Lepton number and expansion of the Universe

We study in a quantitative manner the lepton asymmetry L (net leptons number per photon) in the Universe and its impact on Universe expansion. This is quantified via the standard tool, the effective number of Dirac neutrinos N_ν^eff . Specifically, we present the dependence of N_ν^eff on the ambient temperature and the neutrino chemical potential and show the resultant neutrino yields in the cosmic background. We then obtain the dependence of |L| and the baryon-number to lepton-number ratio, B/|L|, on N_ν^eff . Our results suggest that a value |L| = 0.46 ± 0.06 and B ' (1.33 ± 0.18) × 10⁻⁹|L| may be considered.