On the chaoticity of active-sterile neutrino oscillations in the early   universe
  We have investigated the evolution of the neutrino asymmetry in active-sterile neutrino oscillations in the early universe. We find that there are large regions of parameter space where the asymmetry is extremely sensitive to variations in the initial asymmetry as well as the external parameters (the mass difference and the mixing angle). In these regions the system undergoes chaotic transitions; however, the system is never truly chaotic in the sense that all information about initial conditions is lost. In some cases though, enough information is lost that the final sign of the neutrino asymmetry is stochastic. We discuss the implications of our findings for Big Bang nucleosynthesis (BBN) and the cosmic microwave background (CMB). 
Poul-Erik N. Braad, Steen Hannestad