Neutrino oscillations in the early universe: how call large lepton asymmetry be generated?

The lepton asymmetry that could be generated in the early universe through oscillations of active to sterile neutrinos is calculated (almost) analytic ally for small mixing angles, sin 2 theta < 10(-2). It is shown that for a mass squared difference, delta m(2) = -1 eV(2) it may rise at most by six o rders of magnitude from the initial "normal" value similar to 10(-10) as th e back-reaction from the refraction index terminates this rise while the as ymmetry is still small. Only for very large mass differences, \delta m(2)\ similar to 10(9) eV(2), the lepton asymmetry could reach a significant magn itude exceeding 0.1. (C) 2000 Elsevier Science B.V. All rights reserved.