A Larger Estimate of the Entropy of the Universe

Using recent measurements of the supermassive black hole (SMBH) mass function, we find that SMBHs are the largest contributor to the entropy of the observable universe, contributing at least an order of magnitude more entropy than previously estimated. The total entropy of the observable universe is correspondingly higher, and is Sobs = 3.1+3.0-1.7 × 10104 k. We calculate the entropy of the current cosmic event horizon to be SCEH = 2.6±0.3 × 10122 k, dwarfing the entropy of its interior, SCEH int = 1.2+1.1-0.7 × 10103k. We make the first tentative estimate of the entropy of weakly interacting massive particle dark matter within the observable universe, Sdm = 1088±1 k. We highlight several caveats pertaining to these estimates and make recommendations for future work.