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Regarding Olbers' paradox, the explanation that the article gives contradicts what I heard elsewhere... I heard that the nighttime sky is dark simply because t

Regarding Olbers' paradox, the explanation that the article gives contradicts what I heard elsewhere... I heard that the nighttime sky is dark simply because the universe is to young for the whole thing to get filled with light, though that will happen eventually. Crap, I can't remember where I read that. Somewhere linked off of HN. Nautil.us?

danbruc 10y ago
This will not happen, the accelerating expansion of the universe will make more and more galaxies end up behind the horizon of our observable universe even though the horizon itself keeps expanding.
- pmoriarty 10y ago
  It would happen if there's a big crunch. Also, a lot of astronomy goes on the assumption that the laws of physics are the same far away as they are near by, and that they don't change over time. It's possible that if either of these two assumptions are incorrect that anything goes. I'm also reminded of Arthur C. Clarke's famous dictum that, "when a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong."
  - danbruc 10y ago
    But the expansion of the universe is accelerating which tells us we are not in a big crunch scenario. And we have probed the laws of physics over very long distances and far into the past by looking at very distant objects and have to date not found any strong evidence for varying laws of physics over space or time. There are for example some claims of variations in the fine structure constant but as far as I can tell those claimed variations may still be explained by experimental errors and are therefore not widely accepted.
    - Rzah 10y ago
      If space is curved rather than flat we could still have a big crunch on the far side.
      - jryan49 10y ago
        I think consensus is that it is flat and we keep finding more and more evidence suggesting so.
        daveloyall 10y ago
        I heard that "if you go straight long enough, you end up where you were", as analogous to walking on the surface of a globe. Are you saying that evidence against that has surfaced?
        cygx 10y ago
        The current assumption of the cosmological standard model is spatial flatness, which is compatible with the observations. In principle, space could still curve back on itself on a large enough scale, but assuming there's no big crunch coming, you'd have to go straight for a longer-than-infinite duration to come back to the place you started from.
        daveloyall 10y ago
        Longer than infinite? What does a big crunch have to do with it? Maybe I don't know what a big crunch is... I currently think the big crunch is the idea that gravity will eventually coalesce all matter into a single point.
        cygx 10y ago
        Longer than infinite? In an expanding universe, objects can be separated by a cosmic horizon. Nevertheless, from the comoving perspective, they may very well still move towards each other - but without ever meeting up. However, if you extended conformal time beyond infinity, they would. That's of course not physically possible, so that particular comment was tongue in cheek.
bottled_poe 10y ago
It's dark because space is not a perfect void. A very small concentration of matter over interstellar distances blocks out the light.
- idlewords 10y ago
  You still need a young-ish universe for that to work, or else the intervening matter heats up and starts glowing itself.
  - lisivka 10y ago
    It's impossible because of laws of thermodynamic. Star can heat it surroundings up to temperature of it surface, and with distance this temperature drops in proportion to square of distance. For example, Sun surface has temperature of 5800K, but it can heat surface at Earth only up to 450K, because surface of sphere at Earth orbit is much much larger than surface of Sun. Interstellar light cannot heat for more than fraction of Kelvin.
    - idlewords 10y ago
      You're forgetting to sum over all the visible stars in the sky, which just brings us back to Olber's Paradox. Think of it another way—if you're in an oven at 5800K, there is no arrangement of matter you can put inside that will not eventually come to thermal equilibrium.
      - lisivka 10y ago
        You're forgetting to divide that sum by distances to these stars in square. Thermal equilibrium inside oven and in few light years outside of oven are two very different things.
nevster 10y ago
Olbers' paradox is based on the assumption of an infinite static universe. Taking that assumption you arrive at the paradox. So therefore the assumption must be wrong - ergo evidence for the big bang model which solves the paradox.
mnw21cam 10y ago
The nighttime sky is filled with light. It's called the microwave background radiation. It's the light from the fiery hot stuff that was in existence in the very early universe, and it has travelled vast distances. It just happens that it has been red-shifted so much that it appears dark to your eyes.
- daveloyall 10y ago
  !

danbruc 10y ago

This will not happen, the accelerating expansion of the universe will make more and more galaxies end up behind the horizon of our observable universe even though the horizon itself keeps expanding.

pmoriarty 10y ago
It would happen if there's a big crunch. Also, a lot of astronomy goes on the assumption that the laws of physics are the same far away as they are near by, and that they don't change over time. It's possible that if either of these two assumptions are incorrect that anything goes. I'm also reminded of Arthur C. Clarke's famous dictum that, "when a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong."
- danbruc 10y ago
  But the expansion of the universe is accelerating which tells us we are not in a big crunch scenario. And we have probed the laws of physics over very long distances and far into the past by looking at very distant objects and have to date not found any strong evidence for varying laws of physics over space or time. There are for example some claims of variations in the fine structure constant but as far as I can tell those claimed variations may still be explained by experimental errors and are therefore not widely accepted.
  - Rzah 10y ago
    If space is curved rather than flat we could still have a big crunch on the far side.
    - jryan49 10y ago
      I think consensus is that it is flat and we keep finding more and more evidence suggesting so.
      - daveloyall 10y ago
        I heard that "if you go straight long enough, you end up where you were", as analogous to walking on the surface of a globe. Are you saying that evidence against that has surfaced?
        cygx 10y ago
        The current assumption of the cosmological standard model is spatial flatness, which is compatible with the observations. In principle, space could still curve back on itself on a large enough scale, but assuming there's no big crunch coming, you'd have to go straight for a longer-than-infinite duration to come back to the place you started from.
        daveloyall 10y ago
        Longer than infinite? What does a big crunch have to do with it? Maybe I don't know what a big crunch is... I currently think the big crunch is the idea that gravity will eventually coalesce all matter into a single point.
        cygx 10y ago
        Longer than infinite? In an expanding universe, objects can be separated by a cosmic horizon. Nevertheless, from the comoving perspective, they may very well still move towards each other - but without ever meeting up. However, if you extended conformal time beyond infinity, they would. That's of course not physically possible, so that particular comment was tongue in cheek.

bottled_poe 10y ago

It's dark because space is not a perfect void. A very small concentration of matter over interstellar distances blocks out the light.

idlewords 10y ago
You still need a young-ish universe for that to work, or else the intervening matter heats up and starts glowing itself.
- lisivka 10y ago
  It's impossible because of laws of thermodynamic. Star can heat it surroundings up to temperature of it surface, and with distance this temperature drops in proportion to square of distance. For example, Sun surface has temperature of 5800K, but it can heat surface at Earth only up to 450K, because surface of sphere at Earth orbit is much much larger than surface of Sun. Interstellar light cannot heat for more than fraction of Kelvin.
  - idlewords 10y ago
    You're forgetting to sum over all the visible stars in the sky, which just brings us back to Olber's Paradox. Think of it another way—if you're in an oven at 5800K, there is no arrangement of matter you can put inside that will not eventually come to thermal equilibrium.
    - lisivka 10y ago
      You're forgetting to divide that sum by distances to these stars in square. Thermal equilibrium inside oven and in few light years outside of oven are two very different things.

nevster 10y ago

Olbers' paradox is based on the assumption of an infinite static universe. Taking that assumption you arrive at the paradox. So therefore the assumption must be wrong - ergo evidence for the big bang model which solves the paradox.

mnw21cam 10y ago

The nighttime sky is filled with light. It's called the microwave background radiation. It's the light from the fiery hot stuff that was in existence in the very early universe, and it has travelled vast distances. It just happens that it has been red-shifted so much that it appears dark to your eyes.

daveloyall 10y ago
!