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Is matter not expanding together with the rest of the universe?

  1. May 9, 2012 #1
    I was wondering, if the inflation of the universe can be compared to blowing up a baloon with, say, some ink dots on it representing matter, then, as the baloon grows in size, the ink dots should be getting bigger too since they are part of the same fabric. It does not seem to be the case in the real universe...or is it? Is it only the empty space that is expanding and what would we consider empty in this case?
  2. jcsd
  3. May 9, 2012 #2


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    See this:

  4. May 9, 2012 #3
    Thanks phinds, that's a very good link. However, I was thinking about the pennies glued on the balloon analogy. Pennies are extraneous to the balloon objects, i.e. not part of the balloon fabric (hence, they don't change their size during the baloon inflation). Would it mean that gravity is also somehow extraneous to the expansion as it is so effective in keeping objects together? Or would it be matter and its properties that are extraneous?
  5. May 10, 2012 #4
    I was also thinking, in order to stop the expansion of the universe locally (say, in one galaxy), it would require the force of gravity to be equal to that of the expansion... It also means that if hypothetically, someone was in an intergalactic space where gravity would have little effect, we would expect that person to expand rapidly in relation to the objects in the galaxy...???

    Also, I am curious why would gallaxies collide if the universe is expanding? As far as I understand, it happens due to so-to-say local gravity that exists between the two gallaxies...which means the value of the local gravity should be higher than that of the universe expansion (going against the expanstion). Where could we get such a strong force from in a closed system of the universe?

    Please, could anyone explain it to me? I am not a physicist but I am really curious about these things... :)
  6. May 10, 2012 #5


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    Expansion is governed by the Friedmann metric, it is inappropriate to apply it at small scales - like galaxies.
  7. May 10, 2012 #6


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    This is answered in the link.
  8. May 10, 2012 #7


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    Local clusters are gravitationally bound and some galaxies are moving towards each other locally and will eventually collide/merge

    You don't seem to "get" that the force of the expansionion of the universe is STAGGERINGLY weak on local scales. MANY MANY orders of magniture weaker than gravity, to say nothing of the strong nuclear force.
  9. May 10, 2012 #8
    Could you define "local", please? So, the object in the intergalactic space will not expand due to the strong nuclear force?
  10. May 10, 2012 #9


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    "Local" in this context means a gravitationally bound cluster of galaxies.

    That is correct, a ball bearing in intergalactic space will be totally unaffected by the expansion of the universe, other than that everything will keep receeding from it.
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