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How/why are pocket universes created during eternal inflation?

  1. Oct 12, 2014 #1
    Hi, I'm looking for an explanation of how eternal inflation leads to the creation of universes in a multiverse.

    I've read papers and watched videos on the topic, but I can't seem to get my head around it. I've heard words like decay, expansion and inflation in the same sentence to explain the phenomenon. This really confuses me, and I'd like to understand the process.

    So, could someone please provide me with an explanation that would make sense to a 10th-11th grader in high school like myself who has only knowledge of basic physics and very basic cosmology?

    Thank You.
  2. jcsd
  3. Oct 12, 2014 #2
    Why "pocket"?
  4. Oct 12, 2014 #3
    I'm no cosmologist, but I haven't seen anyone explain it in a sentence. In fact people use a whole lot of text to describe it. Here is one example, I hope it helps: http://scienceblogs.com/startswithabang/2011/10/28/why-we-think-theres-a-multiver/

    Another resource that describes it (at length) is Susskind's youtube Stanford Lectures in cosmology, highly recommended. (You would want the latest of several for most up to date info.)

    Let me try to excerpt the main parts though.

    During the inflation process, the system that is a local volume of spacetime acts like a ball rolling down a potential well:


    Akin to how people describe harmonic oscillators:

    http://osxs.ch.liv.ac.uk/java/model/physchem/Images/SHO-PES.png [Broken]

    [ http://osxs.ch.liv.ac.uk/java/spectrovibcd1-CE-final.html [Broken] ]

    Secretly, what we have sloppily called inflation comes out of some physics, the simplest such is a quantum field. (That is why the inflation potential has field strength instead of a physical dimension such as bond length on the x axis.) Such fields have quantum fluctuations:


    Some parts of the inflating spacetime we looked at will fluctuate towards lower potential energies and stop inflating earlier. Those are the volumes that make local "pocket" or "bubble" universes.

    Some parts of of the inflating spacetime we looked at will fluctuate towards higher potential energies and remain inflating. As it happens, the remaining volumes will end up larger than the volume we started with, despite some parts dropping out of the process. Hence the process can be named "eternal" inflation.

    I hope that helps!
    Last edited by a moderator: May 7, 2017
  5. Oct 12, 2014 #4


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    During inflation, you can imagine that the universe is filled with a special kind of energy density that causes the universe to accelerate in its expansion. In the classical universe, this energy density would be perfectly uniform across the cosmos. As inflation progresses, this energy density slowly drops, eventually reaching a value too small to support more inflation. When this happens, the universe stops inflating and undergoes a more leisurely decelerated expansion, known as the standard big bang cosmology.

    However, we do not live in a classical universe -- quantum mechanics leads to fluctuations in this energy density across space. In this picture, the energy density still falls as inflation progresses, but it does not do so in a clean uniform fashion across the universe. There will be regions where it is higher, or lower, than average. Quantum fluctuations are usually very tiny; very rarely, however, they can be very large. The effects of these are dramatic: even when the average energy density is high enough to support inflation, there will be rare regions where a large fluctuation has driven the energy density too low to support inflation. This region will stop inflating and subsequently undergo the standard big bang cosmology. Such a region might correspond to our observable universe. As inflation progresses and the average energy density drops, more and more regions of the universe will stop inflating. However, there are other rare parts of the universe where a large quantum fluctuation might result in a region with much larger energy than average. These regions will continue to inflate. Even though these large fluctuations are rare, they result in a space that is inflating and therefore growing exponentially. In any given volume of space, they therefore come to dominate the volume.

    This is the idea behind eternal inflation -- that such regions in which quantum fluctuations have driven the energy higher than average are plentiful. Even though our region of the universe has "dropped out" of the inflationary expansion, there are exponentially many regions that are still inflating. And there always will be. Those regions that stop inflating are sometimes said to "decay" or undergo "bubble nucleation". These terms derive from the physics underlying the inflationary expansion based on phase transitions. I can explain them in more detail if you want.
  6. Oct 12, 2014 #5


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    One issue that eternal inflation suffers, as well as virtually every other multiverse conjecture I know of, is all 'alternative' universes are causally disconnected - which is to say they are observationally inaccessible and unable to affect any other universe. Their existence, therefore, smacks more of faith than empirical evidence. Mathematics is the only tool that appears to lend the concept any credibility. I find this unsatisfactory and eminently worthy of suspicion. I believe many people would agree that not all things mathematical possible are necessarily realized in nature - e.g., Boltzmann brains. See Alan Guth's paper http://arxiv.org/abs/hep-th/0702178, Eternal inflation and its implications, for further discussion. Sean Carrols discussion here http://www.preposterousuniverse.com...boltzmann-brains-and-maybe-eternal-inflation/ might also be of interest.
  7. Oct 12, 2014 #6


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    Chronos, I agree. If, however, we come to find that polynomial "chaotic"-type inflation is consistent with observations, then eternal inflation is much easier to swallow. Sure, we won't have any empirical evidence of the separate Hubble patches, but eternal inflation is a mathematical consequence of a theory that would be otherwise well corroborated.
  8. Oct 12, 2014 #7


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    bapowell, I am totally sympathetic. My point is math is a powerful tool and not to be ignored. I merely think it is like any other tool - it has equal power to be enlighten and deceive. It has led us to discoveries our ancestors never imagined, but, also down more than a few blind alleys. Math, unvetted, reveals possibilities, not reality.
  9. Oct 13, 2014 #8
    Thanks for the explanations, you've helped me a lot. :)
  10. Oct 13, 2014 #9


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    There's no reason for this to count for or against a theory. The observable consequences are what matter. That it has other consequences that aren't observable is irrelevant.
  11. Oct 15, 2014 #10
    There is a lot of physics like that though.

