Neutrino oscillations at high densities: cosmological and astrophysical aspects

This doctoral thesis treats the effects of neutrino oscillations in the early universe and supernovae. The main probe for the context of the early universe is big bang nucleosynthesis (BBN). I explain the general link between neutrinos and BBN with an emphasis on the degeneracy between neutrino asymmetry and extra degrees of freedom. I show how the degenerate effects of both oscillations and collisions lead to observables detectable in the near future that brake this degeneracy and draw bounds on neutrino asymmetry and their contribution to the energy density of the universe in the current epoch. Considering astrophysical aspects, I analyze the significance of a proper treatment of neutrino evolution to understand the neutrino signal from the next galactic supernova (SN). I show the type of effects collective neutrino conversions can produce and the numerical difficulties confronted. Finally, I show how that stability analysis can complement a numerical treatment and provide definitive answers in some example cases.