Active-sterile neutrino oscillations in the early Universe and the atmospheric neutrino anomaly

Cosmology cannot rule out the solution nu_mu <-> nu_s to the atmospheric neutrino data and thus only Earth experiments will be able to give a definitive answer. This conclusion holds when a generation of lepton number is taken into account and one assumes that the sterile neutrino is also slightly mixed with an eV-tau neutrino. This result cannot be spoiled by a chaotic generation of lepton domains.