Computing the Entropy Budget in the Observable Universe from Gravitational Waves

Black holes contain more entropy than any other component of the observable universe as has been estimated in Egan & Lineweaver (2010). Gravitational waves (GW) caused by compact object coalescence open a new era in astronomy, which provides new information to update the entropy budget. Recent GW detection, including GW190426 and GW190521, proves the existence of stellar black holes within the pair-instability supernovae mass gap (~ 42M_☉ - 140M_☉) previously listed as tentative components. Increases in entropy due to binary black hole (BBH) mergers, as implied in the second law of thermodynamics, should also be added to the budget. In this study, we adopt the POWERLAW + PEAK model for stellar BBH populations and estimate the remnant population from surrogate model derived from numerical relativity simulations. We update the entropy budget of the observable universe by adding the total entropy change from BBH mergers, contributing the fourth entropy increase. We also compute the entropy density resulted from BBH mergers, which peaks at z ~ 3. With the estimation of BBH entropy, we can constrain the initial mass function of stellar black holes, compute the probability of stars collapsing to black holes in binaries that eventually merge, and detect any special epoch in the universe.