Measurement of Parity Violation in the Early Universe using Gravitational-wave Detectors

TL;DR

This paper introduces a new method to detect parity violation in the stochastic gravitational-wave background, applying it to LIGO data and projecting future detector sensitivities to distinguish cosmological signals.

Contribution

The paper presents a novel technique for measuring parity violation in the SGWB and provides the first upper limit using LIGO data, along with sensitivity estimates for future detectors.

Findings

First upper limit on parity violation in SGWB from LIGO data

New technique to measure polarization in stochastic backgrounds

Projected sensitivity improvements for future gravitational-wave detectors

Abstract

A stochastic gravitational-wave background (SGWB) is expected to arise from the superposition of many independent and unresolved gravitational-wave signals, of either cosmological or astrophysical origin. Some cosmological models (characterized, for instance, by a pseudo-scalar inflaton, or by some modification of gravity) break parity, leading to a polarized SGWB. We present a new technique to measure this parity violation, which we then apply to the recent results from LIGO to produce the first upper limit on parity violation in the SGWB, assuming a generic power-law SGWB spectrum across the LIGO sensitive frequency region. We also estimate sensitivity to parity violation of the future generations of gravitational-wave detectors, both for a power-law spectrum and for a model of axion inflation. This technique offers a new way of differentiating between the cosmological and astrophysical sources of the isotropic SGWB, as astrophysical sources are not expected to produce a polarized SGWB.