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Center of the Universe (thought experiment)

  1. Feb 25, 2010 #1
    First, this thought experiment uses the conditions that the universe undergoes decelerating expansion and then contraction, as is the Big Crunch theory. **I know this is contrary to current evidence, supernovae surveys etc., but the thought experiment assumes a closed universe which will have a Big Crunch scenario as one of its conditions.

    As the universe undergoes contraction, the super-clusters lump up and galaxies pile closer. So, more super-clusters of galaxies clump together. Imagine there are eventually 3 (or some other small number) of major chunks. You could imagine their centers have ulta-massive black holes which are in the range of billions of galaxies in mass. So, we have 3 major centers of mass now at this near-end stage of the Big Crunch. At this point we can easily see that there is a geometrical center between them, and as they are all that comprise the universe at that time, that center is the center of the universe.

    It would be uncertain if there could have been no center until nearing the end, but one would have to assume that if there can be a center at the end, then there must have been a center throughout the existence. Again, try to take the grounds of the Big Crunch and ignore dark energy.
  2. jcsd
  3. Feb 25, 2010 #2


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    A valiant try! :biggrin:

    But your thought experiment is incompatible with the standard cosmology model (currently our best guess as to how the world actually is.)

    And I don't mean because of "dark energy" or the fact that even without that we would not expect a big crunch.

    It is easy to tweak a few parameters in the standard model and get a big crunch. I'm happen to contemplate that with you for the sake of argument.

    The incompatibility is much more fundamental. Your scenario is incompatible with the idea of space used in cosmology---also with its basic principles.

    You have all the matter collapsing down to a unique point, and that point is surrounded by empty space.
    That is not how big crunch would happen in the standard cosmo model (even if we tweaked parameters to make it crunch).

    If you ran your thought movie backwards you would have a big explosion at one unique point in empty space, with matter flying out from that point, out into empty space.
    That is a common misconception of how the big bang looked.

    Probably you should read the Lineweaver article in the Scientific American called "Misconceptions about the big bang" It is the princeton.edu link in my signature at the end of this post. We've all read it here, those who have been around long enough. It's a good place to start understanding.

    In the standard model matter is approximately uniformly distributed throughout the universe at all times. There is no centerpoint. There are two main cases---either space is infinite with an infinite amount of matter, or space is finite--e.g. a hypersphere, like the 2d surface of a balloon except 3d---without edge or boundary---without any point in the universe that you could call the center.

    If you picture the 2d balloonsurface toy model, all space is the 2d surface. You can imagine the balloon contained in a non-existent 3d room and in that fantasy context the balloon has a center. But that centerpoint is not in space because all space is the 2d surface---all existence is concentrated on the surface of the balloon.

    So if you slowly let the imaginary air out, all space and all the galaxies collapse together, but they do not collapse to any point that is on the balloon surface.

    The universe has no centerpoint that you can point to.

    Like a flat 2d creature living on the balloon surface, you cannot point your finger at the center because all the directions you can point in are directions contained in the (locally) flat 2d surface---like 360 degree compass directions on the earth surface.

    As a flat 2d creature, your finger cannot point out into the room, or in towards the imaginary center. Your whole reality is on the surface.

    That's the situation which you have to be able to extrapolate up to 3d. You have to be able to imagine the analogous thing but in one higher dimension.

    That's the finite case. Then there is the infinite space and infinite matter case I already mentioned. Neither one has a center.

    Cosmologists do not imagine the big bang occurred like an explosion from some point blowing stuff out into empty space, and the big crunch is not imagined as the reverse of that.
  4. Feb 25, 2010 #3
    Thank you Marcus

    That is a lot of reading. I will delve into them. However, I do understand all of these things you pointed out already. I really have done a ton of cosmology reading.

    So, if you could take it as a more serious query; look at it again.

    Say we tweak the parameters of the current model and we assume the finite universe. (I am not thinking that everything expanded from one point and will contract back to that one point, I understand the no-center concept.) Still, if the universe begins contracting and there is a finite amount of material, we must assume that there will be huge clumping and decreasing voids. Now, take the thought that there end up being 3 major clumps in the vastly decreased size of the universe. Everything hasn't been contracting towards a center point; it's all just been contracting and growing smaller. However, now that everything is in these three globs (there will be scattered here and there, not cut and dry, but you are getting my drift), there is a geometrical center. And now they will be gravitationally attracted towards one place.
  5. Feb 26, 2010 #4


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    How about this. You google "wright balloon model" and watch it all the way to the big crunch.
    It's a computer generated animation. It goes thru a big bang to big crunch cycle.

    At every moment the forces pulling any particular galaxy are approximately the same in all directions. So there would not be much clumping. Not like all the matter gathering in 3 big lumps before final collapse.

    It isn't all that easy for things to clump. When galaxies collide they tend to go THROUGH each other. there isn't a lot of viscosity, to dissipate energy. it takes time for one blob to absorb another.

    I've never seen a truly realistic simulation of a big crunch but I think the overall collapse would OUTRACE the coalescence of matter into 3 distinct blobs.
    My guess is that what you would see is qualitatively very close to what you see in wright's balloon model. A sphere of roughly uniform distributed matter shrinking down and the matter getting denser and denser. No any one privileged point anywhere on the surface of the balloon.

    Think about all the momentum and angular momentum, in these colliding clusters of galaxies. Where does it go? Even black holes take time to merge because they somehow have to shed the angular momentum as they spiral in to each other.

    You asked me to imagine. I try to simulate it in my head. I see the whole shebang falling together too fast to allow time for the formation of a small number of concentrated objects.

    In the end there is no space, because the balloon surface shrank down along with the matter. Matter and space are essentially the same thing in Gen Rel. As einstein said in 1915, space has no objective physical existence independent of matter. Spacetime is just the web of relations between events. So when all the matter is collapsed down there is no WHERE there. The idea of a point or location is not relevant. In terms of our presentday space, such a location does not exist.

    Lineweaver's SciAm article is not a lot of reading. Just a few pages with plenty of diagrams and illustration. Don't wimp out. It's an easy read. Do you see the astro.princeton.edu link?
  6. Feb 26, 2010 #5


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    With respect, you really are continuing to have major misunderstandings.

    No, this is not true. If there is a finite amount of material, then you are proposing also a finite amount of space, which can occur if you have positive curvature. This was indeed the usual model before dark energy was discovered. Without dark energy, the "big crunch" occurs if matter density is above the critical limit to reverse the expansion rate, in which case you also have positive curvature and finite amount of matter, and space, in total.

    In the big crunch, in this case, the total amount of space in the universe decreases over time.

    The big crunch does not require "globs", and you certainly would never get just three or four "globs" within the entire universe as it contracts. Your thought experiment is inconsistent the very models of space and time that allow for a big crunch, as marcus has explained.

    Cheers -- sylas

    Added in edit: I crossed replies with Marcus. Yes, the contraction of the universe would far out run the clumping. In fact, I think that in a big crunch model with positive curvature and super-critical mass, there is not enough time for a photon to cross the universe and come back to where it started before the singularity of the crunch. I am not 100% sure on that, however.
    Last edited: Feb 26, 2010
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