NEUTRINO OSCILLATIONS IN THE EARLY UNIVERSE WITH AN INVERTED NEUTRINO-MASS HIERARCHY

Numerical simulations of neutrino oscillations in the early Universe are performed for a neutrino-mass hierarchy with the vacuum mass of the second neutrino exceeding that of the first. This situation is equivalent to a normal hierarchy with mixing angle greater than pi/4. We find that a large conversion of electron neutrinos to muon neutrinos occurs, independent of mixing angle. For certain parameter values, this is caused by the MSW effect. In the rest of the parameter region a new mechanism is operative, arising from nonlinear effects involving neutrino-neutrino forward scattering. This nonlinear conversion mechanism causes substantial flavor oscillation even for extremely small mixing angles.