Imprint of cosmic neutrino decoupling in the spectrum of inflationary gravitational waves

Abstract

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