Constraining the cosmic radiation density due to lepton number

Abstract

The cosmic energy density in the form of radiation before and during Big Bang Nucleosynthesis is typically parameterized in terms of the effective number of neutrinos Neff, and it is a key parameters in cosmological models slightly more general than the successful minimal ΛCDM scenario. This quantity, in case of no extra degrees of freedom, depends upon the chemical potential and the temperature characterizing the three active neutrino distributions, as well as by their possible non-thermal features. We summarize here the results of a recent analysis to determine the BBN bound on Neff from primordial neutrino–antineutrino asymmetries, with a careful treatment of the dynamics of neutrino oscillations, and considering quite a wide range for the total lepton number in the neutrino sector, ην=ηνe+ηνμ+ηντ and the initial electron neutrino asymmetry ηνein. Comparing these results with the forthcoming measurement of Neff by the Planck satellite will give insight on the nature of the radiation content of the universe.