Nonlinear neutrino oscillations in the expanding Universe

We derive the equations that govern neutrino and antineutrino behavior in the early Universe. Results are presented from numerical simulations incorporating nonlinear effects of the neutrino-antineutrino background as well as the leading $\mathrm{CP}$-symmetric and $\mathrm{CP}$-asymmetric terms arising from the interactions with electrons and positrons. A broad spectrum of neutrino and antineutrino behavior is observed, ranging from vacuumlike to smooth. Oscillations exhibiting self-maintained coherence also appear. We identify a variety of neutrino properties arising as a consequence of the nonlinear neutrino self-interactions. The nonlinear effects tend to suppress any $\mathrm{CP}$ asymmetry relative to the linear case. An analytical approximation scheme reproducing bulk neutrino behavior is described. We discuss some physical implications of our results.