THE COSMIC MICROWAVE BACKGROUND

As explained in Chapter 1, the dynamical evolution of the universe is characterized by an expansion which results in a decrease of the temperature of the cosmic constituents. Below T ⋍ 1 MeV, the cosmic gas consists of photons and electron and baryon traces which are in thermal equilibrium with the radiation until the temperature reaches a value T_R ⋍ 4000 K, and decoupled neutrinos at “temperature” T_v. When T < T_R, electrons and nuclei recombine forming neutral atoms. Then matter becomes transparent to radiation and the thermal contact between these components is lost…