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Naive Multiverse Question

  1. Aug 13, 2010 #1
    Please bear with me. I'm by no means well versed in these matters. I'm merely an interested layman. This question might not even make sense, but, if it doesn't, maybe someone could help me understand why.

    What I'm wondering is this. If we posit multiple universes -- not in the quantum sense of co-existing in separate dimensions but within the same space; rather, in the more prosaic sense of multiple universes existing side by side in space within this dimension -- would these universes not be expanding towards each other? And if so, what would happen when they collide?

    As I see it, if our universe is finite, that can only mean that beyond it lie other universes. Separated by vast voids, I suppose. Personally I find it impossible to conceive of how either time or space could be finite, how either could have a beginning or an end. If either does have a beginning or an end, it could only be in some limited sense subjectively important within our humble human perspective.
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  3. Aug 13, 2010 #2
    You're struggling with a very common issue, you have to realize that a 'universe' isn't just the gas/stars/galaxies/people in it, a universe is also the very space-time that those things exist in. If you had all of the stuff of our universe, and then a giant void of empty space, and then all of the stuff of another universe --> it would still be one universe because they exist in the same space-time dimensions.

    Essentially if two things have any way of interacting (e.g. colliding), they must be in the same universe. There are a few ways to get around this, but they just come down to semantics and how you define a 'universe.'

    I appreciate where you're coming from with these ideas, but they aren't really based on any science. There isn't (and fundamentally can't really be) any reason to postulate or expect other universes, except that its an interesting idea (you can also make some provoking statistical arguments).

    We don't know if the universe is finite; but we are quite convinced that it had a temporal beginning, and currently has spatial bounds.
  4. Aug 13, 2010 #3


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    (Warning: This sci-fi use of dimension differs from various technical usages of the same term)

    In the universe we have now, if expansion continues to accelerate, this will mean that parts of the universe will become causally disconnected -- there will be no way to send even a light signal from part A to part B or vice versa.

    Do note that this isn't of the form of a boundary. It's not like there's a wall you can't get past -- it's just that even light isn't "fast enough" to outrun expansion.
  5. Aug 13, 2010 #4
    Thanks so much, both of you, for taking the time to respond to my questions.

    I find it impossible to conceive of how the Big Bang could have been the beginning of time. In order for it to have occurred, there must have been some process building up to it, and processes can only occur in time. So, if there had been no time, the Big Bang never would have happened.

    And, to ask another naive question, if the universe has spatial bounds, what lies beyond? If the answer is simply "nothing", I guess I can sort of grasp that.
  6. Aug 13, 2010 #5
    I find this satisfyingly awe-inspiring; it also gives me some sense of what it might mean for the universe to be spatially bounded.
  7. Aug 13, 2010 #6
    Its impossible to find analogies, and use experienced based intuition in deciphering a necessarily unique phenomenon. There is no reason to think we are capable of truly understanding the big bang (if it occurred). The anthropic principle must be considered in these grand questions, our thinking is heavily influenced by the world around us--e.g. 'processes only occurring in time.' If nothing else, quantum mechanics has shown us that there are times when our intuition and even classical concepts of logic simply don't cut it anymore.

    This is again, the same issue I mentioned before. Just because the universe is finite, doesn't mean there is anything "beyond" it. There isn't space-time outside the universe, for other-stuff to be in.
  8. Aug 13, 2010 #7
    Okay, then what if what we currently think of as "the universe" is one of multiple, or infinite, such entities, each expanding independently? Then this spatially bounded "universe" would exist within a larger, perhaps unbounded, universe. And a collision between two or more of these units would have some sort of effect, perhaps reversing expansion leading to a crunch. Or (and now I suppose I'm really going out on a limb) maybe by the time they collided they would be so attenuated the collision wouldn't be particularly violent and they would simply merge in some way.

