Our observed universe is highly non-generic, and in the past it was even more non-generic, or “finely tuned.” One way of describing this state of affairs is to say that the early universe had a very low entropy. … The basic argument is an old one, going back to Roger Penrose in the late 1970′s. The advent of inflation in the early 1980′s seemed to change things — it showed how to get a universe just like ours starting from a tiny region of space dominated by “false vacuum energy.” But a more careful analysis shows that inflation doesn’t really change the underlying problem — sure, you can get our universe if you start in the right state, but that state is even more finely-tuned than the conventional Big Bang beginning. We find that inflation is very unlikely, in the sense that a negligibly small fraction of possible universes experience a period of inflation. On the other hand, our universe is unlikely, by exactly the same criterion. So the observable universe didn’t “just happen”; it is either picked out by some general principle, perhaps something to do with the wave function of the universe, or it’s generated dynamically by some process within a larger multiverse. And inflation might end up playing a crucial role in the story. We don’t know yet, but it’s important to lay out the options to help us find our way.
It’s a very nice paper and Sean’s post is also worth a read. What I didn’t notice before was this comment from Peter Coles:
I remember having a lot of discussions with George Ellis way back in the 90s about this issue. I strongly agree that what inflation does is merely to push the fine-tuning problems back to an earlier epoch where they are effectively under the carpet (or beyond the horizon, if you prefer a different metaphor). In fact we were planning to write a sort of spoof of Galileo’s “Dialogue concerning the Two Chief World Systems” featuring characters with names like “Inflatio” and “Anthropicus” …. but never got around to it.
Dear Peter Coles, Please write that paper!!! I’ve been looking through the inflation literature lately and there seems to be an uncomfortably large portion of it devoted to propaganda, arguing that inflation is inevitable and the only possible solution to the problems of the standard hot big bang. A good example is this exchange of papers (one, two and three), where Hollands and Wald face off against Kofman, Linde, and Mukhanov on the issue of whether inflation can explain the low entropy of our universe. The question of whether inflation can be the last word in cosmology (and initial conditions) is in need of clarification.