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Olbers' Paradox

en.wikipedia.org

21–30 of 55 posts

Re: Olbers' Paradox

#21
Is there a good explanation as to why this paradox isn't explained by universal expansion? From wikipedia: "To any observer in the universe, it appears that all of space is expanding, and that all but the nearest galaxies (which are bound by gravity) recede at speeds that are proportional to their distance from the observer.";

Wouldn't this imply that, there was probably a time in the distant past when the night sky was saturated brilliantly with light, but as things expand apart, we simply see less-and-less because those infinitely distant galaxies are so far away that their light cannot ever reach us?

Additionally, and maybe related; we can only see 13 odd billion or so lightyears away. The universe could be infinite; there could be a point of light at every "pixel" of the night sky, twenty billion light years away; we simply cannot see that far, as their light cannot travel faster than the rate they're expanding away from us. Is that accurate?

Additionally; space is pretty empty, but not empty. Even ignoring all this, its not unreasonable to think that, over truly universal distances, this matters. We look at the night sky and see nothing; we point a powerful telescope at that same place and see millions of objects, but still darkness between them. Is there an argument against the notion that, maybe, the light from objects even further away was just absorbed naturally before it could get here? Or maybe our telescopes are not sensitive enough?

Additionally; the paradox makes an assumption that space is uniform, but even our naïve observations of the universe prove this to be inaccurate. Its tremendously non uniform; mostly thanks to gravity. Stellar matter is not distributed uniformly; it coalesces into galaxies. Galaxies are not distributed uniformly; they coalesce into galactic neighborhoods, fibers which stretch across the cosmos. There are many regions like the Bootes Void, which contain magnitudes fewer galaxies as we'd expect. There's the Great Attractor, an abnormally massive area which affects the placement of galaxies around it. Maybe on an absolutely, truly, insanely large scale, trillions of lightyears, infinite lightyears, that non-uniformity averages out, but again it comes down to; the universe may be infinitely large, but it doesn't seem to be infinitely old.

Re: Olbers' Paradox

#22
post #21

Is there a good explanation as to why this paradox isn't explained by universal expansion? From wikipedia: "To any observer in the universe, it appears that all of space is expanding, and that all but the nearest galaxies (which are bound by gravity) recede at speeds that are proportional to their distance from the observer."; Wouldn't this imply that, there was probably a time in the distant past when the night sky…

> Is there a good explanation as to why this paradox isn't explained by universal expansion?

It is - as described in the second paragraph of the linked article.

> we can only see 13 odd billion or so lightyears away

The radius of the observable universe is ~45 billion light years, not 13.

> but even our naïve observations of the universe prove this to be inaccurate.

On the contrary; at a cosmic scale it is very uniform. Galaxies are just blips: https://en.wikipedia.org/wiki/Cosmological_principle

Re: Olbers' Paradox

#23
post #21

Is there a good explanation as to why this paradox isn't explained by universal expansion? From wikipedia: "To any observer in the universe, it appears that all of space is expanding, and that all but the nearest galaxies (which are bound by gravity) recede at speeds that are proportional to their distance from the observer."; Wouldn't this imply that, there was probably a time in the distant past when the night sky…

I don't know how all of the factors you bring up interact, but as far as the last point: the universe is indeed uniform and isotropic at a large enough scale (~300 million ly), called, nicely enough, the End of Greatness ( https://en.wikipedia.org/wiki/Observable_universe#End_of_Gre... ).

Re: Olbers' Paradox

#24

Earlier quoted context omitted.

> too. It does act like a radiation shield, exponentially damping the distant starlight. This is wrong, damping is not exponential but to the fourth root.

A root is a fractional exponent; I can see that as a valid description.

Exponentially means that it grows like exp(x).

Re: Olbers' Paradox

#25
Reminds me of the classic Barrington J Bayley short story "the Knights of the Limits"...

> ‘No, she’s quite a way off,’ Corngold said, taking a look at a meter. ‘Roughly a googol olbers.’

> ‘Your gadget can see that far? But good God – how do you find a single object at that distance?’

Re: Olbers' Paradox

#26
post #12

So basically, we don't exactly know why the night sky is mostly dark? (At least, that's what I got out of this)

What we know is that an infinitly old and large universe is not possible given our understanding of physics and the sky being dark at night.

That's all.

Re: Olbers' Paradox

#27
post #21

Is there a good explanation as to why this paradox isn't explained by universal expansion? From wikipedia: "To any observer in the universe, it appears that all of space is expanding, and that all but the nearest galaxies (which are bound by gravity) recede at speeds that are proportional to their distance from the observer."; Wouldn't this imply that, there was probably a time in the distant past when the night sky…

[deleted]

Re: Olbers' Paradox

#28
post #5

When I first encountered this idea it quickly jumped to me that the universe does not appear to be "static", or at least our observation of it is constantly changing. This also plays into homogeneity, and the last assumption of being "infinite" is somewhat irrelevant, because if the first 2 aspects don't hold, being infinite still won't make the sky appear 'filled', at least past the point of our observation. Of cour…

[deleted]

Re: Olbers' Paradox

#29

Earlier quoted context omitted.

A root is a fractional exponent; I can see that as a valid description.

Exponentially means that it grows like exp(x).

Also it's an important distinction, because an exponential will always eventually grow (or shrink, depending on sign) faster than a polynomial, no matter how high order.

Re: Olbers' Paradox

#30
post #12

So basically, we don't exactly know why the night sky is mostly dark? (At least, that's what I got out of this)

The paradox itself is that if the universe is both infinite and eternal then we shouldn't have a dark sky. This is 'trivially' solved by demonstrating that either of those conditions aren't true. The article opts to use the explanation given by Edgar Allan Poe to demonstrate this, which is that the universe has a finite age and the speed of light is finite so there's only a finite amount of observable universe, which gives us a universe which was darker in the past and one that will only get brighter in the future as more of the universe becomes observable. This has some problems of course and the model Poe would have been working with would have been one of a cyclic universe of eternal growth and decay. This leads us to the Big Bang theory.

>> The redshift hypothesised in the Big Bang model [...]

>Doesn't sound to me like it's more than a hypothesis, but I could be wrong.

The way the Big Bang theory resolves the paradox is similar to that of how Poe resolved it, with a finite cap on the age of the universe there's only a finite amount of observable universe, and similar to Poe's explanation it presents a problem in that a younger universe would have been immensely bright. However this new issue is resolved through the explanation of the expansion of space which can be observed through the redshift of distant galaxies.

As we do observe a dark sky we know the hypothesis that led to the paradox can't be true, namely that the universe is both infinite and eternal, so the question is less about why we have a dark sky and more about what possible alternate hypotheses resolve the paradox. While the Big Bang theory is just a theory it's important to remember that proof is reserved for maths, a theory is a hypothesis backed up by observational data. General relativity led to the hypothesis of an expanding universe and this was something that was later observed from redshift measurements and from it we derive the Hubble–Lemaître law, that galaxies are moving away from earth with speeds proportional to their distance, in some cases faster than the speed of light, this alone fully resolves the paradox and crucially the Big Bang theory is not incompatible with this observation.

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