We reexamine neutrino oscillations in the early universe. Contrary to previous studies, we show that large neutrino asymmetries can arise due to oscillations between ordinary neutrinos and sterile neutrinos. This means that the big bang nucleosynthesis (BBN) bounds on the mass and mixing of ordinary neutrinos with sterile neutrinos can be evaded. Also, it is possible that the neutrino asymmetries can be large enough (≳10%) to have a significant effect on BBN through nuclear reaction rates.

• Received 14 September 1995

DOI:https://doi.org/10.1103/PhysRevD.53.R5349