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 S_obs = 3.1+3.0-1.7x10^104 k. We calculate the
entropy of the current cosmic event horizon to be S_CEH = 2.6+-0.3x10^122 k,
dwarfing the entropy of its interior, S_CEHint = 1.2+1.1-0.7x10^103 k. We make
the first tentative estimate of the entropy of weakly interacting massive
particle dark matter within the observable universe, S_dm = 10^87-10^89 k. We
highlight several caveats pertaining to these estimates and make
recommendations for future work.
