  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.
