The angular distribution of the cosmic background radiation

The cosmic background radiation is thermal radiation with a temperature of 2.75 K still present throughout the Universe, a relic of its hot, big bang, initial phase. Detailed studies of this radiation, and particularly its angular variations, can provide information obtainable in no other way about the global geometry and expansion of the Universe, about the distribution of mass near our Galaxy, and about the process of galaxy formation. The present review treats searches for such angular variations and the consequences of the resulting measurements of, or upper limits on, variations in the cosmic background radiation.