ABSTRACT
This chapter discusses how within Big Bang cosmological models it is possible to explain in a simple way the existence of the cosmic microwave background (CMB) radiation and its thermal equilibrium spectrum. It also discusses how from the study of CMB temperature anisotropies it is possible to extract a stunningly rich amount of information on the cosmological parameters. The observed CMB thermal spectrum can be elegantly explained by an early hot stage when radiation and matter where coupled together and the conditions for thermal equilibrium were satisfied. the CMB thermal equilibrium is a strong indication that an early hot stage, when matter and radiation were coupled, occurred in the past. The chapter discusses the main features of the observed CMB temperature anisotropies and how in recent years, with the discovery of the acoustic peaks in the power spectrum of primary anisotropies, it became possible to extract precise values of the cosmological parameters.
