A cosmic background of radiation in the frequency range 3 × 1011 hertz to 3 × 108 hertz discovered in 1965. Believed to have emanated from the primordial fireball of the big bang with which the universe is thought to have originated (see Big-bang Theory (Feature)), the radiation has an energy density in intergalactic space of some 4 × 10−14 J m−3. The temperature of the CMB is about 2.7 K. Detailed study of this radiation has been very important in modern cosmology. See COBE; WMAP.