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Ordinary-sterile neutrino oscillations can generate a significant leptonnumber asymmetry in the early Universe. We study this phenomenon in detail. Weshow that the dynamics of ordinary-sterile neutrino oscillations in the earlyUniverse can be approximately described by a single integro-differentialequation which we derive from both the density matrix and Hamiltonianformalisms. This equation reduces to a relatively simple ordinary first orderdifferential equation if the system is sufficiently smooth (static limit). Westudy the conditions for which the static limit is an acceptable approximation.We also study the effect of the thermal distribution of neutrino momenta on thegeneration of lepton number. We apply these results to show that it is possibleto evade (by many orders of magnitude) the Big Bang Nucleosynthesis (BBN)bounds on the mixing parameters, $\delta m^2$ and $\sin^2 2\theta_0$,describing ordinary-sterile neutrino oscillations. We show that the large angleor maximal vacuum oscillation solution to the solar neutrino problem does notsignificantly modify BBN for most of the parameter space of interest, providedthat the tau and/or mu neutrinos have masses greater than about 1 eV. We alsoshow that the large angle or maximal ordinary-sterile neutrino oscillationsolution to the atmospheric neutrino anomaly does not significantly modify BBNfor a range of parameters.Comment: 53 pages, 7 Figures, LaTeX. The equation describing the production of  sterile neutrinos (Eqn.92) has been significantly improved (the corresponding  equation in the previous version was too crude an approximation). None of the  qualitative conclusions change, but there are significant quantitative  changes to the numerical work (see Figures 3-7 in particular

physical review. d. particles, fields, gravitation, and cosmology/physical review. d. particles and fields

Studies of neutrino asymmetries generated by ordinary-sterile neutrino oscillations in the early Universe and implications for big bang nucleosynthesis bounds