  The Cosmic Microwave Background Radiation (CMBR) is the radiation left over
from the hot Big Bang. Its blackbody spectrum and small anisotropy provide
clues about the origin and early evolution of the Universe. In particular, the
spectrum of the CMBR rules out many non-gravitational models of structure
formation, and the anisotropy of the CMBR provides a measure of the
gravitational potential at the time of last scattering, about 300,000 years
after the Big Bang. The density inhomogeneities needed to produce the
gravitational potential perturbations traced by the CMBR have grown to become
the galaxies, clusters of galaxies, and superclusters that we see today.
