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Why does the universe look black?

  1. Sep 13, 2005 #1
    Why is the sky blue? (No pun intended) Also why does the universe look black?
     
  2. jcsd
  3. Sep 13, 2005 #2

    ranger

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    Why is the sky bule
    I'm no expert at this, but I could take a guess as to why the universe is dark. Only about 10% of the universe is visible. The other 90% is made of dark matter. This is matter that cannot be detected from the light which it emits. Since 9/10 of the universe is dark matter, that could probably be why the universe is dark I guess.
     
    Last edited: Sep 13, 2005
  4. Sep 14, 2005 #3

    hellfire

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    The night sky is dark because of the finite age of the universe. In a spatially infinite and eternal universe with an homogeneous distribution of stars (of any density) the night sky would glow, as there is a contribution of infinite stars to the flux of light measured at every point (the flux integral diverges at every point). This is known as Olbers’ paradox. There are other solutions to Olbers’ paradox, but the finite age of the universe is the most important one.
     
    Last edited: Sep 14, 2005
  5. Sep 14, 2005 #4
    actually I would think that the observed expansion resulting in the Hubble volume would be more important- regardless of how old the universe is the light from objects far enough away for expansion to trump c would never reach us so an expanding universe defeats the Olbers' Paradox
     
  6. Sep 14, 2005 #5
    Ok slow down there I don't know that much math. I have a strong interest in math, and I am good at it (at least algebra and geometry) but I'm afraid trigometry and calculus confuses me so far. I have memorized SohCahToa sine = opposite/hypotenuse cosine = adjacent/hypotenuse tangent = opposite/adjacent but even this basic is hard for me, because the theta thing always confuses me.

    Im 17. Please someone explain it in an easier way. I think I know what finite means (not infinite). also how do you know the universe is finite, why isn't it infinite.
     
  7. Sep 15, 2005 #6

    hellfire

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    May bet his helps (from http://homepages.wmich.edu/~korista/bigbang-darksky.html):

    We do not know whether the universe is finite or infinite in space, but we know it had an origin in time (the big-bang). If the universe had an origin in time the light we receive at night can be only due to a finite number of stars (as the speed of light is finite).
     
  8. Sep 15, 2005 #7

    Chronos

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    As I recall, Rayleigh as the first to explain why the daytime sky is blue. Here is a more recent source:

    http://www.sciencemadesimple.com/sky_blue.html

    Olbers paradox [why is the night sky black] is a bit more complicated. The basic answer [courtesy of hellfire] is the universe cannot be both infinitely old and contain an infinite number of stars. setAI's solution using expansion is attractive, but, also runs into similar problems in an infinitely old universe.
     
  9. Sep 15, 2005 #8
    does this mean oneday space will not look black but different colors?
     
  10. Sep 15, 2005 #9

    hellfire

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    The answer depends on the assumptions. If we forget about expansion of space: If the distribution of stars would be always uniform (with the same number of stars per unit of volume), then you are right. However stars will not last forever, star formation in galaxies will stop and all stars will die.
     
  11. Sep 15, 2005 #10

    hellfire

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    The expansion of space alone would be enough to solve Olbers paradox in an infinite universe which expands with constant Hubble parameter. However, I am not sure which one of both (expansion or finite age) actually contributes more to solve Olbers' paradox in our current universe. I guess it should not be difficult to make realistic calculations for the “finite age + no expansion” case, but for the “eternal + expansion” case (de-Sitter model) the flux integral should be difficult and may be without analytic solution (flux depends on redshift and redshift on distance…).
     
    Last edited: Sep 15, 2005
  12. Apr 24, 2006 #11
    It's Not

    The universe is not dark, we are. That is; we don't possess the facilities to perceive all the light that fills the universe. Our eyes are only sensitive to a very small segment of the electromagnetic spectrum, and even at that our perception is very weak. The universe is literally filled with electromagnetic waves. There is no night except in our own perception.
     
  13. Apr 25, 2006 #12

    hellfire

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    The question about darkness of the night sky can be formulated in objective terms independently of our perception. The relevant issue here is the measurable amount of electromagnetic energy flux, as you can read in the posts above.
     
    Last edited: Apr 25, 2006
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