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) analytically 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 orders of magnitude from the initial "normal" value similar to 10(-10) as the back-reaction from the refraction index terminates this rise while the asymmetry 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 magnitude exceeding 0.1. (C) 2000 Elsevier Science B.V. All rights reserved.