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Big Bang and PreExisting Void?

  1. Aug 31, 2010 #1
    How do we know that the Big Bang did not expand into a preexisting void?

    What is the justification for this knowledge/belief?
     
  2. jcsd
  3. Aug 31, 2010 #2
    The BB didn't expand "into" anything (in the way you're suggesting). Space(-time) itself is what was expanding, not just the matter in it. It doesn't make sense to describe it as expanding "into" something.
     
  4. Sep 1, 2010 #3

    Chalnoth

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    If you examine what a universe would look like "from the outside", so to speak, it would look like a black hole. Because the new universe looks like a black hole from the outside, it is unable to expand into the pre-existing universe. But it has no trouble expanding from the perspective of somebody within the new universe.

    That is, if a new universe was generated from a vacuum fluctuation in an existing universe, it would look like a microscopic black hole that came into existence for a moment, then rapidly evaporated away. One way of looking at it is that the space-time of the new universe "pinches off" from the old one, and almost instantly the new universe is all on its own, with no connection to where it came from.
     
  5. Sep 2, 2010 #4

    Chronos

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    Painting on a canvas is a classical argument - suggesting matter must be superimposed over a preexisting 'space'. There is no valid theoretical or observational evidence supporting that premise.
     
  6. Sep 2, 2010 #5
    lets assume that it did expand into a preexisting space. Then where did that space come from? Another Big Bang? Did that Big Bang expand into a preexisting space?

    At some point there had to be a Big Bang that created space itself.
     
  7. Sep 2, 2010 #6
    If cold is the absence of heat, and dark is the absence of light, why can’t an infinite and eternal space be the absence of matter? To me asking the question where did space come from doesn’t make any more sense than asking where did dark or cold come from. There does not seem to be consensus on this forum as to whether space existed prior to or was created by the bb.

    Okay…back to lurk mode for me.

    Tom
     
  8. Sep 2, 2010 #7
    infinite and eternal space of how many dimensions? 3? why not 1,826,548,356,657 dimensions?
     
    Last edited: Sep 2, 2010
  9. Sep 2, 2010 #8
    And it still may safely be assumed that no information could 'pass' through the Big Bang event. i.e. No matter formation 'instructions' at any scale level, or anything else. Everything that took place after has just been random chance out of perhaps a near infinite range of possibilities?
     
  10. Sep 2, 2010 #9
    well, the big bang itself seems to over-ride a fundamental law in biology where spontaneous generation (eg- a universe [XD]) is impossible, unless being that there can be apparently be another creating force to do it.

    Also in statement, having an infante number of dementions seems to sasify this as well, as in an infante number of universes may have existed previously. Furthering that, with an infante number of dementions, there could be an infante number of different results in each of the other universes created from other "big bangs" occuring in other dementions. Furthering that, having an infante number of dementions with universes ending to an infantle variable rate they'd be destroyied/created.

    ..the big-bang was technically random?


    Or can alternate dementions interact? For example, if universe "A" was being destroyed. While inside universe "A" another random universe was being created. (having an infante number of detementions within a demention) Due to the fact a universe may be random. Now, this new "internal" universe, universe "B" was expanding and universe "A" was being destrioed. Apon Universe "A"'s destruction, universe "B" was able to form fully. Any such that was aware of universe "B" was able to excape to it. And any such unaware of universe "B" was destroied.

    (why does this sound like economics, cancer, and the molting of an animals skin.....?)

    Now in this example, there is only now universe "B" but all the such lone in the destruction of universe "A", does this mean the remniants create a NEW universe??? Kinda like the "seeds" of a tree that was chopped down.

    o_O... (why does this sound religious, philosophical, and strangely like the book "the lord of the flies"...?)


    And due to the fact these lone such are in hyper space, they can expand without any other universe holding them back by those universes forces.


    0_o...
     
    Last edited: Sep 2, 2010
  11. Sep 3, 2010 #10
    Getting back to this for a moment, could it not be that a void surrounded the BB which was not comprised of spacetime but rather was a simple void which had no characteristics or properties? Could multiple universes be "housed", surrounded in a void soup of sorts? It outwardly seems to me that the existance of a void of this sort could never be proven or disproven.

    I love this thread, very interesting!
     
  12. Sep 3, 2010 #11
    In such case there should be a border between 'ordinary' and how you call it 'simple' void. You would face many difficult questions like:

    Why both types of void have the same number of dimensions?
    What happens in places where ordinary space is curved?
    What is a border between 2 types of voids?
    Is it sharp or smooth?
    What happens to an object or particle going thru the edge?
    In any case, go thru all theories (QM, EM, Gravity) and incorporate such type of object there.

    Purely hypotetically, if our spacetime could have edges in space, such edges would emit enourmous quantities of hawking radiation because of the pair production, when 1 component of the pair dissapears behind the edge. Contrary to Black Holes, that radiation is not redshifted by gravity and is EXTREMELY intense. Very soon it curves space because of its enourmous energy density and creates real black hole with a 'normal' event horizon :)
     
  13. Sep 3, 2010 #12
    We don't.

    We can however say that the observations of the universe are inconsistent with any void existing any part of the universe we can observe or which influences the behavior of anything that we can observe.
     
  14. Sep 3, 2010 #13
    Let me rephase that to something stronger. The observations say that for the parts of the universe we have any data of, that the universe is not spreading into a void. If the universe was spreading into a void, you'd see the effects of the void. You don't.
     
  15. Sep 3, 2010 #14
    Not true. Anytime someone "assumes" something, you should ask why we are assuming that.

    Some of the current work in cosmology is to work on models of what the universe might of looked like before cosmic inflation happened and to see if any of the "pre-inflation" universe could have any effect on observable things like the CMB power spectrum.

