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Centre of the Universe?

  1. Oct 4, 2012 #1
    So according to the standard theories of Cosmology, the Universe started with a "Big Bang"
    about 14 thousand million years ago? Yet apparently there's no centre to the expansion it's just the same everywhere? How could that be so? doesn't the whole Universe orbit one point? Even if it is expanding like a balloon, it still has to be a point it expands from? A balloon expands from a point, not necessarily a physical point but it still does? My question is... Does the universe have a centre?
     
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  3. Oct 4, 2012 #2
    When we observe the Universe we see it exapanding equally in all directions. So either we *ARE* at the center or everyone everywhere makes the same observation that they are at the center. The latter is likely the truth accepted by science.
     
  4. Oct 4, 2012 #3

    bapowell

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    The universe in the balloon analogy is the surface of the balloon, which has no center.
     
  5. Oct 4, 2012 #4

    bcrowell

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    We have a FAQ about this: https://www.physicsforums.com/showthread.php?t=506991 [Broken]
     
    Last edited by a moderator: May 6, 2017
  6. Oct 4, 2012 #5
    I would say the universe "does not contain its center" in the same way the rubber part of a balloon does not contain its center. If the universe is all of space and its contents AT A GIVEN MOMENT, then there is no center in the universe.

    But as soon as we consider "spacetime", ALL events, past, present and future must be accommodated. The most intuitive placement of the Big Bang event (singularity) is at the center of spacetime.

    If I consider the entire earth, it has a center of iron. If I consider a flat map of the earth's surface, it does not contain a center. (The iron core appears nowhere on the map.) But that's not the same as saying, "There is no center." or "earth has no center." People will be confused as long as we keep up the pretense (of implicitly considering space limited to a single instant).

    We can't deny time as a dimension as long as we allow for "change" of any sort. Expansion would seem foremost among changes in Cosmology.
     
    Last edited: Oct 4, 2012
  7. Oct 4, 2012 #6
    The Universe does not have a center, meaning that it is either infinite or that if you travel in one direction long enough you will end back at the same point again.
     
  8. Oct 4, 2012 #7

    marcus

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    Dave, I'm glad to see you are concerned about public confusion---to your credit. You will not want to contribute to the confusion (which to some extent has been caused by previous well-intentioned efforts to explain.)

    I think you would be doing people a disservice if you were to talk as if a cosmological singularity actually existed in nature---actually occurred.

    A helpful public outreach essay on that confusion can be gotten by googling "tale of two big bangs".

    Singularities occur in various mathematical models (not just the universe), at points, or along lines, along rings, on cylinders. In the most commonly used singular cosmic model the singularity can be pictured as a 3D front of infinite extent. The model fails along a hypersurface of infinite volume (except that everything including volume fails to be defined.

    So "the singularity" is in no sense a "single point", much less a "center point"! It is a disservice to use language that suggests to people that they picture it thus. It's also a mathematical fiction as far as we know. Often simply taken as a symptom that the model is imperfect and needs to be improved so that the "singularity" failure will not occur.

    One of the most important ways that scientific enterprises advance is by acknowledging what is NOT KNOWN. In other words by adhering or at least trying to adhere to the rule "don't say more than you know"---"everything should be made as simple as possible, but not simpler" :biggrin:. Everybody faces this. It's hard work trying to avoid oversimplification.

    I'm just making general observations here not directed at your posts in particular. We all struggle with this---how to avoid overstatement, oversimplification. Because you always pay for it down the line. And yet what we say must be understandable! It has to connect with the our community's imagination.

    I tend to favor what several others have said about no center because our worst misconception is the "explosion" picture. People are always showing up who imagine expansion cosmology as a big explosion of material from some center point outwards into pre-existing empty space. So they ask perennial questions about what direction the center is and how fast is stuff moving away from the center, could stuff move faster than light? Hasn't all the light outraced the matter of the explosion and escaped?

    So our first job is often to counter that "explosion" misconception. There is no direction in the sky where you can point your finger and say "the expansion began over there".

    Of course you realize that very clearly, Dave, and would never say anything suggesting that.

    But there are also unresolved questions about the 4D model. It's probably a good idea not to imply certainties about a "center" of that as well. That's more than I want to go into in this post.
     