    I know this is pure speculation; I guess what I'm wondering is, is there any scientific reason that such things couldn't be true?
  9. Aug 13, 2010 #8
    Yes. You're still applying the concept that universes are bubbles in a greater ocean. If somehow, there are numerous universes, the dimensions/medium in which they mutually-exist is fundamentally different and unrelated to the dimensions we are familiar with. We know from looking around that there are only 3 large spatial dimensions, and one large temporal one. There may be more, small and compact, dimensions; but they couldn't house universes.

    Now, from a semantically (and therefore less appealing and insightful) perspective, if your 'sub-universes' existed in a 'super-universe,' then the 'sub-universes' just wouldn't BE universes anymore. The 'super-universe' would be the only universe. Its worth mentioning, that this is (at least analogous to) what happened at the beginning of the 20th century. We used to think the milky way was the entire universe, only in the 1920s (ish?) did we realize that some of those other "stars" out there were galaxies themselves. But that is very different from them being other universes, they are just other galaxies in the same universe. If using some sort of magical telescope (conceivably some sort of super-ultra-amazing-neutrino telescope) could see beyond the CMB, and it saw something that looked like an entire duplicated of the universe we see around us--we wouldn't call it another universe, it would just be an expansion of universe we know of. On that note, its worth pointing out that the (expected) size of the universe is drastically larger than the size of the universe that we can see, or (expect) to ever be able to see/interact with.
  10. Aug 13, 2010 #9
    It is a rather strange idea that time could have a beginning. But remember that Einstein showed that time is actually part of the fabric of the universe. This is why we speak of spacetime rather than separate the two. We can therefore consider time to be a dimension of our universe. To say that time had a "beginning" simply means that the dimension of time is bounded in the past (or past-incomplete). We might think of time as being like the semi open interval [tex]\left[ 0, \infty \right) [/tex] as opposed to the open (and unbounded) interval [tex]\left( - \infty, \infty\right) [/tex]. Does that help at all?
  11. Aug 13, 2010 #10
    I guess what would make these "bubbles" separate "universes" is that they would be expanding in opposition to one another.

    But I agree that the "super-universe" would simply be the universe. If this idea turned out to be true (ha ha), we might assign a new name to the bubble we'd previously thought of as "the universe", or simply phrase things as you have done and speak of sub-universes within the super-universe.

    All of this helps immensely -- though I confess the terminology of intervals is foreign to me.
  12. Aug 13, 2010 #11
    Happy to help. He just means that we think time to have had a certain 'beginning' (defined as '0') yet could continue on indefinitely (to 'infinity') i.e. from 0 - inf i.e.
    The square bracket means including 0, and the "open bracket" means not including infinity (because you can never include infinity)
  13. Aug 13, 2010 #12
    What if we were able to detect that this "duplicate" was expanding in opposition to our known universe?
    Last edited: Aug 14, 2010
  14. Aug 14, 2010 #13
    CMB = Cosmic Microwave Background.

    You can ignore that part of my question.
  15. Aug 15, 2010 #14
    I was just rereading part of The God Delusion and discovered this is where I originally came across this hypothesis. Dawkins draws the argument from Leonard Susskind, who put it forth in his book The Cosmic Landscape: String Theory and the Illusion of Intelligent Design.

    I gather Susskind, whose work opposes the likes of Lee Smolin and Stephen Hawking himself, is unpopular among astrophysicists. And yet isn't Smolin's hypothesis about daughter universes born of black holes similar in some ways? At least insofar as they both involve a multiverse.
  16. Aug 19, 2010 #15
    We can get rid of this infinite regression of universes within universes by simply conceiving dimensionality as an illusion of the senses. Otherwise the paradox remains and makes our present dimensional perception an impossibility since obviously all other universes would require other universes in which to exist. This conundrum makes an ultimate location at which the laws of loigic break down a necessity.
  17. Aug 20, 2010 #16


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    I would only add that causally connected regions of the universe will never truly become 'disconnected', they will merely redshift into obscurity.
  18. Aug 20, 2010 #17


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    The disconnect is that there will be a time N on the remote observer's watch that will never be seen by the Earthbound observer.

    The reason is not because the light is so dim the Earthbound observer can't make it out -- the reason is because the light will never reach the Earthbound observer at all.
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