MG11 
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Abstract

Constraints on the neutrino asymmetry of the Universe from cosmological data

The present amount of cosmological lepton asymmetry is unknown, and actually nothing, neither from a theoretical nor from an experimental point of view, prevents it to be much larger that the corresponding baryonic asymmetry. Since different models of baryogenesis produce different amount of lepton asymmetry, gathering information on the latter can in principle be used to discriminate between models of baryogenesis. The presence of a lepton asymmetry reflects in a non zero-chemical potential for the cosmological neutrino background (CNB). After discussing how this quantity affects the cosmological evolution, we will discuss the limits that the present cosmological data put on the lepton asymmetry. In particular we will show that CMB data alone cannot efficiently discriminate between models with large (of order 1) and small lepton asymmetry. This results also holds in the framework of cosmological models with non-standard structure of the neutrino sector, in which the nucleosynthesis bound can possibly be evaded.

Effect of cosmological neutrinos on the propagation of primordial gravitational waves

We discuss the interaction between gravitational waves produced in the early Universe, and the cosmological neutrinos (CNs). The gravitational waves entering the horizon after neutrino decoupling are partially absorbed due to the large anisotropic stress of free-streaming neutrinos. This is known to leave a signature on the B modes of the cosmic microwave background anisotropy spectrum (S. Weinberg, Phys. Rev. D 69, 203503, 2004). We extend the treatment to the case in which the neutrino interactions with the electron-positron plasma are not negligible. In particular, we will discuss the influence of the Ns on the propagation of gravitational waves produced before the electroweak phase transition. The implications with respect to the detectability of a cosmological signal by the LISA space interferometer are discussed.