Non-thermal cosmic neutrino background

TL;DR

This paper explores the possibility of a non-thermal cosmic neutrino background for Dirac neutrinos, which could be detectable and provide insights into the early universe, distinct from the standard thermal background.

Contribution

It introduces the concept of a non-thermal relic neutrino background, discusses its potential abundance, and proposes a scenario for its generation during inflationary preheating.

Findings

Non-thermal neutrinos can have a density up to half of photon density.

Such neutrinos can increase the effective number of neutrino species beyond 3.046.

Future experiments like PTOLEMY could detect this non-thermal background.

Abstract

We point out that, for Dirac neutrinos, in addition to the standard thermal cosmic neutrino background (C$\nu$B) there could also exist a non-thermal neutrino background with comparable number density. As the right-handed components are essentially decoupled from the thermal bath of standard model particles, relic neutrinos with a non-thermal distribution may exist until today. The relic density of the non-thermal (nt) background can be constrained by the usual observational bounds on the effective number of massless degrees of freedom $N_\mathrm{eff}$, and can be as large as $n_{\nu_{\mathrm{nt}}}\lesssim 0.5\,n_\gamma$. In particular, $N_\mathrm{eff}$ can be larger than 3.046 in the absence of any exotic states. Non-thermal relic neutrinos constitute an irreducible contribution to the detection of the C$\nu$B, and, hence, may be discovered by future experiments such as PTOLEMY. We also present a scenario of chaotic inflation in which a non-thermal background can naturally be generated by inflationary preheating. The non-thermal relic neutrinos, thus, may constitute a novel window into the very early universe.