  We propose a new mechanism to block the active-sterile neutrino oscillations
in the Early Universe. We show that a typical consequence of theories where the
lepton number is spontaneously broken is the existence of a coherent
cosmological Majoron field with a strength proportional to the lepton and
baryon numbers of the Universe. This field interacts with leptons and changes
the potentials relevant for neutrino oscillations. If the scale of lepton
number symmetry breaking is of the order of 1 GeV then a Majoron field and
lepton number asymmetry of the order of the baryon asymmetry are strong enough
to block the active-sterile neutrino oscillations with the atmospheric neutrino
mass gap which otherwise would bring the sterile neutrino into equilibrium at
the big bang nucleosynthesis epoch.
