Current Status of Eternal Inflation

[ad infinitum]

In 1979 Alan Guth came up with the theory of cosmic inflation. A mechanism by which a proposed field named the inflaton would give rise to a period of massive expansion brought on by quantum fluctuations eventually bringing about the uniform distribution of matter in the universe that we see today. In 2013 it was shown by Andrei Linde that the most popular forms of the inflaton field in the physics community would give rise to the production of a huge amount of singularities making it inconsistent with what we observe today. Is there justifiable evidence today to still believe that eternal inflation is the most likely mechanism

 

[kimbyd]

Could you please link to the 2013 Linde paper you reference here? I'm not sure that conclusion is correct.

From what I can tell, the Planck results do put some significant constraints on possible inflation models, but inflation itself is not a single model: It is an entire class of models, each with different parameters. It's better to think of inflation as a paradigm rather than a single thing. There's a lot of parameter space which is still fully-consistent with the Planck results within inflation. But some specific inflation models are looking much less likely, such as "Natural Inflation". Limiting the model space on inflation is really exciting for inflationary theorists, because if inflation is correct, it provides some limits on which specific models might be correct.

For now, the main observational result is that the cosmological perturbations would be not quite scale-invariant, and observations have borne that out. The main open question that remains is the existence of any tensor perturbations. Measuring that precisely, which probably requires measuring B-mode polarization in the CMB, will place far tighter constraints on possible inflation models.

 

[ad infinitum]

Oops the papers I meant were ones by Anna Ijjas and Paul Steinhardt (2013) as well as one by Kohli and Haslam (2014) which were not written by Linde but were rather a response to Linsde's work over the last decade. While there certainly are still models of inflation consistent with the present data and probably still will be after the upcoming measurements you mentioned, it is my understanding that the remaining models require a great deal of fine-tuning which I had heard is frowned upon it the theroretcal physics community. I believe that inflation was originally propose to deal with explaining the structure of matter in the universe in a way that avoids fine tuning. So I wonder if inflation will become as big of a problem as the very o

 

[kimbyd]

I'm really not sure what you're referring to in these papers. Here are the preprints I could find:
https://arxiv.org/abs/1304.2785 (Ijjas, Steinhardt, Loeb 2013)
https://arxiv.org/abs/1408.2249 (Kohli, Haslam 2014)

I don't see any reference to the production of singularities. The complaints they made in the first paper are:
1) The models which fit the data best have more parameters than those excluded.
2) The models which fit the data tend to require more precise choices for those parameters.
3) The models which fit the data basically require eternal inflation.

Ultimately, I see these kinds of arguments as the kind of thing that are based on a number of assumptions for which it's difficult to come to agreement. For example, Ijjas, Steinhardt, and Loeb argue that the production of eternal inflation strongly disfavors these models, but I would argue that eternal inflation would have a tendency to reduce some of the other problems they mention (such as the resulting inflation tending to be short).

I'd really prefer to wait and see what future observations produce, rather than trying to draw any firm conclusions here. To me, the Planck results are exciting because, for the very first time, they're starting to put some limits on inflation. Unfortunately those results are not precise enough to make those limits very strong, so we can't say much definitively. We really need better observations, particularly of the CMB polarization.

 

[windy miller]

A response to the paper you cite can be found here:
https://arxiv.org/abs/1312.7619
What I find interesting is that both sides of this debate agree that Planck favours models of inflation that are eternal which may undermine common statements that there is no evidence for a multiverse.