Relic neutrino degeneracies and their impact on cosmological parameters

Abstract

Abstract In the standard ΛCDM model, neutrinos are treated as radiation, with their masses and possible degeneracy ignored. In this paper, we compute the impact of a finite relic neutrino degeneracy ξ on the CMB angular power spectra, and obtain constraints on ξ using current cosmological data sets. We find that ξ≈ O(1) is still allowed. We also study the correlations between ξ, the Hubble parameter H 0, and the spectral index n s. Due to these correlations, the CMB constraints on inflation models are loosened when ξ is fitted together with other cosmological parameters, such that some models excluded at 95% confidence level by standard fittings without ξ could be revived. Furthermore, the tension in CMB and local measurements of H 0 is slightly alleviated. Our results suggest that ξ is a non-negligible physical parameter for cosmological analyses.