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What are the Physical effects of the expansion of the Universe

  1. Mar 13, 2012 #1
    1. The problem statement, all variables and given/known data

    I trying to learn general relativity and ran into a few questions while learning about the expansion of the universe.

    I found a really good intuitive explanation of the expansion of the universe here: http://curious.astro.cornell.edu/question.php?number=274

    However, I am still left with a few questions, particularly about this paragraph from the article:

    What exactly does it mean that gravity and atomic forces are stronger than the expansion of the universe? Does this mean that the expansion exerts a small constant force on every particle that forces them away from each other? And if such a small force does indeed exist then shouldn't we be able to find objects on the universe which are at a distance where the force of the expansion of the universe exactly cancels out the force of gravity?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Mar 14, 2012 #2


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    I'm not sure what the exact distance would be. However, galaxies, galactic clusters, and super-clusters seem to hold together.
  4. Mar 14, 2012 #3


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    An analogy that I like is that the effect of "dark energy" on gravitationally bound systems is exactly the same as the effect of an ant pushing on a tank. It's not that the force exerted has a tiny effect, it's that it has no effect at all.
  5. Mar 15, 2012 #4
    I think I understand. So there is a small force. That's the reason one of the possible theories for the end of the universe is that the expansion of space overcomes the force of gravity and electromagnetism cause atoms to rip apart.

    I think they call it "The Big Rip". They're quite fond of repeating that on all those shows about the universe on The History Channel
  6. Mar 15, 2012 #5
    The Force increases in the future because the expansion of the universe is accelerating.
  7. Mar 15, 2012 #6


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    The force does NOT increase as far as is known. Its EFFECTS increase, outside of gravitationally bound systems, because it has a cummulative effect, but inside gravitationally bound systems, there is no evidence that it will ever have any effect. The big rip is total speculation.
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