The energy content in radiation from beyond our Galaxy is dominated by the Cosmic Microwave Background (CMB), discovered in 1965 . The spectrum of the CMB is well described by a blackbody function with T = 2.725 K. This spectral form is one of the main pillars of the hot Big Bang model for the early Universe. The lack of any observed deviations from a blackbody spectrum constrains physical processes over the history of the universe at redshifts z 107 (see previous versions of this mini-review ). However, at the moment, all viable cosmological models predict a very nearly Planckian spectrum, and so are not stringently limited.
Another observable quantity inherent in the CMB is the variation in temperature (or intensity) from one part of the microwave sky to another . Since the first detection of these anisotropies by the COBE satellite , there has been intense activity to map the sky at increasing levels of sensitivity and angular resolution. A series of ground- and balloon-based measurements has recently been joined by the first results from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) . These observations have led to a stunning confirmation of the `Standard Model of Cosmology.' In combination with other astrophysical data, the CMB anisotropy measurements place quite precise constraints on a number of cosmological parameters, and have launched us into an era of precision cosmology.