Gravitational waves in neutrino plasma and NANOGrav signal

TL;DR

This paper explores how gravitational waves generated in neutrino plasmas could explain the NANOGrav signal, proposing a new mechanism involving neutrino asymmetries and discussing the role of magnetic fields.

Contribution

It introduces a novel mechanism for GW generation in neutrino plasmas and assesses its potential to explain the NANOGrav signal, including magnetic field constraints.

Findings

Neutrino plasma instabilities can produce GWs within NANOGrav sensitivity.

GWs from inhomogeneous neutrino plasma cannot account for the NANOGrav signal.

Magnetic fields of at least 10^{-12} G are required to explain the signal.

Abstract

The recent finding of the gravitational wave (GW) signal by the NANOGrav collaboration in the nHZ frequency range has opened up the door for the existence of stochastic GWs. In the present work, we have argued that in a hot dense neutrino asymmetric plasma, GWs could be generated due to the instability caused by the finite difference in the number densities of the different species of the neutrinos. The generated GWs have amplitude and frequency in the sensitivity range of the NANOGrav observation. We have shown that the GWs generated by this mechanism could be one of the possible explanations for the observed NANOGrav signal. We have also discussed generation of GWs in an inhomogeneous cosmological neutrino plasma, where GWs are generated when neutrinos enter a free streaming regime. We show that the generated GWs in an inhomogeneous neutrino plasma cannot explain the observed NANOGrav signal. We have also calculated the lower bound on magnetic fields' strength using the NANOGrav signal and found that to explain the signal, the magnetic fields' strength should have atleast value $\sim 10^{-12}$ G at an Mpc length scale.