Tuning, ergodicity, equilibrium, and cosmology

I explore the possibility that the cosmos is fundamentally an equilibrium system and review the attractive features of such theories. Equilibrium cosmologies are commonly thought to fail due to the “Boltzmann brain” problem. I show that it is possible to evade the Boltzmann brain problem if there is a suitable coarse-grained relationship between the fundamental degrees of freedom and the cosmological observables. I make my main points with simple toy models and then review the de Sitter equilibrium model as an illustration.

Figure 1

Ergodic arguments allow us to predict that entropy will increase into the future, but such state counting arguments would suggest that the past also had higher entropy. Most physicists find it more compelling to assume entropy decreases in the past time direction. This is the “past hypothesis.”

Figure 2

In the dSE model, the equilibrium state fluctuates off a baby universe which inflates, reheats, and undergoes standard cosmological evolution, eventually reequilibrating. The quantum state describes two semiclassical spacetimes between the moment of tunneling until the time of reequilibration. The probability of producing this fluctuation is competitive with the production of an other 1 g localized fluctuation in the matter.