More Is Different: Reconciling eV Sterile Neutrinos with Cosmological Mass Bounds

TL;DR

This paper proposes that sterile neutrinos with self-interactions can reduce the effective number of neutrinos, $N_{eff}$, in the early universe, potentially reconciling eV-mass sterile neutrinos with cosmological mass bounds.

Contribution

It introduces a scenario where sterile neutrino self-interactions suppress their thermalization, decreasing $N_{eff}$ and easing conflicts with cosmological constraints.

Findings

Self-interactions suppress sterile neutrino thermalization.

$N_{eff}$ can decrease after BBN due to flavor equilibrium.

Adding light species relaxes mass bounds on sterile neutrinos.

Abstract

It is generally expected that adding light sterile species would increase the effective number of neutrinos, $N_{eff}$. In this paper we discuss a scenario that $N_{eff}$ can actually decrease due to the neutrino oscillation effect if sterile neutrinos have self-interactions. We specifically focus on the eV mass range, as suggested by the neutrino anomalies. With large self-interactions, sterile neutrinos are not fully thermalized in the early Universe because of the suppressed effective mixing angle or matter effect. As the Universe cools down, flavor equilibrium between active and sterile species can be reached after big bang nucleosynthesis (BBN) epoch, but leading to a decrease of $N_{eff}$. In such a scenario, we also show that the conflict with cosmological mass bounds on the additional sterile neutrinos can be relaxed further when more light species are introduced.