Thermodynamics to infer the astrophysics of binary black hole mergers

Abstract: We introduce the Merger Entropy Index ($\mathcal{I}\mathrm{BBH}$), a new parameter to measure the efficiency of entropy transfer for any generic binary black hole merger in General Relativity. We find that $\mathcal{I}\mathrm{BBH}$ is bounded between an asymptotic maximum and minimum. For the observed population of mergers detected by LIGO and Virgo, we find that $\mathcal{I}\mathrm{BBH}$ is $\lesssim30\%$ of its theoretical maximum. By imposing the thermodynamical consistency between the pre- and post-merger states through $\mathcal{I}\mathrm{BBH}$, we showcase BRAHMA -- a novel framework to infer the properties and astrophysical implications of gravitational-wave detections. For GW190521 -- the heaviest confirmed binary black hole merger observed so far -- our framework rules out high mass-ratio, negative effective inspiral spin, and electromagnetic counterpart claims. Furthermore, our analysis provides an independent confirmation that GW190521 belongs to a separate population.