4 ms·

> As spacetime expands, energy density decreases, space becomes less light-emitting and more transparent, and light already in transit becomes red-shifted. Our

> As spacetime expands, energy density decreases, space becomes less light-emitting and more transparent, and light already in transit becomes red-shifted. Our field of vision is still technically flooded with light, as it always has been, but by now the red-shift is so extreme that the light is in the radio spectrum. You talk about the radiation generated before 1M years in the universe. This article talked about the light from the existing stars in the universe.

goodside 11y ago
The red shift applies to distant stars too. The important point is that even in the extreme scenario where all stars last forever and you wait an infinite amount of time for light to reach you, you will still never see a fully illuminated sky because the stars are receding much faster than the light propagates due to metric expansion. Saying the sky is dark because 1) the Universe is young or 2) stars are short-lived is just false. The Universe gets darker and darker as it expands, despite there being "more time for light to reach us".
notalaser 11y ago
Yes, it's still incorrect! The complete quote is: > Astronomers have concluded that the universe began some 12 to 15 billion years ago. That means we can only see the part of it that lies within 12 to 15 billion light-years from us. There may be an infinite number of stars beyond that cosmic horizon but we can’t see them because their light has not yet arrived. And the observable part of the universe contains too few stars to fill up the sky with light. (Emphasis mine because that's where the error is: 13 billion year-old light has traveled over a distance that is now much higher than than 13 billion light-years) Potential ambiguity (in cases such as this one) is one of the reasons why using light-years for distances may not be a good idea. (By the way, there are real astronomers arguing about this, too, e.g. http://www.astro.ucla.edu/~wright/Dltt_is_Dumb.html http://www.astro.ucla.edu/~wright/Dltt_is_Dumb.html . So it's not just this nerd ranting on HN). GRB 090423 (admittedly, not a star -- it's a gamma ray burst) happened approximately 13 billion years ago. This means that light traveled for 13 billion years to reach us -- but, by the time it got to us, GRB 090423 was no longer 13 billion light-years from us, because all that space got bigger! GRB 090423 "was" about 9.2 Gpc away from us by the time that light reached us, i.e. about 30 billion light-years. If the light that hits us is 13 billion years old, that object (assuming it still exists) is a lot farther than 13 billion light-years from us now.
- vectorjohn 11y ago
  Sure. But that really is just pedantics. It isn't a surprise to anyone to point out that the source of something that was moving away from us 13Gy ago is now further away than it was. You are essentially just giving those words the least charitable interpretation and then picking on that. It isn't wrong, its just not perfectly unambiguous.
  - notalaser 11y ago
    > It isn't wrong, its just not perfectly unambiguous. Yes, it's perfectly wrong, because those objects are not 13Gly away from us. A light-year really is defined as 300,000 km multiplied by whatever number of seconds there are in a year. The object is not at 13e9 x 300,000 x <a lot of seconds> kilometers from us if it took 13 billion years for the light to reach us. > It isn't a surprise to anyone to point out that the source of something that was moving away from us 13Gy ago is now further away than it was. If I wanted to give words the least charitable interpretation, I could point out that their motion has nothing to do with this -- they would still be farther than 13 Gly away from us even if they would not be moving away relative to the Earth :-).

goodside 11y ago

The red shift applies to distant stars too. The important point is that even in the extreme scenario where all stars last forever and you wait an infinite amount of time for light to reach you, you will still never see a fully illuminated sky because the stars are receding much faster than the light propagates due to metric expansion. Saying the sky is dark because 1) the Universe is young or 2) stars are short-lived is just false. The Universe gets darker and darker as it expands, despite there being "more time for light to reach us".

notalaser 11y ago

Yes, it's still incorrect! The complete quote is: > Astronomers have concluded that the universe began some 12 to 15 billion years ago. That means we can only see the part of it that lies within 12 to 15 billion light-years from us. There may be an infinite number of stars beyond that cosmic horizon but we can’t see them because their light has not yet arrived. And the observable part of the universe contains too few stars to fill up the sky with light. (Emphasis mine because that's where the error is: 13 billion year-old light has traveled over a distance that is now much higher than than 13 billion light-years) Potential ambiguity (in cases such as this one) is one of the reasons why using light-years for distances may not be a good idea. (By the way, there are real astronomers arguing about this, too, e.g. http://www.astro.ucla.edu/~wright/Dltt_is_Dumb.html http://www.astro.ucla.edu/~wright/Dltt_is_Dumb.html . So it's not just this nerd ranting on HN). GRB 090423 (admittedly, not a star -- it's a gamma ray burst) happened approximately 13 billion years ago. This means that light traveled for 13 billion years to reach us -- but, by the time it got to us, GRB 090423 was no longer 13 billion light-years from us, because all that space got bigger! GRB 090423 "was" about 9.2 Gpc away from us by the time that light reached us, i.e. about 30 billion light-years. If the light that hits us is 13 billion years old, that object (assuming it still exists) is a lot farther than 13 billion light-years from us now.

vectorjohn 11y ago
Sure. But that really is just pedantics. It isn't a surprise to anyone to point out that the source of something that was moving away from us 13Gy ago is now further away than it was. You are essentially just giving those words the least charitable interpretation and then picking on that. It isn't wrong, its just not perfectly unambiguous.
- notalaser 11y ago
  > It isn't wrong, its just not perfectly unambiguous. Yes, it's perfectly wrong, because those objects are not 13Gly away from us. A light-year really is defined as 300,000 km multiplied by whatever number of seconds there are in a year. The object is not at 13e9 x 300,000 x <a lot of seconds> kilometers from us if it took 13 billion years for the light to reach us. > It isn't a surprise to anyone to point out that the source of something that was moving away from us 13Gy ago is now further away than it was. If I wanted to give words the least charitable interpretation, I could point out that their motion has nothing to do with this -- they would still be farther than 13 Gly away from us even if they would not be moving away relative to the Earth :-).