Sterile neutrinos with eV masses in cosmology -- how disfavoured exactly?

TL;DR

This paper evaluates the cosmological viability of eV-mass sterile neutrinos, showing they are generally disfavoured in standard models but can be compatible under extended frameworks with additional relativistic species or modified dark energy parameters.

Contribution

It systematically analyzes how extended cosmological models can accommodate eV-mass sterile neutrinos, contrasting with standard LambdaCDM constraints and exploring parameter degeneracies.

Findings

Sterile neutrinos are strongly disfavoured in minimal LambdaCDM due to hot dark matter constraints.

Allowing extra relativistic degrees of freedom relaxes these constraints.

Modifications like w<-1 and nu_e degeneracy can further reconcile sterile neutrinos with cosmological data.

Abstract

We study cosmological models that contain sterile neutrinos with eV-range masses as suggested by reactor and short-baseline oscillation data. We confront these models with both precision cosmological data (probing the CMB decoupling epoch) and light-element abundances (probing the BBN epoch). In the minimal LambdaCDM model, such sterile neutrinos are strongly disfavoured by current data because they contribute too much hot dark matter. However, if the cosmological framework is extended to include also additional relativistic degrees of freedom -- beyond the three standard neutrinos and the putative sterile neutrinos, then the hot dark matter constraint on the sterile states is considerably relaxed. A further improvement is achieved by allowing a dark energy equation of state parameter w<-1. While BBN strongly disfavours extra radiation beyond the assumed eV-mass sterile neutrino, this constraint can be circumvented by a small nu_e degeneracy. Any model containing eV-mass sterile neutrinos implies also strong modifications of other cosmological parameters. Notably, the inferred cold dark matter density can shift up by 20 to 75% relative to the standard LambdaCDM value.