Dark Energy and the Entropy of the Observable Universe

The initial low entropy of the universe has allowed irreversible processes, such as the reader reading this abstract, to happen in the universe. This initial low entropy is due to a low value for the initial gravitational entropy of the universe. The standard ΛCDM cosmology has a cosmic event horizon and an associated Gibbons‐Hawking entropy. We compute the entropy of the universe including the entropy of the current event horizon and the entropy of the matter and photons within the cosmic event horizon. We estimate the entropy of the current cosmic event horizon to be $2.6±0.3×10122 k$ and find it to be $∼1019$ times larger than the next most dominant contribution, which is from super massive black holes. We plot an entropy budget as a function of time and find that the cosmic event horizon entropy has dominated other sources of entropy since $10−20$ seconds after the big bang. See Egan & Lineweaver (2009) for details and discussion.