Amplification of isocurvature perturbations induced by active-sterile neutrino oscillations

We show how the generation of a lepton number in the early universe induced by να↔νs, in presence of small baryon number inhomogeneities, gives rise to the formation of lepton domains. The structure of these domains reflects the spectral features of the baryon number inhomogeneities that generated it and interestingly the size of the lepton domains can be super-horizon.