Chiral gravitational waves from thermalized neutrinos in the early Universe

TL;DR

This paper studies how thermalized neutrinos in the early Universe can generate circularly polarized gravitational waves, with potential implications for understanding neutrino properties through future high-frequency gravitational wave observations.

Contribution

It calculates the polarization of gravitational waves induced by chiral neutrinos in the early Universe, highlighting the dependence on neutrino degeneracy and potential detectability.

Findings

Neutrinos induce a non-zero circular polarization in gravitational waves.

The polarization magnitude depends on neutrino degeneracy and wave frequency.

Detection may be possible with future ultra-high frequency gravitational wave detectors.

Abstract

We investigate polarized gravitational waves generated by chiral fermions in the early Universe. In particular, we focus on the contribution from left-handed neutrinos in thermal equilibrium with finite temperature and chemical potential in the radiation dominated era. We compute the correlation functions of gravitational fields pertinent to the Stokes parameter $V$ characterizing the circular polarization of gravitational waves in the Minkowski and expanding spacetime backgrounds. In the expanding universe, we find that the thermalized neutrinos induce a non-vanishing $V$ linear to the neutrino degeneracy parameter and wavenumber of gravitational waves in the long wavelength region. While the magnitude of the gravitational waves generated by thermal neutrinos is too small to be detectable by current and planned third generation gravitational wave detectors, their observations by future generation detectors for ultra-high frequency regimes could provide information on the neutrino degeneracy parameter in the early Universe.