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To the extent of my understanding, Olber’s paradox states that if the universe was static and homogeneous, we should see a star at every point in the night sky and therefore the night sky should be equally as bright as day.

However, since the night sky is dark and non-uniform, it can be said that the universe is not static and not homogeneous. However, if this was already known, what exactly is the “paradox”? Why isn’t it called Olber’s Observation or something else?

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    $\begingroup$ Its the same paradox as what I asked me in the first grade of low school, when our teacher told as universe is infinite. I asked my self, that would mean there are infinite planets where an infinite subset of them will for sure host living species where again infinite species can run space projects resulting in infinite arrival of alien artifacts/species on our planet. So I just thought for many years he can't be right! I developed my own view of whats the universe and within the last years I realized I was closer what science thinks about the universe, then what he was. '^.^ $\endgroup$
    – Zaibis
    Commented Jul 28, 2017 at 8:54
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    $\begingroup$ Static, homogeneous and INFINITE. These were the known (accepted) facts about the universe at that time. Yet the night sky was mostly dark. The facts can't be wrong (so everybody thought), so what else? $\endgroup$
    – Klaws
    Commented Jul 28, 2017 at 11:07
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    $\begingroup$ @Zaibis infinte planets and infinite subset of them with space does not result in an infinite arrival species, as that last bit is a factor of density. If we assume they're all limited by the speed of light, and the age of the universe, even with infinite space faring aliens we're not guaranteed to see any evidence due to low density. $\endgroup$
    – Mooing Duck
    Commented Jul 29, 2017 at 1:04
  • $\begingroup$ The same holds for Olber paradox $\endgroup$
    – Alchimista
    Commented Aug 3, 2017 at 15:57
  • $\begingroup$ The universe can easily be static, homogeneous and infinite and the night sky still be dark given an observation documented more than 100 years after Olbers posed his "paradox": en.wikipedia.org/wiki/Extinction_(astronomy) $\endgroup$
    – pygosceles
    Commented Mar 6 at 18:13

2 Answers 2

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Olber's Paradox was created at a time before the idea of a finite universe was accepted. (It was thought of in the 1600's). In order to resolve Olber's Paradox, you have to introduce the idea that either the universe had a beginning or it is of finite size. (Note: the solution does not require an expanding universe). So, at the time, it was a paradox. Pretty much all astronomers considered the universe to be static and infinite. Therefore, the fact that their observations didn't fit with what they expected made it a paradox.

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    $\begingroup$ Note. It was first suggested in the 1600s, but not by Obler. When first suggested, the universe was thought to be finite in time, but it was uncertain whether light travelled at an infinite speed. $\endgroup$
    – James K
    Commented Jul 28, 2017 at 6:14
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    $\begingroup$ Obler was born 1758, and published in 1823. Kepler stated the problem in 1610. Descarte seems ambivalent on the speed of light, saying that it moves instantly, but later explaining refraction in terms of the acceleration of light. The first measurement of the speed of light was by Romer in 1676. That the universe had a beginning is Gen 1:1. All these men were Christian. $\endgroup$
    – James K
    Commented Jul 28, 2017 at 10:22
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    $\begingroup$ Just a clarification: to my understanding, as astronomers we have not settled on whether the universe is finite or infinite. We simply don't know (yet), and using current cosmological models, Olber's paradox is resolved in either case. $\endgroup$
    – Néstor
    Commented Jul 31, 2017 at 20:35
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    $\begingroup$ @Néstor Olber's paradox is resolved by the fact that the universe had a beginning. So it doesn't matter whether the universe is finite or infinite in this case. $\endgroup$
    – Phiteros
    Commented Jul 31, 2017 at 21:23
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    $\begingroup$ @FKEinternet Alright, there are multiple problems with your statement, but this answer's comments are not the place to discuss it. $\endgroup$
    – Phiteros
    Commented Aug 3, 2017 at 15:18
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The question has been addressed, but for completeness I should like to remark that the most thorough and readily understandable discussion of Olber's Paradox is that of E. Harrison in his book Cosmology, the Science of the Universe (CUP 2000).

It is also worth remarking that we now know that the sky at night is not in fact dark: the light from the cosmic microwave background (CMB) dominates all other sources of radiation, including the sum total of all the stars. Most of the CMB radiation comes out in the microwave region of the electromagnetic spectrum. This was discussed by M. Longair and R.A. Sunyaev in the journal Astrophysical Letters (vol 4 pp65-70 1969) with the following sketchThe intensity of the radiation received from the sky over all of the electromagnetic spectrum. The dashed lines were tentative estimates when this was first drawn. The flux of the 2.7 CMB is shown as a blackbody dominating all other wavelengths.

Updates on this were published in 1990 by M. Ressell and M. Turner (Comments on Astrophysics, vol 14 p323) and by R. Henry in 1999 (Astrophys. J. Vol 516 pp. L49-L52).

So the sky is not dark at night. The light of the night sky is due to the CMB and not due to stars. The finite lifetime of stars, the finite age of the universe and the cosmic expansion have together reduced the contribution from starlight.

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