In this chapter we will describe Big Bang Nucleosynthesis (BBN), the combination of fundamental protons and neutrons into nuclei, as well as calculate more precisely the temperature of recombination.

We include the coupling of a heavy sterile neutrino with active neutrinos in the calculation of primordial abundances of light-nuclei. We calculate neutrino distribution functions and primordial abundances, as functions ...

We study the effects of adding a sterile neutrino to three active neutrinos (3+ 1 scheme) in the calculation of primordial abundances. Taking the normalization constant (a) of the occupation factor of the sterile neutrino ...