    This is important because relating theory to observations is what science is about.
     
  16. Sep 3, 2010 #15
    Dmitry67, thanks for your convincing reply, this stuff makes my brain hurt after 74 years of experience with edges and boundaries. One final idiot question:

    Assuming the existence of multiple universes for a moment, it appears that they cannot be separated from our universe by any sort of boundary, like a border fence. Do they then co-exist within our own universe, separated from us by different dimensions or time? If our universe is infinite/flat, where else could they be? (Brain hurts again.)
     
  17. Sep 3, 2010 #16
    Oldfart, our universes our 4 dimensional, so they can coexist in higher dimensional space without any intersections. That hypotetical super-space is called BULK. However, lets wait for TOE to get clearer picture.

    For the classification of types Universes in Multiverse (I, II, II and IV) google Max Tegmark Mathematical Universe Hypotesis.
     
  18. Sep 3, 2010 #17
    Thanks again, Dmitry67! I checked some of Max's stuff, did not understand it, but then, what's new? Is there a book or two out there that sort of gently leads a layman by the hand through our infinite, expanding universe? You can see by inquiries in this thread that some of us have conceptual problems, need help. (Though I fear that the math involved may greatly limit communication between us.)
     
  19. Sep 3, 2010 #18
    What effects would a void have? Wouldn't said void, by definition, be empty of anything that would cause an effect that could be detected?
     
  20. Sep 5, 2010 #19

    Chronos

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    There is a word they use in physics for anything that is entirely uninteractive - nonexistent.
     
  21. Sep 5, 2010 #20
    Dang, Chronos, I've spent at least ten minutes trying to figure out whether your statement is helpful or simple sarcasm...

    OK, lets see if I've got this figured out. Assuming the universe is flat/infinite, it didn't just get infinite one day but always has been, even at the start of the BB, when it was, lets say, the size of a pea. Correct? OK, us folks that have this mental picture of the BB expanding into a void are mentally sitting in this void, outside the BB, and watching this thingy expand, and thinking yep, it's expanding into a void, alright! But this cannot be a correct way of thinking, as this would mean placing the observer beyond infinity. The observer, and the void for that matter, cannot possibly exist beyond infinity. Correct?

    Having made the void impossible, I am now left with only the minor mental problem of stuffing an infinite universe into a pea...
     
  22. Sep 5, 2010 #21

    Chronos

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    It makes more sense if you picture expansion creating its own space as it evolves, and that space is created in the vast gulfs between galactic clusters. Empty space is endowed with a peculiar property called dark energy, and this dark energy is accelerating the expansion of the universe.
     
    Last edited: Sep 5, 2010
  23. Sep 5, 2010 #22

    bcrowell

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    Looking at this thread, I see that the OP has asked a question, 20 answers have been given, the OP hasn't posted again, and virtually all of the answers are not answers to the OP's question. The OP asked, "how do we know X?," and virtually all the answers are "X is true! X is true! Let me explain X to you!"

    Twofish-quant has tried to answer the OP's actual question here. Hurrah!

    I'm not completely satisfied with twofish-quant's answer, however. Maybe it could be improved upon.

    Our observations can only reach out to a certain distance. If all we know is that we don't see a surrounding void, that doesn't seem like a very strong argument to me, since maybe we just don't see the surrounding void because it's too far away.

    Here is a possible alternative approach.

    (1) General relativity has passed a variety of experimental tests, so we think it's probably pretty accurate. (2) We observe that the redshifts of distant supernovae follow a certain dependence on their distance from us. (3) We also observe that the cosmic microwave background has fluctuations on certain angular scales.

    Smart people have tried very hard to find a model that fits 1, 2, and 3. All they've managed to come up with is a particular model in which there is no preexisting void. In fact, the observations fit that model extremely well. On the other hand, nobody has ever found a model *with* a preexisting void that fits 1, 2, and 3. That makes us suspect that there is no preexisting void.

    Another line of attack is that if we only assume 1 above, then the Hawking singularity theorem http://en.wikipedia.org/wiki/Penrose–Hawking_singularity_theorems , plus some relatively crude observations of the present state of the universe, tells us there must have been a singularity in our past. Such a singularity, as described by GR, has features that are incompatible with the idea of an explosion in a preexisting spacetime. In particular, if GR is correct, then timelike world-lines can't be extended backward through that singularity.
     
  24. Sep 5, 2010 #23
    OK, maybe your explanation "makes more sense", but you are running ahead of me. Is there anything in my previous explanation that is incorrect? And why does your explanation make more sense, it doesn't seem to relate to the "void" issue. Duhh...

    But thanks for getting back!
     
  25. Sep 6, 2010 #24
    Quite true. But what we can say is that there are no voids within *X* billion light years of the earth, right now I think the limit of *X* is 60 billion light years of the earth. There could be voids 1.2 trillion light years from year, or not....

    Who is "us"? All I can say is that with our current knowledge of the universe, there are no detectable anisotropies and inhomogeneities within 60 billion light years. There could very will be massive voids 1.2 trillion years out.

    I can also say that when I do my GR calculations I *assume* that there are no voids at all, because it makes the math easier.

    The trouble is that it is known that GR is wrong once you get to planck's length.
     
  26. Sep 6, 2010 #25

    bcrowell

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    I think we should distinguish logically between a large but finite void and the case where there is an infinite, preexisting, asymptotically flat void. The latter is really what the OP was asking about.

    It is certainly easier to do calculations with an assumption of homogeneity and isotropy, but it's also impossible to cut and paste a Big Bang singularity into an infinite, preexisting, asymptotically flat void. A BB singularity, which we know exists because of the Hawking singularity theorem, has geodesic incompleteness, so it can't arise out of a preexisting void.

    True. Everything I'm saying is in the context of classical GR.
     
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