cosmic microwave background

cosmic microwave background (CMB) A faint, diffuse glow coming from all directions in the sky, most intense at a wavelength of around 1 mm; also known as the microwave background radiation or cosmic background radiation. Its existence was predicted from the Big Bang theory, and it was discovered in 1965 by A. A.Penzias and R. W.Wilson. The CMB originated when the Universe was 300 000 years old and consisted of a plasma at a temperature of approximately 3000 K. The expansion of the Universe since then has redshifted the CMB photons to an apparent temperature of only about 3 K. Measurements by the Cosmic Background Explorer satellite (COBE) showed that the CMB has a black-body spectrum with a temperature of 2.73 K. COBE detected random irregularities in temperature of the order of 1 part in 10⁵ (10 μK). The CMB is regarded as one of the three most important pieces of evidence for the Big Bang, the others being the recession of the galaxies and the cosmic abundance of helium.