Sensitivity to a Frequency-Dependent Circular Polarization in an Isotropic Stochastic Gravitational Wave Background

TL;DR

This paper evaluates the frequency-dependent sensitivity of various gravitational wave detection methods to circular polarization in an isotropic stochastic gravitational wave background, highlighting the importance of broad frequency coverage.

Contribution

It provides a unified framework for assessing sensitivity to circular polarization across multiple detection techniques, improving understanding of their combined potential.

Findings

Adding an Einstein Telescope-like interferometer enhances sensitivity by a factor of two.

Current and planned observations cover a critical frequency range for polarization detection.

There exists a sensitivity gap between pulsar timing arrays and indirect methods for certain frequencies.

Abstract

We calculate the sensitivity to a circular polarization of an isotropic stochastic gravitational wave background (ISGWB) as a function of frequency for ground- and space-based interferometers and observations of the cosmic microwave background. The origin of a circularly polarized ISGWB may be due to exotic primordial physics (i.e., parity violation in the early universe) and may be strongly frequency dependent. We present calculations within a coherent framework which clarifies the basic requirements for sensitivity to circular polarization, in distinction from previous work which focused on each of these techniques separately. We find that the addition of an interferometer with the sensitivity of the Einstein Telescope in the southern hemisphere improves the sensitivity of the ground-based network to circular polarization by about a factor of two. The sensitivity curves presented in this paper make clear that the wide range in frequencies of current and planned observations ($10^{-18}\ {\rm Hz} \lesssim f \lesssim 100\ {\rm Hz}$) will be critical to determining the physics that underlies any positive detection of circular polarization in the ISGWB. We also identify a desert in circular polarization sensitivity for frequencies between $10^{-15}\ {\rm Hz} \lesssim f \lesssim 10^{-3}\ {\rm Hz}$, given the inability for pulsar timing arrays and indirect-detection methods to distinguish the gravitational wave polarization.