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Why isn't the pull of gravity neutral on large scales?

  1. Dec 17, 2016 #1
    I'm only an interested layman so please forgive my lack of knowledge but assuming the universe is flat, infinite and the cosmological principle holds then why wouldn't the attractive force between galaxies cancel out?

    If I think of a 2d analogy such as the surface of a sphere, and there were galaxies spread evenly on the surface, then geometrically speaking it would be impossible for everything to move towards each other (or away from each other) unless the size of the 3D sphere changed with time.

    So does the fact that gravity and indeed dark energy produce the results observed mean we must live in a 4 dimensional spacial universe? Or is it correct to view space time as that extra spacial dimension?
     
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  3. Dec 17, 2016 #2

    phinds

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    What makes you think they don't?

    Yes, so? It DOES change with time. See the link in my signature.

    Absolutely not. Why would it?
    What extra dimension? None is needed.
     
  4. Dec 17, 2016 #3

    Drakkith

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    The key is that matter is not distributed uniformly on a local scale. If one piece of matter is even slightly closer to one of its neighbors than to the others, then it will feel a net force in that direction and will accelerate. And it turns out that such non-uniformity definitely existed in the early universe, as all matter existed as an extremely hot, extremely dense plasma.
     
  5. Dec 17, 2016 #4

    Drakkith

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    Gravity isn't canceled. Otherwise galaxies wouldn't form, let alone galaxy clusters and superclusters.
     
  6. Dec 17, 2016 #5
    Newtons theorem. I couldn't understand how it could ignore the effects of gravity outside the shell.
     
  7. Dec 17, 2016 #6
    Yes I sort of understand that. But as mentioned above I still can grasp how newtons theorem would work, as wouldn't it have to be based on matter not being uniformity spread, not an isotopic and homogeneous universe as used in the FRW equation?
     
  8. Dec 17, 2016 #7

    Drakkith

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    Maybe I'm misunderstanding what you're asking. Are you asking why galaxies are attracted to each other, how the universe can expand, or something else?
     
  9. Dec 17, 2016 #8

    phinds

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    Right. "locally" (on cosmological scales) things are lumpy, but overall they are smooth (the Cosmological Principle) and the Milky Way isn't being moved in any one direction by anything other than the Local Group, right?
     
  10. Dec 17, 2016 #9

    Drakkith

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    That's right. Our galaxy is under the influence of gravity from the local group, the virgo cluster and supercluster, and possible a larger structure, but once you get beyond that scale the universe has started to look very smooth and gravity starts to "cancel out".
     
  11. Dec 17, 2016 #10
    Yes, exactly. I can understand how locally things aren't uniform so there could be a strong attraction in a particular direction. That even makes sense on much larger scales, as I would imagine there would always be variation (as per quantum physics). But on larger scales, assuming the cosmological principle then I'm struggling to understand how the universe would 'contract' due to gravity, which is the assumption used in the FRW equation as I understand it.
     
  12. Dec 17, 2016 #11

    Drakkith

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    I'm sorry, @phinds, I may have misunderstood your earlier post. Can we chalk it up to fatigue? I'm actually up way too late and should probably go to bed. o:)

    Oh, I see what you're asking. Unfortunately I'm not an expert on General Relativity, so I can't really answer this.
     
  13. Dec 17, 2016 #12

    phinds

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    No problem.
     
  14. Dec 17, 2016 #13

    phinds

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    I can't answer your question mathematically (it's a good one) but I can tell you that up until quite recently (maybe 15 years ago) it was believe that the universal expansion WOULD at the very least slow down and reach a steady state or more likely go into reverse and cause "the big crunch". BUT ... "dark energy" was discovered and "Einstein's Biggest Blunder" turned out to not be a blunder and the "Cosmological Constant" in his equations is given as one possible explanation for dark energy. So apparently the math does work out in the end.
     
  15. Dec 17, 2016 #14

    Bandersnatch

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    This looks like the crux of the problem. Is the issue with not understanding the derivation of the shell theorem, its conclusions intuitively, or its application to the derivation of Friedmann equations?
     
  16. Dec 17, 2016 #15

    Bandersnatch

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    The first part is correct, the second doesn't follow. It is possible to fully describe an expanding space without having to embed it in a higher dimension. We tend to do that with an expanding 2-sphere as in your example, but it is just a visual aid, not a mathematical necessity. Furthermore, one needs to keep in mind that the universe might be actually flat and infinite, which makes the embedding in a higher dimension no longer useful (so it cannot be time either, as it'd only work in one special group of cases with positive curvature).
     
  17. Dec 17, 2016 #16

    Chronos

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    @rede96 You have essentially restated Mach's principle.
     
  18. Dec 18, 2016 #17
    Yes, that is certainly one area I am struggling to come to grips with. It's not so much the principle, as I accept that, even though I don't fully understand it. It is more how it works when applied simultaneously to all matter in the universe. For example if I choose myself to be in the 'center' and look at the gravitational effects on a star some arbitrary distance away, then I know the gravitational forces on that star is equivalent to all the matter within that shell. However there could be someone else on the other side of that star, an equal distance away, whose shell contains the same amount of matter as my shell. So which way does the star in the middle move?

    Assuming a universe that was perfectly isotropic and homogeneous, as every point in the universe is equally valid, if I choose all points simultaneously then it would seem the gravitational effects would cancel out. Or more accurately there would be equal gravitational forces in all directions.
     
  19. Dec 18, 2016 #18

    Drakkith

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    As stated already, these forces would cancel out when you're looking at the largest scales of the universe. But at local scales, such as within galaxies and galaxy clusters, the forces certainly don't cancel out and you have a net force on the star.
     
  20. Dec 18, 2016 #19

    Bandersnatch

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    Drakkith, the question is not about that, it's about the Newtonian derivation of the Friedmann equations, which has as a feature this apparent 'paradox' the OP is talking about.

    I don't think I can give a good answer to the OP now, though. I'll think about it some more.
     
  21. Dec 18, 2016 #20

    Drakkith

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    My apologies. I just woke up. Rereading this thread a bit more, I see now this was already expanded on up above.
     
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