Circular Polarization of Primordial Gravitational Waves in String-inspired Inflationary Cosmology

TL;DR

This paper proposes a mechanism in string-inspired inflationary cosmology to generate circular polarization in primordial gravitational waves, involving Gauss-Bonnet and Chern-Simons terms, with potential detectability by future observatories.

Contribution

It introduces a two-field inflation model that produces detectable circular polarization of gravitational waves, overcoming limitations of single-field models.

Findings

Circular polarization generated during super-inflation via Gauss-Bonnet term.

Single-field inflation models conflict with CMB observations.

Two-field inflation model predicts detectable polarization by BBO or DECIGO.

Abstract

We study a mechanism to produce the circular polarization of primordial gravitational waves. The circular polarization is generated during the super-inflation driven by the Gauss-Bonnet term in the string-inspired cosmology. The instability in the tensor mode caused by the Gauss-Bonnet term and the parity violation due to the gravitational Chern-Simons term are the essential ingredients of the mechanism. We also discuss detectability of the produced circular polarization of gravitational waves. It turns out that the simple model of single-field inflation contradicts CMB observations. To circumvent this difficulty, we propose a two-field inflation model. In this two-field model, the circular polarization of gravitational waves is created in the frequency range designed by the Big-Bang Observer (BBO) or the deci-hertz gravitational-wave observatory (DECIGO).