  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.
