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 co nsiderations necessarily lead to the creation of spatial domains of lepton number with opposite signs. Lepton number gradients at domain boundaries ca n open a new channel for Mikheyev-Smirnov-Wolfenstein resonant production o f sterile neutrinos (nu(s)). This enhanced nu(s) production allows consider able tightening of big bang nucleosynthesis constraints on active-sterile n eutrino mixing.