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?

• 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. '^.^ – Zaibis Jul 28 '17 at 8:54
• 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? – Klaws Jul 28 '17 at 11:07
• @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. – Mooing Duck Jul 29 '17 at 1:04
• The same holds for Olber paradox – Alchimista Aug 3 '17 at 15:57

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.

• 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. – James K Jul 28 '17 at 6:14
• @JamesK Do you have references to support your assertion? – FKEinternet Jul 28 '17 at 9:44
• 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. – James K Jul 28 '17 at 10:22
• 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. – Néstor Jul 31 '17 at 20:35
• @FKEinternet Alright, there are multiple problems with your statement, but this answer's comments are not the place to discuss it. – Phiteros Aug 3 '17 at 15:18

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 sketch

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.

My answer to your question is mainly not related to physics or cosmology, but to the question what a paradox actually is.

If you think back to several other paradoxes, you see that a paradox can be several different types of arguments. For example, mathematical proofs that end up with $1=2$ may be called paradoxes, and they simply hide the error very well from the casual reader. Paradoxes like Zeno's Turtle are not themselves based on errors, but show up areas of research/knowledge that are simply missing (here: infinite sums), and where applying conventional wisdom just leads to wrong results. Theseus' Ship contains no error at all, and just makes use of unclear definition of seemingly simple words (here: "same").

# Relation to Olber's

So. Asking "what is the paradox in Xyz" assumes that there is some particular "weirdness" going on, or that there is one specific bit in the argument that makes it a paradox, but this does not need to be the case. In Olber's Paradox, the problem was simply that the assumptions (infinite size, homogenous distribution, static (i.e., unmoving, undying) stars) lead to a result which differs from what we see each night. It is a simple Reductio Ad Absurdum, and the term "paradox" here simply means that the result (that the universe cannot be infinitely large) was very unexpected or surprising at the time.

Back then, it was not obvious to everybody that you could challenge whether the infinity or static properties of the universe are really true; hence the grand word "paradox". There could have been the possibility that, as in Zeno's Turtle, all the assumptions would have been actually true, and there was just an additional bit of knowledge in addition that we were missing (which did not turn out to be the case).

# Remarks

You do not need infinite time, so it is reconcilable with christian creation. This is because at that time people did not really know for sure that light moves at finite speed. They had no concept of event horizons and the like. They did not know that stars are born or can die (Thomas Digges interpreted a supernova as a star moving closer and thus starting to be visible). There were strong supporters of "infinite+static", i.e. Digges, Bruno, Galilei.

• It's also German. – Barmar Jul 28 '17 at 15:17
• I was very confused for a minute, thinking there was some new form of the word paradox that I did not know. – zephyr Jul 28 '17 at 15:18