In our current best model, the Universe is infinite and ever expanding. In this model, it is thought that every line of sight eventually ends at the photosphere (outermost visible layer) of a star. If so, it may be argued that the night sky should be brilliant - as bright as a typical star. The fact that the night is dark is known as Olbers' Paradox.
Here are some explanations:
1) Light dilutes in strength as distance^2.
2) The dust between the stars blocks the light.
3) The expanding Universe "reddens" the starlight to longer wavelengths, since space expands as the light waves pass through it.
Why are all of these wrong or incomplete? E.g., for number 3, why then is the night sky not brilliant at long wavelengths?
What is the most complete explanation, and what poet found the answer?
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Submitted by Steven Lord
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Solution:
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1) Yes, lights dilutes as 1/d^2, but if every line off sight intersects a star, the brightness of light seen in any direction is unaffected by the distances of the star. This is counter-intuitive, but true, and has to due with the difference between "surface brightness" and "intensity" of light. Suggestion: google wikipedia, "surface brightness". If every line of sight ended in a typical stellar photosphere (3000 K degrees) we would also be heated up to 3000 K and turn into plasma.
Surface brightness is "flux" of light per unit of angular area. Flux (or flux density, if you wish to denote it per unit wavelength) is energy seen through an aperture. To try to make it simple, imagine you had a lot of 3x5" cards that were at 3000K and thus emitting light. (These are our stellar surfaces.) You could place each them at any distance, with all distances different, and, so long as they overlapped to completely fill your field of view, the brightness you see will be the same as if the cards were all equidistant, say, forming a small sphere around your head.
2) To stay in equilibrium, the dust between stars would eventually heat up as hot as the incident stars, to thousands of degrees K, and the problem would persist identically.
3) The reddening due to expanding space does redistribute the stellar energy to longer wavelengths but does not diminish it. The night sky is not brilliant even at the infrared and far infrared wavelengths. So the expansion _does_ cause distant starlight to become infrared light but does not explain why the deep is not brilliant at infrared wavelengths and also why all the dust still doesn't heat up to 3000 K.
The answer is that the Universe is not old enough for the most distant starlight to have reached us. (As one person answered - parts of the Universe beyond 13.7 billion light years are receding from us faster than the speed of light due to the expansion of space.) We can only see outward to 13.7 billion light years and the Universe is (likely) infinite. We cannot yet see the stars located beyond that distance due to the finite speed of light. In fact, due to the expansion we will see less and less each year. This is sometimes called the trend toward "Cosmic Loneliness".
Edgar Allen Poe was one of the first to offer this explanation of not seeing the distant stars in this work:
Eureka: A Prose Poem,” published in 1848:
“Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by the Galaxy–since there could be absolutely no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing that the distance o the invisible background [is] so immense that no ray from it has yet been able to reach us at all.”
links:
YouTube video, website:
youtube http://www.youtube.com/watch?v=gxJ4M7tyLRE
https://cultureofcuriosity.wordpress.com/2012/11/28/olbersparadox/
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