  9. Oct 4, 2012 #8

    Jorrie

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    Isn't "the center of spacetime" a rather meaningless concept? If you insist, I would rather say that it is "here and now", like the usual origin chosen for a spacetime diagram. Space and time stretch from there to + and - infinity. Granted, Minkowski spacetime may not represent the real universe, but neither does the balloon analogy (BA) that you seem to favor; it is actually not a spacetime representation at all.

    But we don't - all our models include time.
     
  10. Oct 4, 2012 #9

    marcus

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    That's a really clear way to put it. Concise too. Thanks! We're talking about the standard cosmic model(s)--the usual topics discussed here involve the models most cosmologists use and that you normally find in the literature.

    That means matter distributed approximately UNIFORMLY throughout all existing space. No edge to the region occupied by matter. No edge to space itself--no "space outside of space".
    These things would just add unnecessary complication to the model and there is no evidence for them so the Ockham simplicity criterion applies. One's always seeking the simplest model with the best fit to observation.

    That's on a 3D level and well-understood by most of the people you see around here.

    There's also a problem with the concept of "center" on a 4D level. You didn't address that in your post, so I'll try to say something (Brian Powell and Ben Crowell may have better ways of putting this or it may be covered in FAQ). I think the problem in that case is that there are different competing ideas.

    In the old classic picture there certainly is no center! speaking in a 4D sense because the model breaks down, blows up, is simply not defined at the start of expansion, which could extend over an infinite 3D hypersurface region according to the old classic picture.
    So that corresponds to nobody's idea of "center" It is not even part of the manifold---the start of expansion is off the spacetime continuum on which the model is defined.

    But there are different ideas of how to fix that, and the professional community has not yet CHOSEN a favorite.

    So I feel as if I can't make any absolute unqualified statement about there being no "center". Most likely it won't be a POINT, in any case. But when some consensus does develop around one of the new models that resolve (get rid of) the singularity then maybe there WILL be something corresponding at least to a "central slice". Or maybe there won't be!

    The central mathematical object might be a central infinite volume 3D slice or a central finite volume 3D hypersphere or torus---I don't think one can say---but there might be a central *something* in whatever consensus cosmic model emerges. Or might not! Who knows? Maybe some new mathematical object will be invented :biggrin: something to replace the classical idea of a manifold (the classical space or spacetime continuum) and that will turn out to be the "center". Or there won't be a center in the new picture. I strongly believe we cannot foresee the future of math physics research---one of the nice things about it is the inherent unpredictability.

    So one can say that in a 4D sense there is no center so far. But there might be, when a consensus emerges around some model which includes the start of expansion in a nonsingular way.
     
    Last edited: Oct 4, 2012
  11. Oct 4, 2012 #10
    So, what you're trying to say is that at this moment there might possibly be a centre but based on the current models of Cosmology, there's no centre?
    Also in reply to Flatland, if the Universe is infinite then wouldn't you be the centre of the universe, wouldn't everyone be the centre of the Universe? If you get where I'm coming from.
     
    Last edited: Oct 4, 2012
  12. Oct 4, 2012 #11

    bapowell

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    In an infinite universe, there are no edges. If you have no edges, how do you define the center? Center of what??
     
  13. Oct 4, 2012 #12

    marcus

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    I think we can rule out the idea of a center POINT. It's pretty clear that will never be a feature of the accepted cosmic model.

    But if you want to enlarge the idea of "center" to include an infinite 3D volume then in a 4D model of space time you could have a "central" 3D volume at the moment of bounce when the universe reaches maximum density.

    That would be "central" in the sense that it would be the spatial slice sandwiched BETWEEN the contracting phase and the expanding phase.

    So that is a feature of one of the cosmic models that is being studied and which people are thinking of ways to extract testable predictions from. That doesn't mean it's right! Just one of those proposed and being worked on by 30 or 40 or so people, peer-review published about, discussed at major conferences etc etc.

    the thing is, there are various COMPETING models that have a "central" spatial slice. Space, at the moment of the bounce does not necessarily have to be infinite volume. It could be a finite 3D volume analogous to the surface of ball or balloon---a 3D hypersphere. the universe collapses until it reaches maximum density in that finite hypersphere volume and then rebounds.

    A hypersphere that is not embedded in any higher D space has no center so there is no center POINT anywhere in spacetime in that picture. But the whole hypersphere itself is "central" because it occurs between the contracting phase and the expanding. It is the universe at the moment when expansion starts.

