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Inflation and the mulitverse

  1. Apr 8, 2010 #1
    It seems many forms of inflation, such as eternal inflation and chaotic inflation imply a multiverse, but do they all?
  2. jcsd
  3. Apr 9, 2010 #2


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    No. Some recent models of inflation do not.

    For example just in the past year two Nobel laureates have independently proposed mechanisms/explanations for inflation that do not involve assuming an inflaton field. These are two separate proposals. One by Steven Weinberg and a different one by George Smoot and co-authors. Neither proposed mechanism involves the kind of proliferation that you see in older plans such as eternal and chaotic. They are barebones down to business inflation, without multiverse.

    I haven't seen anything essentially new coming from the multiverse people for quite a while.
    Same with the string Landscape. Seems to have declined in interest and been put on back burner.

    Evidently there's a growing interest in getting the effects of inflation with a bare minimum of assumptions. No elaborate fairytales, as little exotic as possible. As I mentioned several recent ideas dispense with an inflaton---which is a type of field that has not yet been observed: a bit on the exotic side. Why make up such a thing if you can get away without assuming it? I'd say that is how the pendulum is swinging at the moment, against multiverse and such. Keep in mind though, it could swing back some time in future!

    Ashtekar has a new inflation model. No multiverse, but unlike Weinberg and Smoot's it does involve an inflaton field.

    You can find these guys' new inflation papers online just by doing an arxiv search

    AFAICS the newer inflation people are not ruling out multiverses, just ignoring them. They don't need to assume them so have no need to waste time thinking or talking about them. A kind of "Occam's razor" thing. The scientific version of "keep it simple--if you don't observe it and do not have a logical need for it, then don't worry about it."

    I'm not talking about pop-sci and multiverse hype. That's more media less actual science.
    Last edited: Apr 9, 2010
  4. Apr 9, 2010 #3


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    Er, well, just because those presenting the various inflation models don't mention them doesn't mean that the theories don't imply such a thing. Dropping the inflation doesn't get rid of the myriad of other reasons to believe that there is a multiverse in the sense of disconnected regions with different low-energy physics.
  5. Apr 9, 2010 #4


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    Eternal inflation (when defined loosely) strikes me as the opposite of what Marcus says. It is the model with the least amount of assumptions.

    From the beginning, it was clear that once you create a mechanism to start inflation (usually described by an inflaton field but possibly something more elaborate), it is in some sense generic.

    Being generic is good of course, b/c it means that its plausible that such a thing took place for our universe and we don't have to worry about invoking the anthropic principle to explain why it in fact did. However its not good in the sense that you have to answer the question, if inflation started here, why can it not start at some other causally seperated place?

    From there, one way or the other you end up with a multiverse.

    No one, to my knowledge, has ever figured out a completely satisfying answer to get around that (the papers listed above do not even address it, and are decidedly not precluding the multiverse concept at all).
  6. Apr 9, 2010 #5


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    Even within models of inflation that utilize an inflaton field (as opposed to the entropic model or that of Weinberg), one finds different predictions for eternal inflation. As the OP mentioned, Linde's chaotic inflation is eternal, as are many 'large field' models in which the inflaton is initially displaced from the origin by a distance of order the Planck scale. However, the other popular set-up, generally termed 'new inflation' behaves rather differently. Inflation arising from fields that resemble those governing spontaneous symmetry breaking are good examples of new inflation, and these are all typically 'small field' models -- the field starts at the top of the Mexican hat and rolls down. There has historically been much discussion about just how natural this is: why does the field prefer to sit at the top as opposed to any other piece of the potential? If the field is initially in thermal equilibrium with the rest of the universe before inflation, then symmetry restoration can stabilize the inflaton about the point of symmetry, thereby providing a reason for why it should be at the top of the hill later. However, there's little, if any, reason to suppose that the inflaton was every in thermal equilibrium with anything else in the universe due to its supremely weak coupling.

    So, some field theoretic models can be proven eternal, while others can not.
  7. May 23, 2010 #6
    Somewhat related to OP's question - are there any models of inflation (if the curvature of the universe turns out to be perfectly flat) which propose a finite multiverse, as opposed to an infinite one?
  8. May 23, 2010 #7


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    Well, first of all, inflationary models typically don't consider the universe to be perfectly flat. Inflation makes the universe flat, so this assumption is completely unnecessary. That said, I believe that the generic physical models of inflation tend to propose a finite universe/multiverse. This universe/multiverse may extend infinitely into the future, but has finite volume for any finite time after the start of inflation. I think when people say "infinite" with respect to inflation, they don't mean infinite in a mathematical sense, but instead in the more colloquial "really really big" sense.
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