Effects of an Intermediate Mass Sterile Neutrino Population on the Early Universe

TL;DR

This paper investigates how decaying sterile neutrinos in the early Universe influence cosmological parameters, potentially challenging standard models and affecting processes like nucleosynthesis and neutrino spectra.

Contribution

It introduces detailed simulations of sterile neutrino decay effects on early Universe physics, connecting BSM models with observable cosmological parameters.

Findings

Constraints on sterile neutrino models from $N_{\rm eff}$ values.

Altered active neutrino spectra impacting BBN.

Potential deviations from standard cosmological predictions.

Abstract

The hot and dense early Universe combined with the promise of high-precision cosmological observations provide an intriguing laboratory for Beyond Standard Model (BSM) physics. We simulate the early Universe to examine the effects of the decay of thermally populated sterile neutrino states into Standard Model products around the time of weak decoupling. These decays deposit a significant amount of entropy into the plasma as well as produce a population of high-energy out-of-equilibrium active neutrinos. As a result, we can constrain these models by their inferred value of $N_{\rm eff}$, the effective number of relativistic degrees of freedom. In this work, we explore a variety of models with $N_{\rm eff}{}$ values consistent with CMB observations, but with vastly different active neutrino spectra which will challenge the standard cosmological model, affect lepton capture rates on free nucleons, and may significantly affect Big Bang Nucleosynthesis (BBN).