Measuring the circular polarization of gravitational waves with pulsar timing arrays

TL;DR

This paper investigates how pulsar timing arrays can detect the circular polarization anisotropies in the stochastic gravitational wave background, which can reveal information about astrophysical sources and early universe processes.

Contribution

It provides a realistic assessment of current and future PTA sensitivities to circular polarization anisotropies in the SGWB, highlighting the potential for detection.

Findings

Astrophysical SGWB polarization detectable with near-future datasets.

Cosmological SGWB polarization detectable with advanced SKA experiments.

PTAs are insensitive to the polarization monopole, relying on anisotropies for detection.

Abstract

The circular polarization of the stochastic gravitational wave background (SGWB) is a key observable for characterising the origin of the signal detected by Pulsar Timing Array (PTA) collaborations. Both the astrophysical and the cosmological SGWB can have a sizeable amount of circular polarization, due to Poisson fluctuations in the source properties for the former, and to parity violating processes in the early universe for the latter. Its measurement is challenging since PTA are blind to the circular polarization monopole, forcing us to turn to anisotropies for detection. We study the sensitivity of current and future PTA datasets to circular polarization anisotropies, focusing on realistic modelling of intrinsic and kinematic anisotropies for astrophysical and cosmological scenarios respectively. Our results indicate that the expected level of circular polarization for the astrophysical SGWB should be within the reach of near future datasets, while for cosmological SGWB circular polarization is a viable target for more advanced SKA-type experiments.