Intrinsic handedness in O1-O4a black-hole mergers: probing orbital precession, remnant retention in dense environments and cosmological mirror asymmetry

Abstract

Precessing binary black-holes generically produce an imbalance of right- and left- handed gravitational waves, reflecting the breaking of mirror symmetry by the merger dynamics. We study this phenomenon using the observer-independent quantity $V_{\mathrm{GW}}$, a gravitational analogue of the optical Stokes parameter that quantifies the intrinsic handedness of the emitted radiation. Using 91 LIGO-Virgo-KAGRA black-hole mergers from the O1-O4a observing runs, we find that $92\%$ of the analyzed events favour non-vanishing $V_{\mathrm{GW}}$, indicating a predominance of precessing dynamics across the events. Through a recently established relation between $V_{\mathrm{GW}}$ and the remnant black hole recoil, we further constrain the retention of merger remnants in dense stellar environments, finding that at most $8\%$ could remain gravitationally bound to globular or nuclear star clusters and subsequently participate in hierarchical merger channels. We finally investigate the cosmological distribution of black-hole merger handedness. The observed $V_{\mathrm{GW}}$ distribution is consistent with symmetry under $V_{\mathrm{GW}}\to -V_{\mathrm{GW}}$, and yields an average value $⟨V_{\mathrm{GW}}⟩=-1.9_{-6.6}^{+6.1}\times 10^{-3}$ ($90\%$ credibility), consistent with the absence of a preferred handedness and with expectations from large-scale statistical isotropy. In particular, the inclusion of O4a events reduces uncertainties in $⟨V_{\mathrm{GW}}⟩$ by $\sim 40\%$ with respect to O1-O3 events. These results establish black-hole merger handedness as a unified probe of orbital precession, remnant recoil, hierarchical formation, and cosmological mirror symmetry.

14 pages, 10 Figures, 3 Appendixes