So basically, we don't exactly know why the night sky is mostly dark? (At least, that's what I got out of this)
Doesn't sound to me like it's more than a hypothesis, but I could be wrong.
To quote the above article
>The redshift hypothesised in the Big Bang model would by itself explain the darkness of the night sky even if the universe were infinitely old.
Redshift is what explains this. There could be different explanations for the redshift but redshift is 100% an observed phenomena.
It's hard to imagine a lot of things, including that the speed of light is the same for all observers, but that's not particularly relevant. Any discussion of the nature of the universe including the big bang itself is going to involve things that are hard to imagine.
Ope, right you are. Got my history a little mixed up there.
> It's hard to imagine a lot of things, including that the speed of light is the same for all observers, but that's not particularly relevant.
I mean, sure, but if light was infinitely fast it would lack every single quality that might make it redshift. Is it possible? Sure, anything is possible, but you'd need something that isn't a wave and doesn't act like a wave to independently behave exactly like a wave in a single specific circumstance. I'm sure someone could try and construct such a theory, but it would need a lot of epicycles to make it work. If light was infinitely fast, it really wouldn't be light anymore, at least not as we understand it.
ed: I guess relativity does kind of account for redshift in a way that looks the largely the same as it would with the standard Doppler effect combined with a classical electromagnetic wave, but for different reasons. Still, infinite speed is a bigger leap
That being said, rereading the original comment I see that yeah, even if redshifting wasn't tied to relative velocity the fact that distant galaxies are more redshifted absolutely solves Olber's paradox. I short circuited from "observe redshift" to "universe is expanding" due to velocities, but as long as the energy of distant light fades out fast enough obviously it doesn't matter why.
https://en.wikipedia.org/wiki/One-way_speed_of_light
I am not suggesting that light is instantaneous, only that there is nothing inconsistent with it being so, and that consequently trying to reason about these matters on the basis of what appeals to our intuitions is not sufficient to come to any solid understanding of this phenomenon.
>> 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.
I've never really felt like I understood the paradox, since the way people explain it, it sounds to me like they're just denying the idea that an infinite sum can have a finite value.
Like, why couldn't the brightness of the sky in an infinite universe be any value at all, depending on the density?
The argument against an infinite universe that makes sense to me is that it would collapse on itself. But as a thought experiment, the stars could be massless and/or fixed in place.
> To show this, we divide the universe into a series of concentric shells, 1 light year thick. A certain number of stars will be in the shell 1,000,000,000 to 1,000,000,001 light years away. If the universe is homogeneous at a large scale, then there would be four times as many stars in a second shell, which is between 2,000,000,000 and 2,000,000,001 light years away. However, the second shell is twice as far away, so each star in it would appear one quarter as bright as the stars in the first shell. Thus the total light received from the second shell is the same as the total light received from the first shell.
The argument that the universe would collapse in on itself is made using similar math (gravity decreasing with the square of the distance is by no accident the same as brightness) so if you buy one you sorta have to buy the other. Of course, the universe probably is infinite and isn't collapsing in on itself, but that's because of dark energy (OK yes, there are all sorts of universes that obey relativity, and some of them are infinite and not collapsing, but if we live in one of those no one's found the solution that fits our observations. The dominant thinking was that the universe would eventually collapse until we discovered it's actually expanding).
Doesn't it imply the observer has existed forever? Which doesn't seem to me like an obvious assumption.
That's all.
If the universe is infinitely large and old, and light works as well believe it does, then light should come from all directions. That's the entire point of this paradox.
Light as we know it moves without limit. If anything is blocked by it, the blocking object would heat up until it glowed just as brightly.
Black holes say no.
Or, more precisely, an infinite number of stars spread out rather uniformly.
I was just pointing out that the thread keeps referring to ‘infinitely large’ universe. Perhaps it’s just shorthand for ‘infinite number of stars’. No big deal. Obviously, if you had 10 stars in an infinitely large universe, there is no ‘paradox.’