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

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.

arXiv comment11 pages, 2 figures. V2: References Added

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• Reproducing neutrino effects on the matter power spectrum through a degenerate Fermi gas approach
• Gravitational waves in Palatini gravity for a non-minimal geometry-matter coupling
• The role of longitudinal polarizations in Horndeski and macroscopic gravity: Introducing gravitational plasmas