Parity violation in stochastic gravitational wave background from inflation

TL;DR

This paper explores a new parity-violating gravity model during inflation that produces a chiral gravitational wave background with a localized bump, potentially detectable by future space-based observatories like LISA and Taiji.

Contribution

It introduces a novel gravity model with the Nieh-Yan term coupled to an axion-like field, leading to distinctive chiral gravitational wave signatures during inflation.

Findings

The model predicts a localized bump in the gravitational wave spectrum.

The chiral gravitational wave background is detectable by LISA and Taiji.

The chirality of GWs can be tested through detector correlation.

Abstract

We study the inflationary implications of a novel parity-violating gravity model, which modifies the teleparallel equivalent of general relativity by introducing the Nieh-Yan term coupled to an axion-like field. The parity-violating Nieh-Yan term results in the velocity birefringence of gravitational waves (GWs) and triggers the tachyonic instability only for one of the two circular polarization states. We consider that the inflaton is identified as the coupled axion-like field with a wiggly potential characterized by steep cliffs connected by smooth plateaus. During inflation, the temporary fast roll of axion on the cliff-like region leads to the significant enhancement of the tensor perturbations in one polarization state with the wave numbers that exit the horizon around this period. In this setup, the resulting energy spectrum for GWs presents a sizable localized bump involving the contribution of only one polarization state. This chiral GW background is detectable by LISA and Taiji, and its chirality can be determined by correlating two detectors, which provide an opportunity to probe the inflation and test the gravity model.