Update on neutrino mixing in the early universe

From the current cosmological observations of cosmic microwave background (CMB) and nuclear abundances, we show, using an analytic procedure, that the total effective number of extra neutrino species $\Delta N_{\nu }^{\mathrm{tot}}<\mathrm{0.3.}\mathrm{}$ We also describe the possible signatures of nonstandard effects that could be revealed in future CMB observations. This cosmological information is then applied to neutrino mixing models. Taking into account the recent results from the Sudbury Neutrino Observatory (SNO) and SuperKamiokande experiments, disfavoring pure active to sterile neutrino oscillations, we show that all four neutrino mixing models, both of $2+2$ and $3+1$ type, lead to a full thermalization of the sterile neutrino flavor. Moreover such a sterile neutrino production excludes the possibility of an electron neutrino asymmetry generation and we conclude that $\Delta N_{\nu }^{\mathrm{tot}}\simeq 1,$ in disagreement with the cosmological bound. This result is valid under the assumption that the initial neutrino asymmetries are small. We suggest the existence of a second sterile neutrino flavor, with mixing properties that generate a large electron neutrino asymmetry, as a possible way out.