Chiral gravitational waves from thermalized neutrinos in the early Universe

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.

Probing neutrino interactions and dark radiation with gravitational waves

“…Gravitational waves also interact with other types of matter, including relativistic axions [90,91], nonrelativistic and collisional matter [92][93][94][95], and vector fields [96][97][98][99]; some of these scenarios are not captured by the kinetic theory treatment we review. We neglect any chemical potential that would be requisite to describe, e.g., chiral fermions and polarized gravitational waves [100][101][102][103][104].…”