Cosmological mirror symmetry and gravitational-wave helicity

Abstract

Our current understanding of the Universe relies on the hypothesis that, when observed at sufficiently large scales, it looks statistically identical regardless of location or direction of observation. Consequently, the Universe should exhibit mirror-reflection symmetry. In this essay, we show that gravitational-wave astronomy provides a unique, observer-independent test of this hypothesis. In particular, we analyze the average circular polarization emitted by an ensemble of binary black hole mergers detected by LIGO-Virgo, which we compute using a novel geometric and chiral observable in general relativity. We discuss current results and future prospects with upcoming detections and technical advancements. Moreover, we show that this circular polarization and the helicity of the remnant black hole are linearly correlated, drawing a conceptual parallel with Wu experiment in particle physics.

Figures (4)

• Figure 1: Linear correlation found between GW circular polarization ($V_{\rm GW}$) and the helicity of the final black hole in BBH simulations from the SXS (left) and RIT (right) catalogues. The helicity is understood as the projection of the recoil velocity $\vec{K}$ along either the total angular momentum direction $\hat{J}$ or the final black hole spin $\hat{a}_{\rm f}$.
• Figure 2: Left panel: Posterior probability distribution of the gravitational Stokes parameter (\ref{['VGW']}) for the 47 LIGO-Virgo events considered in islam2023analysisgwtc3fullyprecessing. The black curve denotes the prior distribution. Only GW200129 (in green) shows strong evidence for a nonzero $V_{\rm GW}$. Right panel: Posterior probability distribution (in blue) for the average value of $V_{\rm GW}$ across the 47 LIGO-Virgo events. The prior distribution is shown in gray. While minimally informative, the posterior remains strongly consistent with zero.
• Figure 3: Evidence for non-zero $V_{\rm GW}$ in each analyzed event. The plot shows the relative Bayes factor comparing the $V_{\rm GW}\neq 0$ scenario against the $V_{\rm GW} = 0$ one, based on the posterior parameter samples released by islam2023analysisgwtc3fullyprecessing.
• Figure 4: Value of $V_{\rm GW}$ as a function of sky location for the 47 BBHs analyzed in islam2023analysisgwtc3fullyprecessing. The density of points is proportional to the joint posterior probability for a given sky-location across the 47 BBHs, while the color represents the corresponding $V_{\rm GW}$ value.