A possible signature of cosmic neutrino decoupling in the nHz region of the spectrum of primordial gravitational waves

TL;DR

This paper investigates how cosmic neutrino decoupling leaves a distinctive, frequency-dependent signature in the primordial gravitational wave spectrum around 1 nHz, which could potentially be detected by Pulsar Timing Arrays.

Contribution

It introduces a detailed analysis of the neutrino decoupling effect on gravitational wave damping, highlighting a spectral feature near 1 nHz.

Findings

Neutrino decoupling causes a frequency-dependent damping of GWs.

A spectral feature appears just below 1 nHz in the GW spectrum.

Detection of this feature by PTAs is unlikely.

Abstract

In this paper we study the effect of cosmic neutrino decoupling on the spectrum of cosmological gravitational waves (GWs). At temperatures T>>1 MeV, neutrinos constitute a perfect fluid and do not hinder GW propagation, while for T<<1 MeV they free-stream and have an effective viscosity that damps cosmological GWs by a constant amount. In the intermediate regime, corresponding to neutrino decoupling, the damping is frequency-dependent. GWs entering the horizon during neutrino decoupling have a frequency f ~ 1 nHz, corresponding to a frequency region that will be probed by Pulsar Timing Arrays (PTAs). In particular, we show how neutrino decoupling induces a spectral feature in the spectrum of cosmological GWs just below 1 nHz. We briefly discuss the conditions for a detection of this feature and conclude that it is unlikely to be observed by PTAs.