25 ms·

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

by nusaru 5y ago

mabbo 5y ago
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.
- Kranar 5y ago
  An infinitely large observable universe is not possible, however an infinitely large and infinitely old universe is still possible with dark sky at night.
  - mabbo 5y ago
    Howso? 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.
    - lurquer 5y ago
      'Infinitely large' isn't a prerequisite to the paradox. Rather, it's an infinite number of stars. Or, more precisely, an infinite number of stars spread out rather uniformly.
      - Kranar 5y ago
        It's not possible to have an infinite number of stars in a finite region. However, you could have a finite number of stars that form a cover over some point.
        lurquer 5y ago
        I agree. 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.’
    - ravi-delia 5y ago
      Rapid expansion looks the same as infinitely large and finitely old, in this case. Our universe, as it turns out, also seems to be expanding in addition to being finitely old. If it were not, the sky would slowly be getting brighter, but it will in fact only get darker from here on out. The space beyond the edge of the observable universe will be forever out of reach, even to light.
    - dragonwriter 5y ago
      > Light as we know it moves without limit. Black holes say no.
      - ravi-delia 5y ago
        Considering gravity decreases at the same rate as brightness, that's a cure that would be significantly worse than the disease. Black holes would need to significantly outnumber stars, and we really don't see that.
ravi-delia 5y ago
We have a fairly firm grasp on exactly why. The universe is finitely old (rather, has looked basically the way it does for a finite length of time) and rapidly expanding. Either could handle the issue alone, but both make sure of it. We are independently confident both in the fact that the early universe looked like a hot, roughly uniform soup and that the universe is currently expanding. The whys and hows are less clear on both fronts, but these at least are physical observations we make. My astrophysics friends inform me that there's lots of debate on the exact timeline and if inflation happened the way that it is often described, but my physics knowledge tends smaller so that's about the limits of my knowledge, at least until someone finds a way to slap a nice God-fearing operator over this whole mess.
bckr 5y ago
> The darkness of the night sky is one of the pieces of evidence for a dynamic universe, such as the Big Bang model. That model explains the observed non-uniformity of brightness by invoking spacetime's expansion, which lengthens the light originating from the Big Bang to microwave levels via a process known as redshift; this microwave radiation background has wavelengths much longer than those of visible light, and so appears dark to the naked eye.
- nusaru 5y ago
  > 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.
  - AnotherGoodName 5y ago
    Well redshift alone explains the paradox. You don't even need to accept the big bang theory. You can just say "stars that are further away are more redshifted" and that alone explains this. Which we know is true. That's not a hypothesis, redshift can be observed directly. And that fact alone explains the apparent paradox. The most distant stars are redshifted away. 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.
    - Kranar 5y ago
      Saying "stars that are further away are more redshifted" is Hubble's law and leads almost directly to the big bang. It alone, however, does not explain the paradox. For example even with red shifting, you could have a bright night sky if there are infinitely many stars and the speed of light is instantaneous.
      - ravi-delia 5y ago
        It doesn't really, no? Physicists were well convinced of the big bang for ages before it popped out that the expansion was speeding up, not slowing down. Also, it's pretty hard to imagine a universe where light travels infinitely fast, but also redshifting still exists.
Aengeuad 5y ago
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.
- vba616 5y ago
  >The paradox itself is that if the universe is both infinite and eternal then we shouldn't have a dark sky 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.

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

by nusaru 5y ago

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.

Kranar 5y ago
An infinitely large observable universe is not possible, however an infinitely large and infinitely old universe is still possible with dark sky at night.
- mabbo 5y ago
  Howso? 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.
  - lurquer 5y ago
    'Infinitely large' isn't a prerequisite to the paradox. Rather, it's an infinite number of stars. Or, more precisely, an infinite number of stars spread out rather uniformly.
    - Kranar 5y ago
      It's not possible to have an infinite number of stars in a finite region. However, you could have a finite number of stars that form a cover over some point.
      - lurquer 5y ago
        I agree. 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.’
  - ravi-delia 5y ago
    Rapid expansion looks the same as infinitely large and finitely old, in this case. Our universe, as it turns out, also seems to be expanding in addition to being finitely old. If it were not, the sky would slowly be getting brighter, but it will in fact only get darker from here on out. The space beyond the edge of the observable universe will be forever out of reach, even to light.
  - dragonwriter 5y ago
    > Light as we know it moves without limit. Black holes say no.
    - ravi-delia 5y ago
      Considering gravity decreases at the same rate as brightness, that's a cure that would be significantly worse than the disease. Black holes would need to significantly outnumber stars, and we really don't see that.

ravi-delia 5y ago

We have a fairly firm grasp on exactly why. The universe is finitely old (rather, has looked basically the way it does for a finite length of time) and rapidly expanding. Either could handle the issue alone, but both make sure of it. We are independently confident both in the fact that the early universe looked like a hot, roughly uniform soup and that the universe is currently expanding. The whys and hows are less clear on both fronts, but these at least are physical observations we make. My astrophysics friends inform me that there's lots of debate on the exact timeline and if inflation happened the way that it is often described, but my physics knowledge tends smaller so that's about the limits of my knowledge, at least until someone finds a way to slap a nice God-fearing operator over this whole mess.

bckr 5y ago

> The darkness of the night sky is one of the pieces of evidence for a dynamic universe, such as the Big Bang model. That model explains the observed non-uniformity of brightness by invoking spacetime's expansion, which lengthens the light originating from the Big Bang to microwave levels via a process known as redshift; this microwave radiation background has wavelengths much longer than those of visible light, and so appears dark to the naked eye.

nusaru 5y ago
> 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.
- AnotherGoodName 5y ago
  Well redshift alone explains the paradox. You don't even need to accept the big bang theory. You can just say "stars that are further away are more redshifted" and that alone explains this. Which we know is true. That's not a hypothesis, redshift can be observed directly. And that fact alone explains the apparent paradox. The most distant stars are redshifted away. 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.
  - Kranar 5y ago
    Saying "stars that are further away are more redshifted" is Hubble's law and leads almost directly to the big bang. It alone, however, does not explain the paradox. For example even with red shifting, you could have a bright night sky if there are infinitely many stars and the speed of light is instantaneous.
    - ravi-delia 5y ago
      It doesn't really, no? Physicists were well convinced of the big bang for ages before it popped out that the expansion was speeding up, not slowing down. Also, it's pretty hard to imagine a universe where light travels infinitely fast, but also redshifting still exists.

Aengeuad 5y ago

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.

vba616 5y ago
>The paradox itself is that if the universe is both infinite and eternal then we shouldn't have a dark sky 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.