Circular Polarization of the Astrophysical Gravitational Wave Background

TL;DR

This paper reveals that the astrophysical gravitational wave background can exhibit detectable circular polarization due to source fluctuations, impacting future searches for primordial signals.

Contribution

It demonstrates that Poisson fluctuations induce measurable polarization in the astrophysical background, which must be considered in cosmological gravitational wave studies.

Findings

Poisson fluctuations generate circular polarization in the astrophysical background.

Third-generation interferometers can detect this polarization with high confidence.

Methods are proposed to distinguish primordial signals from astrophysical foregrounds.

Abstract

The circular polarization of gravitational waves is a powerful observable to test parity violation in gravity and to distinguish between the primordial or the astrophysical origin of the stochastic background. This property comes from the expected unpolarized nature of the homogeneous and isotropic astrophysical background, contrary to some specific cosmological sources that can produce a polarized background. However, in this work we show that there is a non-negligible amount of circular polarization also in the astrophysical background, generated by Poisson fluctuations in the number of unresolved sources, which can be detected by the third-generation interferometers with signal-to-noise ratio larger than one. We also explain in which cases the gravitational wave maps can be cleaned from this extra source of noise, exploiting the frequency and the angular dependence, in order to search for signals from the early Universe. Future studies about the detection of polarized cosmological backgrounds with ground- and space-based interferometers should account for the presence of such a foreground contribution.