Leptonic Domains in the Early Universe and Their Implications

We extend a treatment of the causal structure of space-time to active-sterile neutrino transformation-based schemes for lepton number generation in the early universe. We find that these causality considerations necessarily lead to the creation of spatial domains of lepton number with opposite signs. Lepton number gradients at the domain boundaries can open a new channel for MSW resonant production of sterile neutrinos. The enhanced sterile neutrino production via this new channel allows considerable tightening of Big Bang Nucleosynthesis constraints on active-sterile neutrino mixing, including the proposed $\nu_\mu \to \nu_s$ solution for the Super Kamiokande atmospheric $\nu_\mu$ deficit, and the four-neutrino schemes proposed to simultaneously fit current neutrino experimental results.