CosmoCoffee

Carroll and Tam have written a nice and very readable paper revisiting Penrose’s entropy problem as well as work related to Liouville’s theorem in the context of inflation. They look at the claim that inflation evolves from generic initial conditions. This often cited claim is radically at odds with Liouville’s theorem, which requires a given number of states of the universe to evolve into the same number of states at a later time. In other words, it is not possible for a large number of (highly curved and inhomogeneous) initial states to evolve into a small number of smooth and homogeneous “final” states. Thus it is not clear whether the main argument by which inflation is usually motivated actually holds up! (Of course inflation has the appealing byproduct that it can produce density perturbations along the way.)

The paper does not provide a resolution of the problem, but is interesting in that it highlights an often over-looked issue that is certainly in need of a solution, if we want to claim that inflation explains the initial conditions of the hot big bang cosmology.