The cosmic microwave background: from the spectrum to the anisotropies

The spectrum and the temperature anisotropies of the cosmic microwave backgound (CMB) are both essential, quantitative tools for cosmology. The former contains a record of the thermal history of the Universe back to a temperature T similar to1 keV. In this context, I discuss the process of thermalization, i.e., the establishment of a Planck spectrum, as well as the Compton and mu -distortions. The fundamental ingredients necessary for understanding the angular power spectrum of the temperature anisotropies are then presented. The first generation anisotropy data is shown to be consistent with the basic expectations of Inflation: a flat Universe with scale-invariant perturbations. This data set also prefers a rather large baryon density. The recent BOOMERanG and MAXIMA-1 data confirm these first indications with higher precision. (C) 2001 Elsevier Science B.V. All rights reserved.