Leptogenesis and tensor polarisation from a gravitational Chern-Simons term

TL;DR

This paper explores how a gravitational Chern-Simons term coupled to an axion in string theory can produce observable gravitational wave polarization and baryon asymmetry, but emphasizes the need for string theory input for precise calculations.

Contribution

It highlights the importance of string theory input in computing effects of the gravitational Chern-Simons term on polarization and baryogenesis.

Findings

Vacuum fluctuations become circularly polarized during axion variation.

Polarization could be observable via the Cosmic Microwave Background.

String theory input is necessary for accurate predictions.

Abstract

Within an effective field theory derived from string theory, the universal axion has to be coupled to the the gravitational Chern-Simons (gCS) term. During any era when the axion field is varying, the vacuum fluctuation of the gravitational wave amplitude will then be circularly polarised, generating an expectation value for the gCS term. The polarisation may be observable through the Cosmic Microwave Background, and the vacuum expectation value of the gCS term may generate the baryon asymmetry of the Universe. We argue here that such effects cannot be computed without further input from string theory, since the `vacuum' in question is unlikely to be the field-theoretic one.