Impact of Entropy on Black Hole Astrophysics

Abstract

As an irreversible natural process, mergers between two black holes will increase the entropy of the universe. By imposing thermodynamical constraints derived from general relativity and quasi-circular hypothesis, we showcase BRAHMA – a novel framework to infer the properties and astrophysical implications of binary black hole mergers in LIGO-Virgo-KAGRA (LVK). We apply the framework as an IMR (inspiral-merger-ringdown) consistency test to 10 heaviest binary black hole merger events reported by LVK Collaboration and perform a systematic investigation on the consistency between phenomenological waveform and ringdown models (analysis data available on Zenodo). In doing so, we obtain astrophysical insights into the origins of black holes for GW190521 and GW191109, two of the heaviest confirmed merger events. We also show the high consistency of the NRSur7dq4 waveform and Kerr221 ringdown model, providing insights into the timing of the ringdown stage. For events without QNM measurements due to low SNR of ringdown, we use the post-merger conditions inferred from phenomenological waveforms to compute the IBBH (Merger Entropy Index: measures the efficiency of entropy transfer during BBH merger) distribution across all GW events. IBBH shows high differentiability in formation channels, which can be used as a tool to classify compact binary populations. Therefore, we employ IBBH to identify compact objects in the lower mass gap (2.5 ∽ 5𝑀⊙), including the newest discovery GW230529 in the 4th observing run.