I Plateau models of inflation after Planck

[windy miller]

Reading some of the critics and defenders of inflation in various papers, one thing I note they have in common is that both seem to agree the plateau models of inflation are favoured by Planck. According to one of the critics of inflation "The plateau-like potentials selected by Planck2013 are in the class of eternally inflating models, "
Steindhardt et al, https://arxiv.org/abs/1304.2785

Can someone explain how plateu models ensure inflation is eternal?

 

[Bandersnatch]

As far as I understand, if the potential is too steep, the inflation field decays sufficiently fast that quantum fluctuations can not 'bump' it back often enough.
Imagine you have a snapshot of an inflating (comoving) volume of space. To keep inflation eternal, the proportion of that volume in which inflation continues versus the part in which the field decays and inflation ceases must be large enough for the exponentially expanding regions to 'replace' the decaying parts.
A plateau ensures that the scalar field decays slowly, so that in sufficiently large fraction of space quantum fluctuations are not overcome by quick decay of the field.

 

[windy miller]

As far as I understand, if the potential is too steep, the inflation field decays sufficiently fast that quantum fluctuations can not 'bump' it back often enough.
Imagine you have a snapshot of an inflating (comoving) volume of space. To keep inflation eternal, the proportion of that volume in which inflation continues versus the part in which the field decays and inflation ceases must be large enough for the exponentially expanding regions to 'replace' the decaying parts.
A plateau ensures that the scalar field decays slowly, so that in sufficiently large fraction of space quantum fluctuations are not overcome by quick decay of the field.

So given Planck favours Plateau models , something I get from the Planck team, the defenders of inflationary theory ( Guth, Nomura etc ) and the detractors of inflationary theory ( Steinhart, Loeb etc ) why should I not consider the fact that there IS evidence for a multiverse? After all, if the data favours these sort of models and they lead to eternal inflation then we have a multiverse. I am not trying to claim this is a knock down argument for a multiverse or definitive evidence, but it surely means that the evidence is not zero ,as we do often hear.

 

[Bandersnatch]

why should I not consider the fact that there IS evidence for a multiverse?

I think the argument is that while the data clearly disfavours other inflationary models, this doesn't necessarily mean that those that remain are sensible. To use a silly analogy, it might be like when you're trying to guess what kind of monster lives your bed, and everyone agrees that the limited space definitely eliminates anything larger than a goblin, but it doesn't necessarily mean that what remains is a sound theory (there might not be any monster).
I've only skimmed the abstract of the article linked in the OP, but it looks like Steinhardt outlines his arguments as to why what remains might not be a good theory.

 

[Chalnoth]

So given Planck favours Plateau models , something I get from the Planck team, the defenders of inflationary theory ( Guth, Nomura etc ) and the detractors of inflationary theory ( Steinhart, Loeb etc ) why should I not consider the fact that there IS evidence for a multiverse? After all, if the data favours these sort of models and they lead to eternal inflation then we have a multiverse. I am not trying to claim this is a knock down argument for a multiverse or definitive evidence, but it surely means that the evidence is not zero ,as we do often hear.

I don't think eternal inflation says anything one way or another about a multiverse, at least not an interesting one. All that you get from eternal inflation is a huge universe.

What you need to also get a multiverse that is more interesting than just being really big is some mechanism to have different physical laws in different locations. And if you have a mechanism to have different physical laws in different locations, you will have a multiverse regardless of whether inflation is eternal.

I think our best bet for nailing down whether or not physical laws can vary from place to place is more in-depth studies of high-energy physics. Right now, our current best-fit model, the Standard Model, includes spontaneous symmetry breaking. This means that a portion of the results of this model are determined randomly, which is precisely what is needed for a multiverse. Pretty much every proposed model that has been considered to replace the Standard Model includes even more spontaneous symmetry breaking. If we find support for those models, and no evidence of any non-random mechanism to break the symmetries of the higher-energy model, then that's evidence in support of a multiverse where the physical laws are different from place to place.

 

[windy miller]

Of course I guess it depends on how you define the phrase multiverse. I don't think its a phrase that only has one definite meaning. So if we define it as separate space time generated by a different big bang ( and I am defining big bang as what happened once the universe started to expand from a super hot state) then I take that to mean eternal inflation implise a multiverse. This could of course be a very dull multiverse where the constants are all the same. As Chalnoth rightly points out , whether these other pocket universes as Guth calls them have different constants will depend on some deeper theory. But just as the existence of these other space times with their own big bang does not as far I can see.