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Why galaxies are not expanding

  1. Jul 16, 2012 #1
    I'm pretty sure this answer is not known but it always good to double check. Dark energy causes space to expand but the space inside a galaxy does not. Does anyone know why? I'm pretty sure the answer is not known, so I have a guess as to why this might be the case. Particles send out gravitational waves that cause the space around them to attract other particles. When these particles are not present the space is free of this attraction and will therefore expand. In brief, where there are massive objects space contracts or at least does not expand, where there are no massive objects space expands. Not much of an explanation but it's just a conjecture anyway.
     
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  3. Jul 16, 2012 #2
    I imagine it is similar to the reason why a slinky, when held vertically from one end over the grand canyon, will not stretch to reach the canyon floor. Instead it will stretch to the point where the force of gravity equals the spring force of the slinky, and remain in that state indefinitely. Someone please correct me if I am mistaken.
     
  4. Jul 17, 2012 #3

    Ibix

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    I'm not certain that your cosmology is 100% textbook, but the answer to why space does not expand inside galaxies is simple: it does. It's just that gravity is strong enough that the expansion of space does not drag matter along with it. Shovel's comparison is apt; another one would be to place coins, representing galaxies, on a rubber sheet, representing space. Stretch the sheet - why do the coins not stretch too? Because the internal forces holding the coins together are stronger than the frictional forces trying to tear them apart.

    Remember that the expansion of space is very slight. Every second you get an extra kilometre of space spread over about fifty megaparsecs - and our galaxy is only about thirty kiloparsecs across. That means that the expansion of the space under the galaxy is around half a metre per second. In that same second, the Earth has moved around twenty kilometres round the Sun, the speed it has to move not to spiral in. Gravity is much stronger.
     
  5. Jul 17, 2012 #4

    Excellent explanation. I'm really glad I dispelled that illusion. Thanks.
     
  6. Jul 17, 2012 #5

    phinds

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  7. Jul 17, 2012 #6

    Drakkith

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    Well, remember that we cannot observe space itself expanding. Even the concept of "space itself" expanding is nearly nonsensical. Instead, we can only observe the behavior of matter and light within space. If a galaxy isn't expanding I don't think we can say that space IS expanding or ISN'T expanding inside it, only that the expansion process does not cause objects which are bound to recede from each other.
     
  8. Jul 17, 2012 #7

    DaveC426913

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    I always go back to the "pennies on a balloon" analogy.

    Take a half-filled balloon.
    Glue some pennies to it.
    Now blow up the balloon.
    The pennies move away from each other (just like galaxies in the universe).
    But the pennies themselves do not expand. Why not?
    Because the cohesive forces within the penny easily overcome the expansive force of the balloon.
     
  9. Jul 17, 2012 #8

    phinds

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    See post #5
     
  10. Jul 17, 2012 #9

    Chalnoth

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    Well, perhaps the simplest way of describing it is simply to point out that if you have a galaxy sitting in space with nothing around it, it stays stable due to the self-gravity of the stuff that makes up the galaxy. This fact doesn't change when you place the galaxy in an expanding universe, because the gravity of all the other objects in the universe has no net effect on the galaxy.

    In fact, if you go through and carefully calculate what gravity says will happen in an expanding universe where some parts of the universe are more dense than others, you find that the more dense regions become stable while the less dense regions continue expanding. The exact same laws of gravity that govern the expansion say that more dense regions don't expand at all.
     
  11. Jul 17, 2012 #10
    No, dark energy does not cause space to expand. That's a general relativistic effect caused by an even distribution of matter. The solution of the Einstein Field Equations that is homogeneous and expanding is the FRW metric. However, the FRW metric only applies for homogeneous distributions of matter, which a galaxy is not. Instead, the Schwarzschild metric is used inside of a galaxy, where space isn't expanding.

    Dark energy is a different business. It exerts a force at every point in space, just like a vacuum energy (it may or may not be that). It also ha the effect of being a negative pressure. That thing about homogeneous negative pressures is that they 'pile on' to whatever the universe is already doing. If the universe is expanding, dark energy will accelerate it. If it's contracting, it'll accelerate that too.

    So, it has an extremely small presence inside of galaxies, although really negligible because of how weak it is.
     
  12. Jul 17, 2012 #11
    well if dark energy causes the expansion to accelerate, what causes the expansion in the first place? i read in a textbook once that the answer is not know as to what causes space to expand.
     
  13. Jul 17, 2012 #12
    I mentioned it at the beginning of my post. Any homogenous arrangement of matter causes space to expand in general relativity.
     
  14. Jul 17, 2012 #13

    Chalnoth

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    This is down to the initial conditions of our universe. We don't yet know how those initial conditions were set up. There are models, but this is still under investigation. And it is, unfortunately, probably going to be very difficult to get a handle of how the initial conditions for our universe were set up.
     
  15. Jul 17, 2012 #14

    bcrowell

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    This may be of interest: http://www.lightandmatter.com/html_books/genrel/ch08/ch08.html#Section8.2 [Broken] (subsection 8.2.6).

    If absolutely everything expanded at the same rate (atoms, rulers, galaxies, separations between superclusters), then the expansion would be undetectable.

    Actually, bound systems like galaxies and solar systems do expand a little -- just not very much. Numerical examples are given in the link, with references to papers.
     
    Last edited by a moderator: May 6, 2017
  16. Jul 17, 2012 #15

    DaveC426913

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    The article describes the larger effect of pennies moving away from each other, but doesn't really draw an analogy to the pennies themselves not expanding - as an analogy to why galaxies do not expand. Perhaps the article could add something like that.
     
  17. Jul 17, 2012 #16

    Chronos

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    Actually, as Ben noted, even gravitationally bound systems are slightly affected affected by expansion.
     
  18. Jul 18, 2012 #17

    Chalnoth

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    They're slightly affected by dark energy. But bound systems are indeed stable in an expanding universe. The rate of expansion determines how easily systems can become bound systems, however.
     
  19. Jul 18, 2012 #18
  20. Jul 18, 2012 #19

    bcrowell

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    The question isn't whether they're stable, it's whether they expand. The answer is that they do, but only slightly. The link I gave at #14 has footnotes to references, the most relevant being this: http://arxiv.org/abs/astro-ph/9803097v1
     
  21. Jul 18, 2012 #20
    bapowell:
    Not sure what 'expand' means in the stated context, but it seems different than this:

    In another thread Wallace, of these forums says:
    Post #63, thread # 162727:

     
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