early universe not homogeneous

George Jones

It is that thought that because of quantum fluctuations, the early universe was inhomogeneous. Expansion of the universe (including inflation) blew up these quantum fluctuations into the very small temperature variations we observe in the cosmic microwave background, and then into the clusters and superclusters of galaxies that we observe today. The universe expanded by a (linear) factor of (about) [itex]10^{30}[/itex] during inflation and another factor of (about) [itex]10^{30}[/itex] between the end of inflation and now. This means that structure on the order of the Planck length at the start of inflation has now expanded to
[tex]10^{-35}m \times 10^{60} = 10^{25} m = 10^9 light-years[/tex]
This is fascinating stuff. For more details and unanswered questions, see

http://www.physicsforums.com/showthr...66#post1810866.

bcrowell

George, you're making a lot of strong affirmative statements here, but my understanding as a nonspecialist is that inflation still lacks strong empirical confirmation (although it has made some correct predictions), and it also has severe, possibly insoluble, theoretical issues. The other thing that makes me a little uneasy about your post is that it could give the impression that there is some kind of ab initio calculation using inflation that can predict that the inhomogeneity of the CMB should have its observed value of about 10^-4. AFAIK that isn't the case, is it?

Chalnoth

Well, how "confirmed" it is depends upon your point of view. The primary confirmation is that of a nearly scale-invariant spectrum of CMB fluctuations. There are other potential explanations for these, but inflation remains the simplest by far, in particular because it doesn't depend upon most of the details of beyond standard model physics (other explanations tend to assume a particular theory, such as loop quantum cosmology or string theory).

As for its problems, well, I'm pretty sure it doesn't have any in general. Some particular inflation models have severe issues, but not all. One thing to recognize here is that inflation is not one model, but an entire class of models.

bcrowell

There was a recent discussion of this on PF: http://physicsforums.com/showthread.php?p=3214329

Again, I'm not a specialist, but here's a brief summary of my impressions of what some of the problems are with inflation:

It gets rid of some fine-tuning problems, but it creates others. E.g., fine-tuning is required to keep temperature variations from being too big.

Penrose argues that obtaining a flat universe with no inflation is higher in probability than obtaining one with inflation.

Eternal inflation robs the theory of its predictive value. Attempts to fix this by applying a measure don't work, because the choice of measure is ad hoc, has no explanatory value, and is itself in need of explanation.

Chalnoth

As I said, particular models have problems. But inflation isn't a particular model, it's an entire class of models. For instance, de Sitter equilibrium cosmology (described here) appears to circumvent these issues.

bcrowell

I'm perfectly happy to accept the possibility that someone will find a version of inflation that works. But the idea is 30 years old now, and its history is starting to look a lot like the history of string theory. Initially there is a wonderful idea, and then more and more fundamental difficulties start to crop up.

Chalnoth

I don't see your point. Most theorists still consider string theory to be our best bet for a grand unified theory.