Why does the universe look black?

  • #1

Main Question or Discussion Point

Why is the sky blue? (No pun intended) Also why does the universe look black?
 

Answers and Replies

  • #2
ranger
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http://www.sky-watch.com/articles/skyblue.html [Broken]
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.
 
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  • #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 [URL [Broken] paradox[/url]. There are other solutions to Olbers’ paradox, but the finite age of the universe is the most important one.
 
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  • #4
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hellfire said:
This is known as [URL [Broken] paradox[/url]. There are other solutions to Olbers’ paradox, but the finite age of the universe is the most important one.
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
 
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  • #5
hellfire said:
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 [URL [Broken] paradox[/url]. There are other solutions to Olbers’ paradox, but the finite age of the universe is the most important one.
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.
 
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  • #6
hellfire
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May bet his helps (from http://homepages.wmich.edu/~korista/bigbang-darksky.html):

Here is an analogy. Imagine standing in the middle of a large, mature oak forest that itself sits in a vast grassland plain. Let's assume it is late autumn and the trees are leafless. For a given average number of standing trees per acre, the further the forest stretches away from you, the fewer gaps you can see between the trunks of the trees to the plains and horizon outside the forest. For a sufficiently large forest your view to the outside becomes fully blocked, and everywhere you look, as far as you can see, your sight line runs into a tree. You'll agree, too, that denser forests need not be as large in extent before your view to the outside is blocked. Let's call this distance at which your view to the outside just becomes filled with tree trunks the lookout limit. Interchange tree trunks with stars and you have yourself an awfully bright sky.

Obviously, we don't live in a blast furnace. So what's gone wrong? For starters, one or more of our assumptions we (and many others) made is wrong. There is also one more important concept related to this paradox: as you gaze up at the sky, you look out into space and back into time, simply because the speed of light is a finite quantity.
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).
 
  • #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.
 
  • #8
does this mean oneday space will not look black but different colors?
 
  • #9
hellfire
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QuantumTheory said:
does this mean oneday space will not look black but different colors?
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.
 
  • #10
hellfire
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Chronos said:
setAI's solution using expansion is attractive, but, also runs into similar problems in an infinitely old universe.
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…).
 
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  • #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.
 
  • #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.
 
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