We discuss the interplay between lepton asymmetry L and neutrino oscillations in the early Universe. Neutrino oscillations may suppress or enhance previously existing L. On the other hand L is capable to suppress or enhance neutrino oscillations. The mechanism of L enhancement in MSW resonant neutrino oscillations in the early Universe is numerically analyzed. L cosmological effects through neutrino oscillations are discussed. We discuss how L may change the cosmological BBN constraints on neutrino and show that BBN model with electron-sterile neutrino oscillations is extremely sensitive to L - it allows to obtain the most stringent constraints on L value. We discuss also the cosmological role of active-sterile neutrino mixing and L in connection with the indications about additional relativistic density in the early Universe, pointed out by BBN, CMB and LSS data and the analysis of global neutrino data.
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