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Detecting the presence of other universes

  1. May 8, 2015 #1
    In the inflationary Big Bang model, our universe is one of the many pocket universes in the multiverse. How can we detect the existence of the other pocket universes?

    I presumed that all the pocket universes exist in the same physical space. So in principle, they can exert influence on one another, gravitationally for instance. But if the expansion rate of the space between two pocket universes exceeds the speed of light, does that mean that the two pocket universes will cease to have any physical effect on each other? In other words, information about the other pocket universes is available in the past but not in the future. Does this result in a paradox?
     
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  3. May 8, 2015 #2

    Chronos

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    That is a big question and no satisfactory answer currently exists.
     
  4. May 8, 2015 #3

    Drakkith

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    I don't think this is strictly true. Does 'regular' inflation predict the same effects that 'chaotic' inflation does?

    That depends on what you mean by 'multiverse'. See here: http://en.wikipedia.org/wiki/Multiverse#Categories
     
  5. May 8, 2015 #4
    I'm stating the mainstream view, held by most cosmologists as stated by Alan Guth in his lectures:

     
  6. May 8, 2015 #5

    Drakkith

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    In the specific scenario given by Guth, it looks to me like the different 'pocket universes' would indeed exist in the same physical space.
     
  7. May 8, 2015 #6

    Chronos

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    Reguarding causal connection, causality is the gift that keeps on giving. Once a causal link is established it cannot be severed. There are many galaxies [most in fact] that have receded beyond detectability that we still observe. We will not receive any photons they currently emit, but, we will continue to receive photons they emitted prior to the moment they exited the observable universe forever. They will merely redshift beyond detectability at some point in the distant future. It is not unlike a remote observer watching an astronaut fall into a black hole. The hapless fellow will never appear to reach the event horizon, merely redshift into obscurity. Under most multiverse theories, other universes are causally disconnected at the instant they form. A causal disconnect is virtually demanded under any reasonable defiinition of a universe. Some theorists, such as Laura Mersini-Houghton, hypothesize something like a 'surface of last causal connectivity' could remain as a detectable signature.
     
  8. May 8, 2015 #7

    Chalnoth

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    It's highly unlikely that we'll ever be able to find direct evidence. There is some indication that bubble collisions (i.e., collisions with other "pocket universes") in the early universe might be visible in the CMB. But so far there isn't any solid evidence of that occurring.

    Our best bet is to gain a deeper understanding of high-energy physics. Spontaneous symmetry breaking is good indirect evidence of other regions of space which have different low-energy laws of physics.

    Not likely. Even in the many worlds of quantum mechanics, where this is the case, the other worlds are sufficiently isolated that direct evidence is highly unlikely. Other models have the regions with different low-energy physics existing beyond our cosmological horizon where direct detection seems hopeless.
     
  9. May 9, 2015 #8

    PhanthomJay

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    But indirectly, it was hypothesized quite some time ago that dark matter may in fact be gravitational energy 'leaking' into our universe from a parallel brane universe, perhaps from short (Planck scale?) hi frequency gravity waves which , unlike light and other electromagnetic radiation, could theoretically penetrate through the higher order dimensions via black holes. Is this hypothesis still being seriously pursued by the m-theory community? I think Hawking is amongst them ( or was).
     
  10. May 9, 2015 #9
    I think there is some confusion between chaotic inflation and eternal inflation. Even some big name comsologists like George Ellis seem to make this mistake. Eternal inflation leads to a multiverse, chaotic inflation is eternal inflation, but eternal inflation is not necessarily chaotic inflation. See this paper for an explanation:
    http://arxiv.org/pdf/gr-qc/0409055.pdf
    According to Guth, virtually all models of "regular inflation" are eternal. i think there is something of a myth going around that only chaotic inflation leads to a multiverse, this is not right at all.
    see Also Guths paper here:
    http://arxiv.org/abs/hep-th/0702178
    Of course Guth could turn out to be wrong. but I believe that most people that work on inflation do agree.
     
  11. May 9, 2015 #10

    Chalnoth

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    That would be exceedingly difficult to demonstrate. The problem with dark matter stemming from interactions with matter in a closely-located brane is it would be exceedingly difficult to demonstrate this. We could falsify it by detecting a dark matter particle, but if the dark matter particle remains elusive, I'm not sure it would be possible to distinguish between this idea and the WIMP concept, or a number of other, more exotic dark matter ideas.
     
  12. May 12, 2015 #11

    PhanthomJay

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    Thank you Chalnoth for your valued insight.
     
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