Intimate Relationship Between Sterile Neutrino Dark Matter and $\Delta N_{\rm eff}$

TL;DR

This paper investigates how sterile neutrino dark matter models with neutrino self-interactions predict a minimum increase in early universe radiation, which can be tested by upcoming cosmological observations.

Contribution

It establishes a link between sterile neutrino dark matter relic density and a lower bound on N_{ m eff}, providing testable predictions for future experiments.

Findings

N_{ m eff}} > 0.12 at CMB epoch

Lower bound will be tested by CMB-S4

Improved X-ray searches will strengthen constraints

Abstract

The self-interacting neutrino hypothesis is well motivated for addressing the tension between the origin of sterile neutrino dark matter and indirect detection constraints. It can also result in a number of testable signals from the laboratories to the cosmos. We explore a model of neutrino self-interaction mediated by a Majoron-like scalar with sub-MeV mass, and show that explaining the relic density of sterile neutrino dark matter implies a lower bound on the amount of extra radiation in early universe, in particular $\Delta N_{\rm eff}>0.12$ at the CMB epoch. This lower bound will be further strengthened with an improved $X$-ray search at the Athena observatory. Such an intimate relationship will be unambiguously tested by the upcoming CMB-S4 project.