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Why is the sky dark at night?

  1. Sep 6, 2010 #1
    I just watched this http://www.youtube.com/watch?v=TFwtJC9_dXs" that said it's because the universe has a finite existence and the light from most of the stars hasn't reached us yet and this would be an argument against an infinitely old universe. Is this correct?
    Last edited by a moderator: Apr 25, 2017
  2. jcsd
  3. Sep 6, 2010 #2


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    No. It's simply because the sun is on the other side of the Earth.
  4. Sep 6, 2010 #3
    Exactly. It wouldn't be so dark at all if your eyes could detect visible light better.

    But seems like a misunderstanding, perhaps a rephrase and better explanation of your question is in order.
  5. Sep 6, 2010 #4

    Doc Al

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    I haven't watched that clip, but I think that's essentially the correct answer to what was called Olber's paradox.
    Last edited by a moderator: Apr 25, 2017
  6. Sep 6, 2010 #5
    Yes, that's what the explanation was in the clip. But is it in conflict with an infinite/eternal universe?
  7. Sep 6, 2010 #6

    Doc Al

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    I'd say so. Perhaps a cosmologist will chime in with more details.
  8. Sep 7, 2010 #7


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    This particular answer to the paradox requires the assumption of a static universe. If we have an expanding universe, light from distant stars is redshifted, and Olber's paradox never occurs. It is the expansion, then, that solves the problem.

    Now, for entirely different reasons, the expansion of the universe requires an expanding universe to have a finite age, because if you extrapolate back in time too far, you end up with a singularity. Nobody expects this singularity to be real, but somewhere between here and the singularity must be some sort of physical process (currently unknown) that generates a region of expanding space-time.
    Last edited by a moderator: Apr 25, 2017
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