Relic neutrino decoupling including flavour oscillations Original Research Article

In the early universe, neutrinos are slightly coupled when electron–positron pairs annihilate transferring their entropy to photons. This process originates non-thermal distortions on the neutrino spectra which depend on neutrino flavour, larger for image than for image or image. We study the effect of three-neutrino flavour oscillations on the process of neutrino decoupling by solving the momentum-dependent kinetic equations for the neutrino spectra. We find that oscillations do not essentially modify the total change in the neutrino energy density, giving image in terms of the effective number of neutrinos, while the small effect over the production of primordial 4He is increased by image, up to image. These results are stable within the presently favoured region of neutrino mixing parameters.