Forgive me if any of my terminology is off I am no physicist but I have wondered about this question for a while.

I have read in various books that if the universe was infinite, and uniformly created the whole sky would be white. This is because in every possible location in the sky there would be a star emitting light. Sort of like pixels, filling a screen.

I thought about this and wondered, doesn't this therefore imply that light is never absorbed by another medium?

What if a distant stars light is absorbed, therefore never reaching earth, giving the illusion that there are less stars?

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    $\begingroup$ This is known as Olber's paradox. It requires the universe to be eternal, infinite, static, and uniform. In that scenario, matter that absorbs light is irrelevant, since it gets heated up by all the light to the point that it's glowing like everything else. See astronomy.stackexchange.com/search?q=olber%27s+paradox $\endgroup$ – PM 2Ring Jan 16 '19 at 16:25
  • $\begingroup$ Hubble's constant does away with Olber's paradox. Visible universe is currently 38 or so billion light years across; farfrom infinite. Everything else is receding faster thanlight. $\endgroup$ – Wayfaring Stranger Jan 17 '19 at 18:57

Olber's paradox describes an infinite, static, eternal, and homogenous universe. If there were a cloud of opaque gas situated in some region of the sky, light coming from all conceivable directions via the paradox would bombard the cloud until either it begins emitting light itself via the radiation it absorbs or its particles scatter to allow light to pass through.


The reason that there isn't stars at every single point of the outlook from the earths surface is because of something called dark energy which is expanding the universe (the main point being that the universe in not shrinking). The further something is from something else the more the expantion effect applies. We can for an example see this because of something called "redshift" which is basically light being stretched to lower wavelengths.

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If something is far enough so the relative expation for us compared to a obj is faster then light we can't see it. The part inside of that range is called the "observable universe". Only in the obeservable universe light can interact with you. This takes out a big chunk of stars making it not being obs-infinite. The stars left are too spread out to cover the entire "sky". Also the look of stars in the observalbe universe varys a lot. The stars feathest away have a very low ratio of photons reaching the earth compered to stars closeby. this is because light disperches is squared.

There are other factors as well like the dispersion of light in the atmoasphear!

Stein aus!

  • $\begingroup$ The sky was dark before anything was known about dark energy. $\endgroup$ – Rob Jeffries Jan 16 '19 at 21:49
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    $\begingroup$ The limit of the observable universe is defined by how far information can have travelled in the age of the Universe. While the distance itself is affected by the scale factor, the physical limit is unaffected by expansion regardless of whether it’s caused by dark matter. $\endgroup$ – Chappo Hasn't Forgotten Monica Jan 17 '19 at 11:18
  • $\begingroup$ Im not saying that dark matter has anything to do with the expaition itelf, that's stupid. Instead im implying that the reason that photons from a long distanse can't reach us becaj $\endgroup$ – Das Stein Jan 17 '19 at 15:49
  • $\begingroup$ U know what if i can't edit after 5 min i woun't bother, either way i wrote dark energy not dark matter. The fastest information transmission without interdimesional shortcuts is the speed of light! $\endgroup$ – Das Stein Jan 17 '19 at 16:00
  • $\begingroup$ “The part inside of that range is called the ‘observable universe’. No, you’re confusing this with the cosmological horizon. And re my earlier comment, yes I meant dark energy. $\endgroup$ – Chappo Hasn't Forgotten Monica Jan 17 '19 at 23:07

Those books have it wrong apparently, and are not up-to-date on astronomy.

We can't even see the stars from the other side of our galaxy. Why? There's just not enough photons coming in from them, they're too faint.

If one would want to place so many stars in every line of sight to fill the sky 'white', either the universe would collapse because those stars would represent so much mass that it couldn't be stable, or time-of-flight effects come into play, if the stars were farther apart.

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    $\begingroup$ The book probably described Olber's paradox which itself relies on a static, homogenous, eternal, and infinite universe. With these conditions, regardless the density of the universe or light-delay involved, the sky would appear white and infinitely hot. $\endgroup$ – Brayden Fox Jan 16 '19 at 20:16
  • $\begingroup$ @BraydenFox: Even then, it only depends on the stellar density of the homogeneous universe whether you can see far away stars or not. My point about you not even seeing the other end of the galaxy stands. $\endgroup$ – AtmosphericPrisonEscape Jan 16 '19 at 21:17
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    $\begingroup$ If, situated behind the span of the galactic disk, there were an infinite number of stars, then an infinite number of photons would reach and subsequently penetrate the galactic disk. Distances do not matter because this posited universe is eternal in age. In every conceivable direction, an observer will find a photon. $\endgroup$ – Brayden Fox Jan 16 '19 at 21:22
  • $\begingroup$ @BraydenFox: That's true for an evening sky as well. If your brightness it not high enough, your eye will not register it. $\endgroup$ – AtmosphericPrisonEscape Jan 16 '19 at 21:24
  • $\begingroup$ I am surprised by the down votes. Many are the causes for what solves the paradox. I don't say there is debate, but surely papers treating this are still coming out. Density of photons is among the above reasons, probably interlocked wit time of existence and expansion. I've stopped thinking about it but I realized that the easy way the paradox is solved is a somehow simplified one. Up voted $\endgroup$ – Alchimista Jan 19 '19 at 9:25

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