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 −1.7 × 10 104 k. We calculate the entropy of the current cosmic event horizon to be S CEH = 2.6 ± 0.3 × 10 122 k, dwarfing the entropy of its interior, S CEH int = 1.2 +1.1 −0.7 × 10 103 k. We make the first tentative estimate of the entropy of weakly interacting massive particle dark matter within the observable universe, S dm = 10 88±1 k. We highlight several caveats pertaining to these estimates and make recommendations for future work.
We review the cosmic evolution of entropy and the gravitational origin of the free energy required by life. All dissipative structures in the universe including all forms of life, owe their existence to the fact that the universe started in a low entropy state and has not yet reached equilibrium. The low initial entropy was due to the low gravitational entropy of the nearly homogeneously distributed matter and has, through gravitational collapse, evolved gradients in density, temperature, pressure and chemistry. These gradients, when steep enough, give rise to far from equilibrium dissipative structures (e.g., galaxies, stars, black holes, hurricanes and life) which emerge spontaneously to hasten the destruction of the gradients which spawned them. This represents a paradigm shift from "we eat food" to "food has produced us to eat it".
The energy densities of matter and the vacuum are currently observed to be of the same order of magnitude: (Ω mo ≈ 0.3) ∼ (Ω Λo ≈ 0.7). The cosmological window of time during which this occurs is relatively narrow. Thus, we are presented with the cosmological coincidence problem: Why, just now, do these energy densities happen to be of the same order? Here we show that this apparent coincidence can be explained as a temporal selection effect produced by the age distribution of terrestrial planets in the Universe. We find a large ( ∼ 68%) probability that observations made from terrestrial planets will result in finding Ω m at least as close to Ω Λ as we observe today. Hence, we, and any observers in the Universe who have evolved on terrestrial planets, should not be surprised to find Ω mo ∼ Ω Λo . This result is relatively robust if the time it takes an observer to evolve on a terrestrial planet is less than ∼ 10 Gyr.
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