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I Many worlds and cosmological evolution

  1. Mar 20, 2017 #1
    Hello guys,

    I have a question that came up to my mind while thinking about the evolution of our universe and quantum mechanics. It involves cosmology so maybe that was the right spot to post it, and of course the moderators can do it if they feel it's more adequate for the thread, but my opinion is that it's more based on quantum physics so I think this is the right sub-forum.

    There are many plausible cosmological scenarios for the evolution of our universe, big rip, big crunch, heat death etc. It is currently accepted that the heat death is the most likely one. My question is - does the same cosmological evolution and the same end apply to all of the universal wave function, or to say it better - to all Everettian worlds? Will all the worlds thermodynamically end at the same time because of heat death - and there will be no bizzare worlds where civilizations live forever etc.

    Thanks in advance
     
  2. jcsd
  3. Mar 20, 2017 #2
    I do not subscribe to the many world interpretation MWI, but given that MWI exists as most describe it, then all possible universes exist and an infinite number of them (0%) will survive indefinitely - simply because they will be doing highly unlikely things like having black hole that evaporate rapidly, entropy that decreases, etc.

    But not counting the endless supply of such outliers, all universes are likely to suffer the same basic fate.
     
  4. Mar 20, 2017 #3

    PeterDonis

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    We don't know because we don't know what the initial state of the universal wave function was. It could have been a state which only had amplitudes for universes that lead to heat death, or it could have been a state that had amplitudes for all of the other possibilities as well. Or it could even have had amplitudes for possibilities we haven't thought of, or possibilities that don't have any simple classical interpretation at all. We have no way of testing any of these speculations, so there's not much we can say about them.
     
  5. Mar 21, 2017 #4
    But isn't it the case that heat death/big crunch etc. depend exclusively on the cosmological constant and the properties of matter/spacetime - so the initial conditions you mentioned are the same for all the worlds? In other words, how can the initial state be such that the cosmos undergoes different evolution? The only possibility I have in mind is the possibility of a different vacuum which comes to existence in some other branch. Do you have any reference for the claims that different branches can experience different cosmological evolutions?

    Thanks
     
  6. Mar 21, 2017 #5

    Nugatory

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    Only in classical (as opposed to quantum-mechanical) models of the evolution of the universe - you've asked for a quantum-mechanical one, and for that we'd also need to know "the initial wave function of the universe" .
     
  7. Mar 21, 2017 #6
    But what can be quantum-mechanical and yet so influential that the entire evolution of one universe would be different?

    For instance, imagine a world where the cosmological constant is the same as ours, what in the wavefunction of that world would be different than in our world so the world would not end in a heat death?

    In modern cosmology - is it supposed that all the branches undergo the same evolution - or in other words, is anything known about the initial wavefunction?

    Thanks in advance
     
  8. Mar 21, 2017 #7

    Nugatory

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    There's only one wave function in MWI, so it makes no sense to talk about the "wavefunction of that world" as opposed to the wave function of "our world". Read PeterDonis's reply again.
     
  9. Mar 21, 2017 #8
    My bad, the appropriate question to ask is "what would be different in that branch of the universal wavefunction?"
     
  10. Mar 21, 2017 #9

    PeterDonis

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    We don't know, because we don't have a full quantum mechanical theory of the universe. Without such a theory we can't know what the possible initial wave functions for the universe were. We don't even know the correct Hilbert space.

    We don't know because we don't know the universal wave function--or even the Hilbert space of all possible wave functions. See above.
     
  11. Mar 22, 2017 #10
    Is the Hartle- Hawking no boundary proposal one such example of a many worlds theory where the initial wavefunction determines different evolution of the worlds?
     
  12. Mar 22, 2017 #11

    PeterDonis

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    To the extent I can understand it, yes, I think so.
     
  13. Mar 23, 2017 #12
    does MWI mean that every time we performed the double slit experiments.. the universe is duplicated? Do the billions and billions of galaxies got duplicated as well? But information travels at light speed, so how could the galaxy billions of light years away got created the moment we do a double experiment here on earth?
     
  14. Mar 23, 2017 #13

    PeterDonis

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    No. The "splitting" (which is a potentially misleading way of describing the evolution of the wave function in a quantum measurement--see next comment) is a local process.

    Also, it does not "duplicate" anything. There is one wave function. It just happens to be one where, when you try to describe it in classical terms, you have to describe it as containing multiple "branches" in which different results of the measurement occur.

    They don't. But billions of years after the double slit experiment, when information about what the results were reaches an observer in the distant galaxy (in a highly idealized thought experiment where the observer there can see how the experiment here comes out), that observer will "split" as well--one version of him for each way the double slit experiment came out.
     
  15. Mar 23, 2017 #14
    Wow. This makes MWI more plausible. Is this the original Everett formulation or the latter Wallace etc versions?
    We mostly heard of the "excess baggages" of MWI where simple devices (like double slits) can create billions of universes and worlds. So this is a misconception.
     
  16. Mar 23, 2017 #15

    PeterDonis

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    Both. All formulations of the MWI use the same math, and I'm just describing what the math says.
     
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