    The insides of black holes are also causally disconnected so the most analogous example. It seems odd to think that the event horizon would encompass nothing - they have a radius and the black hole has a mass, a spin and a charge.

    Other examples I can think of is (the size of) point-like particles, the wavefunction, et cetera et cetera.
  12. Oct 15, 2014 #11
    ...with big difference that concept of a wavefunction is very useful in science while concept of a "pocket universe" is very useful in science fiction
  13. Oct 16, 2014 #12


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    Which examples do you have in mind that have observational tension?
  14. Oct 16, 2014 #13
    ... Most(if not all) of the argument on multiverse is associated with the interpretation of the visual representation of the indeterminate statistical distribution of particle--faraday wave. It would then be treated as multiple state with disconnected realities--Copenhagen. This type of formalism is later used to established a universal interpretation(cosmology) leading to prediction such as Multiverse.

    But nature is tricky. There is also a 'possibility' that QM is deterministic-- that can raise doubt on discontinuity. 'IF' the mimicry of experimental pilot wave theory is directly interpreted or holds consistent. We can assume that such multiple state will have 1 unitary state that appears to be smirred around. The convenient thing about this notion is that we eliminate inconceivable huge inserts. IMO, If the prediction is somewhat dubious like multiverse/MWI and so on. It is only natural to check whether we are interpreting the statistics right. BTW I'm not saying that multiverse or any version of it is not possible since the notion is perfectly reasonable relative to the interpretation. I'm just casting a doubt on the interpretation of the premise.
    Last edited: Oct 16, 2014
  15. Oct 16, 2014 #14
    Well, the concept of inflation is very useful and the predictions that can't be tested is still accepted. "Pocket universes" is very useful in inflation and multiverses are useful in the string theory landscape, FWIW.

    But more than that, such notions doesn't append to the black hole example.
  16. Oct 16, 2014 #15


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    This discussion is not about MWI or multiverse theories arising from possible interpretations of quantum mechanics. The OP is asking specifically about multiverse models within the context of eternal inflation.
  17. Oct 16, 2014 #16
    Sorry for stressing the issue a bit. I'm aware of eternal inflation. We already knew the conventional wisdom(Guth)--as a quantum scalar field w/or out tunneling. Inflation field produces tiny variation of densities from the poking of Quantum fluctuation but instead of slowly rolling down on its potential during inflation. The poke causes the field to go higher than low in eternal fashion-- inflation continues forever. Hence, we get eternal inflation.

    On the side-note. I'm just making an argument of which the basis for all this model comes from the statistical approach of how we interpret quantum fluctuations in the first place. Of course we won't have problems if the fluctuations is considered a standard actual event in time and get along with it on whatever the consequence might be. But what if we have experiment that challenge that approach. Should we just ignore it?
  18. Oct 16, 2014 #17
    The concept of inflation is very useful for cosmological theory of our universe I agree. OTOH, concept of a pocket universe brings nothing of importance and isn't testable per definition. And if you're saying that parts of one completely unproven theory (string theory) allow the existance of a completely esoteric concept of multiverse, than fine, I have no further objections.
  19. Oct 16, 2014 #18


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    Pilot wave theory reduces to MWI with one of the "worlds" labeled as "real". It has all of the exact same predictions as MWI, except that it adds the concept that one of the states is special. All of the other worlds exist within pilot wave theory. They're just not labeled real, as if that somehow makes a difference.

    As I've said a number of times, "I don't like what this theory predicts, therefore it probably isn't true!" is not a valid argument.
  20. Oct 16, 2014 #19
    There is no such "definition". And it would be a wrong definition anyway, since there have been several ways proposed how to test it in the literature. (Say, testing weak anthropicity, bubble collisions, et cetera.)

    I didn't say that. I said that multiverses have found use in the exploration and potential testing of string theory as a physics of branes.

    Here is another instance of making common but false claims: string theory is not "unproven". It has passed tests (say, predicting QCD flux tubes or the correct entropy of black holes), but its prediction has been surpassed by simpler theory (QCD) and postdiction doesn't make it competitive but catching up.

    The correct question to ask is perhaps if it is physical. I'm reminded of energy concepts, that start out in some results of pure mathematics of differential equations and just happens to map to physics. It's even worse than string theory, because the math is without units. ... but that doesn't say that string theory is physics.
    Last edited: Oct 16, 2014
  21. Oct 16, 2014 #20
    For metastable vacuum bubble collisions. Not for pocket universes that remain intact, growing, to constitute "multiverse"
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