Relic Neutrino Degeneracies and Their Impact on Cosmological Parameters

TL;DR

This paper investigates how a finite relic neutrino degeneracy parameter affects cosmological observations, showing it can relax constraints on inflation models and slightly reduce the Hubble tension.

Contribution

It introduces the consideration of relic neutrino degeneracy in cosmological models and analyzes its effects on CMB data and parameter constraints.

Findings

Relic neutrino degeneracy parameter $\xi$ is still compatible with current data.

Allowing $\xi$ relaxes constraints on inflation models.

Including $\xi$ slightly alleviates the Hubble tension.

Abstract

In the standard $\Lambda$CDM 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 $\xi$ on the CMB angular power spectra, and obtain constraints on $\xi$ using current cosmological data sets. We find that $\xi \approx O(1)$ is still allowed. We also study the correlations between $\xi$, the Hubble parameter $H_0$, and the spectral index $n_s$. Due to these correlations, the CMB constraints on inflation models are loosened when $\xi$ is fitted together with other cosmological parameters, such that some models excluded at 95% confidence level by standard fittings without $\xi$ could be revived. Furthermore, the tension in CMB and local measurements of $H_0$ is slightly alleviated. Our results suggest that $\xi$ is a non-negligible physical parameter for cosmological analyses.