    I think we can say confidently that neither space nor spacetime has a center POINT. And we do not expect the accepted cosmic model to ever change so radically that it would have a center point. But it is possible some day that mankind's prevailing cosmic model evolves so that it has some central subset or region. I don't think you can rule that possibility out.

    Models like that are in fact being developed and studied. No central point but something like a central spatial slice sandwiched between the contracting and expanding phase.
     
    Last edited: Oct 4, 2012
  14. Oct 5, 2012 #13
    I really don't want to bring more confusion into this thread. But it would be not fair not to mention one of the models (which is very likely to be true, IMHO). Unfortunately, I don't remember who is the author(s) of that idea.

    The sea of false vacuum, infinitely expanding at tremendous speeds, but decaying in some places, creating finite-size bubbles of the true vacuum. Every bubble is the universe, and from the inside, it is homogeneous and infinite, but from the outside, from the multiverse point of view bubble is always finite in size and there is a 'starting point', which can be called, if you want, 'a center of the universe'.
     
  15. Oct 5, 2012 #14
    Hi marcus, I read A Tale of Two Big Bangs, a quick read that anyone here could understand. (The author sounds familiar!) Thanks

    I don’t find I have a problem with either Big Bang interpretation, as a single point which does not yet fit with the laws of physics OR as a relatively compact region in spacetime. I don’t think it changes my assertion that the region of the Big Bang phase in spacetime should be considered central to the 3-sphere of the balloon analogy (BA).

    I'd be happy to suggest an approach to resolving the problem of infinities that would occur at a singular point, but that can’t be done if there is not even considered a possibility of a central region. That is, if you consider the universe to be flat (uncurved), i.e. having no center (not even a possibility of one), then there would be no point in trying to explain how such a center could work.

    We encounter a similar situation with black holes. If I ask an astronomer where the massive black hole (BH) at the center (approximately) of our galaxy is located, I would expect to be given a point location based upon accepted comsmological landmarks. But one might argue that such a point doesn’t really exist. That a BH represents a tear in spacetime, that the locations felt to be inside the event horizon are inaccessible to our existence and that it would be a disservice to speak of them. OK, no problem. I’m perfectly happy to accept the region bounded by the event horizon as the approximate location of the BH. But that does NOT mean the Milky Way is not disk shaped or that we can’t talk about its center.

    I get the impression that you wish to relegate the BA as just an oversimplified teaching aid, and that it can’t represent the structure of spacetime. That might be true. I’m saying, “Please, not so fast.” I believe there is undeveloped value in that model which rises to its own defense as a candidate structure. I will try never to use the word “explosion” regarding the Big Bang, as I simply don’t require it.
    Good! That’s close enough for now. I’ll work up from there in my other thread.
     
  16. Oct 5, 2012 #15
    That is a symmetry loving, mathematical perspective. Quite understandable (I love symmetry too) but…
     
  17. Oct 5, 2012 #16

    russ_watters

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    Why would we? Whenever someone asks this question, they are pretty much always talking about space, not spacetime. I say "pretty much" because the thought that anyone would consider spacetime had never occurred to me, but since it occurred to you, there must be at least one person out there who thinks it is relevant!
     
  18. Oct 5, 2012 #17

    Jorrie

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    The "but..." quote considers a 'toy model' with enough matter and radiation to close the universe spatially - a situation which does not agree with observation.

    I'm not sure where you are heading, but my impression is that you are attempting to justify the balloon analogy as more than a beginner's teaching aid. If so, it may be time to put all the cards on the table so that we can see the whole picture.
     
  19. Oct 5, 2012 #18
    Yo, Phil-eE, How 'bout them E'gils! (I'm in Newtown Sq., Temple U. for post grad.)
    Einstein and Minkowski surely got us thinking of space and time as aspects of the same manifold. It's been quite useful.

    With respect to the balloon analogy (BA), expansion occurs over time. Even if it's possible, I wouldn't want to avoid considering time's structural role as a legitimate separator of events (with space an optional contributor).

    Then again, I could just be assuming that everyone thinks the way I do. I've been wrong about that before.
     
    Last edited: Oct 5, 2012
  20. Oct 7, 2012 #19
    Putting my cards on the table.

    I believe that in hypothetical (n + 0.5) manifolds “space” is inherently closed (spherical) while time is open (outward). For any wishing justification of a such a radial time manifold, I offer a thread here.

    For those OK with a hypothetical expanding-space, radial-time model, I “put my cards on the table” around post #13 here.
     
    Last edited: Oct 7, 2012
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