The microwave background radiation

The microwave background radiation is a remnant of the early Universe. It provides us with information on physical processes and (limits on) initial conditions that have determined the formation of the galaxies and clusters of galaxies that one sees today. Its spectrum matches that of the radiation from a blackbody at T -- 2.75 K with no statistically significant distortions measured at any frequency, which is strong evidence of a hot Big Bang model for the evolution of the Universe. It is highly isotropic: ΔT/T<10/sup -4/ over all angular scales except for a dipole component which can be attributed to motion with respect to the preferred reference frame in which the radiation would appear isotropic. This lack of anisotropies provides important constraints on key cosmological parameters such as Hubble's constant and the density parameter for the different theoretical scenarios for the Universe and its contents. A recent (small-scale) anisotrophy is produced as the radiation interacts with the ionized gas in dense clusters of galaxies; this is the Sunyaev-Zel'dovich effect which might eventually become an important cosmological tool. These lectures discuss briefly these topics but are not a review of the subject. Instead, the author concentrates mainly on recent measurements by the Princeton group, with special emphasis on the experimental difficulties involved and future (likely) improvements