The impact of lepton asymmetry and sterile neutrinos on the Primordial Nucleosynthesis

In this paper we have determined the relation between the number of additional relativistic degrees of freedom, $\Delta N_{\mathrm{eff}}$, and the lepton asymmetry parameter of neutrinos, $\xi$, at the epoch of Primordial Nucleosynthesis, at which the theoretically predicted value of the primordial $^4$He abundance, $Y_p$, agrees with the value determined from observations. It is shown that the presence of light fully or partially thermalized sterile neutrinos will not contradict the observational data on the Primordial Nucleosynthesis at the value $\xi\approx$ 0.05, which can be generated in the early Universe in the framework of the existing models of leptogenesis.