What is the Cosmic Microwave Background (CMB)?
The CMB is a relic radiation left over from the Big Bang; it is the signature of a once young, hot, dense Universe. The CMB was discovered in 1965 as excess noise in a microwave receiver. The CMB formed when the Universe was about 300,000 years old, long before the first stars and galaxies formed. Superimposed on a nearly uniform background are tiny variations of about 10 parts per million. These variations in the density of the early Universe look like hot and cold spots through a microwave telescope. These small variations eventually grew into galaxies and clusters of galaxies. The variations in the CMB were first discovered by the COBE satellite in 1991 and we have been conducting experiments to measure it with greater precision ever since.
