Testing mirror symmetry in the Universe with LIGO-Virgo black-hole mergers

Abstract

Precessing black-hole mergers can produce gravitational waves with net circular polarization, understood as an imbalance between right- and left-handed amplitudes. According to the Cosmological Principle, such emission must average to zero across all binary mergers in our Universe to preserve mirror-reflection symmetry at very large scales. We present a new, independent gravitational-wave test of this hypothesis. Using a novel observable based on the Chern-Pontryagin pseudo-scalar, we measure the emission of net circular polarization across 47 black-hole mergers recently analyzed by Islam. et. al. with a state-of-the art model for precessing black-hole mergers. The average value obtained is consistent with zero. Remarkably, however, we find that at least $82\%$ of the analysed sources must have produced net circular polarization, which requires orbital precession. Of these, GW200129 shows strong evidence for mirror asymmetry, with a Bayes Factor of 12.6 or, equivalently, $93.1\%$ probability. We obtain consistent (although stronger) results of $97.5\%$ and $94.3\%$ respectively using public results on this event from Hannam et. al. and performing our own parameter inference. This finding further implies indirect evidence for spontaneous emission of circularly polarized photons out of the quantum vacuum. Forthcoming black-hole merger detections will enable stronger constraints on large-scale mirror asymmetry and the Cosmological Principle.Comment: 9 pages, 6 Figures, 3 Appendixe