Chirality oscillation of primordial gravitational waves during inflation

TL;DR

This paper demonstrates that coupling a massive scalar field to the gravitational Chern-Simons term during inflation causes primordial gravitational waves to oscillate between chiral modes, affecting their power spectra and CMB polarization.

Contribution

It introduces the concept of chirality oscillation in primordial GWs induced by a scalar field coupled to the Chern-Simons term during inflation.

Findings

Chirality oscillation causes amplitude conversion between left- and right-handed GWs.

Permanent amplitude differences lead to distinct oscillating patterns in GW power spectra.

Implications for CMB B-mode polarization observations are discussed.

Abstract

We show that if the gravitational Chern-Simons term couples to a massive scalar field ($m>H$), the primordial gravitational waves (GWs) will show itself the chirality oscillation, i.e., the amplitudes of the left- and right-handed GWs modes will convert into each other and oscillate in their propagations. This oscillation will eventually develop a permanent difference of the amplitudes of both modes, which leads to nearly opposite oscillating shapes in the power spectra of the left- and right-handed primordial GWs. We discuss its implication to the CMB B-mode polarization.