Circularly polarized scalar induced gravitational waves from the Chern-Simons modified gravity

TL;DR

This paper studies circularly polarized scalar-induced gravitational waves in Chern-Simons modified gravity, revealing potential parity violation signatures in the early universe with significant polarization but modest energy density contributions.

Contribution

It provides semi-analytic formulas for the energy density of SIGWs in Chern-Simons gravity, highlighting the polarization effects and their observational implications.

Findings

Circular polarization degree can be as large as unity.

Contribution to energy density from CS term is comparable to GR.

Potential to test parity violation in early universe gravitational waves.

Abstract

We investigate the scalar induced gravitational waves (SIGWs) in the Chern-Simons (CS) modified gravity during the radiation dominated era. The SIGWs are circularly polarized, which provide us a tool to test the possible parity violation in the early universe. We derive the semianalytic expressions to evaluate the fractional energy density of the SIGWs, $\Omega_{\mathrm{GW}}$, which receives contributions from the general relativity (GR) and the correction due to the parity-violating term, respectively. We find that the degree of the circular polarization of the SIGWs can be as large as of order unity, although the contribution to $\Omega_{\mathrm{GW}}$ from the CS term is at most of the same order as that from the GR.