Introduction: the Cosmic Microwave Background radiation

The Cosmic Microwave Background (CMB hereafter) is the oldest observable electromagnetic radiation of our Universe. It was emitted about 300000 years after the Big Bang (z 1#1 1000), when the Universe became transparent to photons. The main cause for this radiation is the combination of hydrogen atoms at this time from the proton and electron plasma. Thus the light emitted was hot (1#1 3000 K) but became gradually colder with the expansion of the Universe, until being observed nowadays in the millimeter domain (2.73 K). This radiation was discovered in 1965 by Penzias & Wilson in radiowaves. The CMB is a very isotropic blackbody emission, but exhibits small fluctuations (2#2 relative), whose large angular scales were observed by the COBE satellite (Smoot et al. 1992). The primordial quantum fluctuations were amplified by physical processes in the cosmic soup at that time, such as acoustic wave phenomena. These processes, and the way the information came to us today, are highly dependent on the cosmological parameters of the Universe, so that the CMB map aspect and the CMB power spectrum shape can provide powerful informations about the characteristics, history and fate of our Universe. These fluctuations are also the origin of the large-scale structure of the Universe (cluster and galaxy formation, by gravitational collapse).