  We study post weak decoupling coherent active-sterile and active-active
matter-enhanced neutrino flavor transformation in the early universe. We show
that flavor conversion efficiency at Mikheyev-Smirnov-Wolfenstein resonances is
likely to be high (adiabatic evolution) for relevant neutrino parameters and
energies. However, we point out that these resonances cannot sweep smoothly and
continuously with the expansion of the universe. We show how neutrino flavor
conversion in this way can leave both the active and sterile neutrinos with
non-thermal energy spectra, and how, in turn, these distorted energy spectra
can affect the neutron-to-proton ratio, primordial nucleosynthesis, and
cosmological mass/closure constraints on sterile neutrinos. We demonstrate that
the existence of a light sterile neutrino which mixes with active neutrinos can
change fundamentally the relationship between the cosmological lepton numbers
and the primordial nucleosynthesis He-4 yield.
