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Why isn't it possible that gravity is repulsive at long distances?

  1. Jan 29, 2008 #1
    Hello. I'm still only an undergraduate studying physics, so there's still much I don't know. Hence why it's probably likely that I can't figure out the answer to this question myself. Anyway, I've looked into the subject of dark energy a little bit and from what I've seen we still know very little about it. From what we've observed it seems the general thought is that the universe is accelerating in it's expansion. So my question is: How do we know that gravity is not the cause of the expansion? From all the places I've read about people suggesting this idea, I've seen them told that research has shown that gravity can't be the cause. Yet, I've not seen an explanation of why this is the case. I was just hoping someone could give me an explanation of why we know, with very little doubt, that we don't have the equation for gravity wrong. Why it can't be that distances within a galaxy are too small that the "other part" of the equation of gravity is so small it is neglected, but when it comes to intergalactic distances it causes a repulsive force. Like I said before, I am not trying to present this as a theory, but just trying to get a specific explanation of why it is not possible, because I have not yet found one.

    Thank you for your time.
     
  2. jcsd
  3. Jan 29, 2008 #2

    Danger

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    Hi, dude;
    I don't really know much about this stuff, but if you go by the Einsteinian theory of gravity being space-time warpature, then there really isn't any circumstance within our Universe in which it could be negative.
     
  4. Jan 29, 2008 #3

    dst

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    It's quite fun to imagine that we are also the nanoscale of well, something. Could anything actually create gravitational curvature like that? Maybe as in a saddle surface?
     
  5. Jan 29, 2008 #4

    Danger

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    I guess now that 'Broke Back Mountain' is a classic, anything is possible in a saddle...
     
  6. Jan 29, 2008 #5

    f95toli

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    It is definitly possible. It is at least being seriously considered and I don't think it has been ruled out by anyone, but ultimately we need more data before we can say for sure.
    As far as I know we don't have a good explanation for why the expansion is accelerating (it was a VERY surprising discovery), and I don't think it can be explained within the framework of "orthodox" GR.
     
  7. Jan 29, 2008 #6

    Danger

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    Welcome to your education. Nothing can be fully explained within any frameworks that we're intimately familiar with. Let's get on to the scientific approach to things.

    edit: I just realized that I probably came over as sounding like at total ******* on that last post, and that's not what I wanted. Sorry, F95toli. And my further apologies to anyone that I offended with my posts. I'm pretty drunk, so my coherency is in doubt.
     
    Last edited: Jan 29, 2008
  8. Jan 29, 2008 #7

    rbj

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    :smile:

    i dunno, Danger, why they ever stripped you of your Funniest PFer award. you deserve that forever.
     
  9. Jan 29, 2008 #8

    Danger

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    I didn't get 'stripped of it'; someone came along who deserved it more. It was an honour while I had it, but those days are behind me. I don't care who has the ribbon, as long as people keep making me smile. I can't see that PF will ever disappoint me on that level, or for educating me. Dammit, you're my family... oh, go **** off before I start crying...
     
  10. Jan 30, 2008 #9

    rbj

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    you said "****"! oh, my virgin ears!
     
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