Fermions and gravitational gyrotropy

TL;DR

This paper explores how chiral fermion densities in the early universe could rotate gravitational wave polarization, leading to observable effects in the cosmic microwave background and potentially revealing new physics from the early cosmos.

Contribution

It introduces a mechanism for gravitational gyrotropy caused by fermionic chiral densities and discusses its implications for CMB polarization and early universe physics.

Findings

Chiral fermion densities can rotate gravitational wave polarization.

E-B and T-B correlations in CMB can be generated by primordial gravitational waves.

Detectability of these effects is limited for low tensor-to-scalar ratios.

Abstract

In conventional general relativity without torsion, high-frequency gravitational waves couple to the chiral number density of spin one-half quanta: the polarization of the waves is rotated by $2\pi N_5 {\ell_{\rm Pl}^2}$, where $N_5$ is the chiral column density and $\ell_{\rm Pl}$ is the Planck length. This means that if a primordial distribution of gravitational waves with E-E or B-B correlations passed through a chiral density of fermions in the very early Universe, an E-B correlation will be generated. This in turn will give rise to E-B and T-B correlations in the cosmic microwave background (CMB). Less obviously but more primitively, the condition Albrecht called "cosmic coherence" would be violated, changing the restrictions on the class of admissible cosmological gravitational waves. This altered class of waves would, generally speaking, probe earlier physics than do the conventional waves; their effects on the CMB would be most pronounced for low ($\lesssim 100$) multipoles. Rough estimates indicate that if the tensor-to-scalar ratio is less than about $10^{-2}$, it will be hard to constrain a spatially homogeneous primordial $N_5$ by present data.