Anisotropies of the temperature of the Cosmic Background Radiation (CBR) give us unique information on the universe at redshifts of about 1000. By using observational limits on these anisotropies we can constrain the parameters of cosmological models and the spectrum and amplitude of the initial perturbations. Additional information on the thermal history of the universe is provided by distortions of the CBR spectrum. In this paper we give an overview of physical processes leading to anisotropies in popular cosmological models.

Introduction

The remarkable isotropy of the CBR in a universe containing so many structures is one of the most fascinating cosmological observations. Strong limits put on possible anisotropies of the CBR let us impose important constraints on models of global structure of the universe, and on models of galaxy and cluster formation.

The difference between the CBR and all the other radiations investigated in astronomy is that the former existed from the beginning of the universe, and the latter were created only after first objects were formed. Also, the CBR has now a 2.74K blackbody spectrum, and the radiation of astronomical objects is usually not blackbody.

Following the discovery of the CBR (Penzias and Wilson (1965)), progress in observations has been rapid. Currently, anisotropies on the level of 10−4 – 10−5 are detectable on a wide range of angular scales and frequencies. (Observations of anisotropy are usually made by comparing intensities of radiation detected by two antennas, separated by an angle θ, the angular scale.