There are quite a few challenges for cosmologists trying to discover evidence of what it was like early in the life of our universe, but while the inflationary universe scenario was an almost universally accepted model, there are still problems, especially for observational astrophysicists who don’t see any evidence of a flat universe, as inflation would predict.

While eternal inflation offers solutions, it presents problems. The most important one involves the unitary or Liouville problem. In classical mechanics, the Liouville Theorem states that if you take a number of event states and evolve them forward in time, you’ll end up with the exact number of states you started out with. No new states are created and none are destroyed. If there are a number of states that qualify as the initial conditions necessary for inflation, eternal inflation would evolve them forward into time, and we’d get a collection of universes that would grow in time. The problem arises that as the collection grows, there will be an increasing identical number of states, which didn’t begin as a single, tiny, inflating patch. So while it might be true that you can generate an infinite number of universes, at the same time, the fraction of such states that actually in a single inflating patch quickly goes to zero.

The final problem is called the holography/complementarity problem, which arose when physicists thought about black hole entropy and proposed horizon complementarity. This idea stated that one observer couldn’t talk about things happening outside their own horizon. In cosmology, it means that things need to be studied locally, one pocket universe at a time, not all of them.

[via Cosmic Variance, images via Nature, University of Notre Dame and Wikipedia]