orbits the centre of the Milky Way galaxy, and as the Milky Way is pulled by the gravitational attractions of other nearby galaxies and dark matter. Scientists using the COBE satellite discovered in 1992 other irregularities in the cosmic background radiation on smaller distance scales, that have since been confirmed by many other experiments. These ripples are interpreted as being due to small fluctuations in the density of matter in the Universe, that are believed to have led to the formation of galaxies and stars, probably with the aid of amplification by dark matter. Such density fluctuations had been predicted by theories of cosmological inflation. New observations Recent observations of the CMB radiation include some from Antartica and there are plans for future satellite telescopes, for instance ESA's Planck mission. A more detailed discussion of the physics of the cosmic background radiation, and how observations of it may be used to probe this theory and the formation of structure in the Universe is available at Max Tegmark's Web page.

Map of the early universe resulting from observations taken over four years by the COBE satellite