Leptonic Domains in the Early Universe and Their Implications

We apply causality considerations to active-sterile neutrino transformation-based schemes for lepton number generation in the early Universe. These considerations necessarily lead to the creation of spatial domains of lepton number with opposite signs. Lepton number gradients at domain boundaries can open a new channel for Mikheyev-Smirnov-Wolfenstein resonant production of sterile neutrinos ( νₛ). This enhanced νₛ production allows considerable tightening of big bang nucleosynthesis constraints on active-sterile neutrino mixing.