  We investigate neutrino flavor transformation in the early universe in the
presence of a lepton asymmetry, focusing on a two-flavor system with 1 - 3
mixing parameters. We identify five distinct regimes that emerge in an
approximate treatment neglecting collisions as the initial lepton asymmetry at
high temperature is varied from values comparable to current constraints on the
lepton number down to values at which the neutrino-neutrino forward-scattering
potential is negligible. The characteristic phenomena occurring in these
regimes are (1) large synchronized oscillations, (2) minimal flavor
transformation, (3) asymmetric (neutrino- or antineutrino-only) MSW, (4)
partial MSW, and (5) symmetric MSW. We examine our numerical results in the
framework of adiabaticity, and we illustrate how they are modified by
collisional damping. Finally, we point out the existence of matter-neutrino
resonances in the early universe and show that they suffer from
non-adiabaticity.
