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I Ultimate fate of the Universe

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  1. Dec 29, 2016 #1
    Are there any plausible models based on current evidence about the universe which ends in some way and doesn't last forever?

    I am aware that the accelerated expansion leads to eventual heat death for eternity, but are there any other plausible ideas what can happen?
     
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  3. Dec 29, 2016 #2

    phinds

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    It doesn't seem likely since that would contradict the currently understood cosmological model, which as you state is accelerated expansion forever. The "Big Crunch" scenario has been discarded as a viable model.
     
  4. Dec 29, 2016 #3

    Chronos

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    Based on a few unproven [and likely unprovable] assumptions - all matter in the universe may eventually evaporate [don't hold your breathe, it will take an incredibly long time]. The universe will then resemble the state that existed prior to the big bang and could fluctuate itself back into existence.
     
  5. Dec 29, 2016 #4
    Interesting.

    Wouldn't that open the prospect of Boltzmann brains (when the universe resembles the pre-big bang state)?
     
  6. Dec 29, 2016 #5

    phinds

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    Wouldn't it be more appropriate to say "and could then change state ... " ? I mean, it would BE in existence, it just wouldn't have anything much IN it.
     
  7. Dec 29, 2016 #6

    Chronos

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    Observational evidence does not support the case for Boltzmann brains as anything more than a mathematical curiosity. In theory, a tossed die has a non zero probability of coming to rest perfectly balanced on the apex of one corner, but, you could toss dice continuously for billions of years and never observe such an outcome.
     
  8. Dec 29, 2016 #7
    I agree with you and your example is quite good.

    The only problem is the concept of infinity, since BB-s and similar low likelihood events (like tossing a coin million times heads in a row) will eventually come to existence just because of infinity. Or perhaps I am wrong?
     
  9. Dec 29, 2016 #8

    Chronos

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    Division by zero [or multiplication by infinity] turns probabilities into gibberish. We routinely renormalize predictive models to fit observed outcomes. There is a non zero probability you could quantum tunnel right through the walls of a bank vault, but, you might plausibly attribute the crime to a Boltzmann brain. While all things possible can happen, not all things that can happen do happen - nor are they compelled to do so.
     
  10. Dec 30, 2016 #9
    Well...there´s Murphy´s law:smile:
     
  11. Dec 30, 2016 #10
    So it is plausible to assume that a Boltzmann brain will indeed never happen nowhere and we may dismiss its probability?
     
  12. Dec 30, 2016 #11

    jbriggs444

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    If the universe is infinite [and the parts are suitably independent] and is roughly the same everywhere then it is almost certain that a Boltzmann brain will happen somewhere. The "almost certain" part is because the fact that a thing can happen does not make it a logical certainty that it will happen. It only makes it a statistical certainty.
     
  13. Dec 30, 2016 #12
    Isn't it the case, as Chronos mentioned, that using probability in combination with infinity makes no sense? So it makes no sense to say 'anything that can happen eventually will happen'?
     
  14. Dec 30, 2016 #13

    jbriggs444

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    Probability makes sense even with infinite sets. There is a formal way of assigning a probability measure to subsets of infinite sets. For instance, https://en.wikipedia.org/wiki/Measure_(mathematics)
     
  15. Dec 30, 2016 #14
    So in that case, you are implying that there are infinite Boltzmann brains in the far future (since the universe is infinite in time). What would you suggest to get rid of the paradox/absurdity?
     
  16. Dec 30, 2016 #15

    jbriggs444

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    While the universe may be infinite in time, it is not uniform in time. There is no implication that if you wait long enough that a Boltzmann brain will form.

    By contrast, the universe does seem to be uniform in space.

    The intended meaning was that if you consider a universe with infinite spatial extent [with suitable caveats about uniformity and independence], a Boltzmann brain is nearly certain to already exist somewhere. Of course, that somewhere is hideously unlikely to be within our observable universe, since the observable universe is decidedly finite in spatial extent.
     
    Last edited: Dec 30, 2016
  17. Dec 30, 2016 #16
    Ah, my bad, now I understand.

    Thank you. :biggrin:
     
  18. Dec 31, 2016 #17
    But when you say already, what do you mean?
    Each region of space may wait forever and still not yield a BB or another unlikely event if the universe is uniform in time. How is not being uniform in time compatible with being uniform in space in this context?
     
  19. Dec 31, 2016 #18

    jbriggs444

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    By "already" I mean "at a time coordinate earlier than now". To make that meaningful, I am assuming co-moving coordinates.

    We actually observe a universe which is not uniform in time but which is uniform in space. What's the problem?
     
  20. Dec 31, 2016 #19
    But wouldn't that mean that at every moment there are BB-s that are brought into existence simply because at every moment the space is infinite. So going forward in time would mean that actually the number of BBs increases?
     
  21. Dec 31, 2016 #20

    jbriggs444

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    No.

    Yes, with probability one, under the assumptions of a spatially infinite and isotropic universe, the rate at which BB's come into existence is infinite. But the rate at which they go *poof* is also infinite. One might expect the number to decrease rather than increase.

    But the number in existence would be infinite and simply saying "number of BB's decreases" would not be sensible. Instead, one could use a term like "asymptotic density decreases".
     
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