Sean Carroll in Cosmic Variance:
My inaugural column for Discover discussed the lighting-rod topic of the inflationary multiverse. But there’s only so much you can cover in 1500 words, and there are a number of foundational issues regarding inflation that are keeping cosmologists up at night these days. We have a guest post or two coming up that will highlight some of these issues, so I thought it would be useful to lay a little groundwork. (Post title paraphrased from Andrei Linde.)
This summer I helped organize a conference at the Perimeter Institute on Challenges for Early Universe Cosmology. The talks are online here — have a look, there are a number of really good ones, by the established giants of the field as well as by hungry young up-and-comers. There was also one by me, which starts out okay but got a little rushed at the end.
What kinds of challenges for early universe cosmology are we talking about? Paul Steinhardt pointed out an interesting sociological fact: twenty years ago, you had a coterie of theoretical early-universe cosmologists who had come from a particle/field-theory background, almost all of whom thought that the inflationary universe scenario was the right answer to our problems. (For an intro to inflation, see this paper by Alan Guth, or lecture 5 here.) Meanwhile, you had a bunch of working observational astrophysicists, who didn’t see any evidence for a flat universe (as inflation predicts) and weren’t sure there were any other observational predictions, and were consequently extremely skeptical. Nowadays, on the other hand, cosmologists who work closely with data (collecting it or analyzing it) tend to take for granted that inflation is right, and talk about constraining its parameters to ever-higher precision. Among the more abstract theorists, however, doubt has begun to creep in. Inflation, for all its virtues, has some skeletons in the closet. Either we have to exterminate the skeletons, or get a new closet.