Right, so this is the standard misunderstanding about what it means for space itself to be expanding. Thesetwo Wikipedia article might be a good place to start, but in brief: relativity forbids information to pass through space faster than light, but when space itself expands then the distance between two objects can increase faster than c without a problem. (The second link quotes a number of 2 trillion years for the time when no galaxies not currently gravitationally bound to us will be visible.)
I don’t think redshifting destroys information.
Well, technically I guess it just lowers the information density, which means less information can be gathered by observers on Earth (and less is available inside the observable universe, etc.) And then eventually the wavelength will be greater than the size of the observable Universe and thus undetectable entirely.
Thanks for the links. It all makes a lot more sense to me now (though at 2 trillion years, the timescales involved are much longer than I had considered).
One last quibble: Relativity does not forbid the space between two objects (call them A and B) from expanding faster than c, it’s true. But a photon emitted by object A would not be going fast enough to outrace the expansion of space, and would never reach B. So B would never obtain any information about A if they are flying apart faster than light.
But because the expansion of the Universe is accelerating, the apparent receding velocity caused by the expansion is increasing, and, for any object distant enough, will at some point become greater than c, causing the object to disappear beyond the cosmological horizon.
This, obviously, assuming that the current theories are correct in this respect.
But a photon emitted by object A would not be going fast enough to outrace the >expansion of space, and would never reach B. So B would never obtain any >information about A if they are flying apart faster than light.
I think that was the point, but since the expansion is accelerating this was not always the case.
A and B are retreating faster than light now (in our reference frame), so the light they are emitting now will not reach each other.
However, the A and B are far apart, say 5 billion light years. 5 billion years ago A and B were receding more slowly—perhaps half the speed of light, so the light emitted 5 billion years ago from A is now reaching B. Hence, B currently sees light from A.
Five billion years in the future this will not be the case. Sometime in the next 5 billion years B will observe A to redshift all the way to zero and wink out.
Right, so this is the standard misunderstanding about what it means for space itself to be expanding. These two Wikipedia article might be a good place to start, but in brief: relativity forbids information to pass through space faster than light, but when space itself expands then the distance between two objects can increase faster than c without a problem. (The second link quotes a number of 2 trillion years for the time when no galaxies not currently gravitationally bound to us will be visible.)
Well, technically I guess it just lowers the information density, which means less information can be gathered by observers on Earth (and less is available inside the observable universe, etc.) And then eventually the wavelength will be greater than the size of the observable Universe and thus undetectable entirely.
Thanks for the links. It all makes a lot more sense to me now (though at 2 trillion years, the timescales involved are much longer than I had considered). One last quibble: Relativity does not forbid the space between two objects (call them A and B) from expanding faster than c, it’s true. But a photon emitted by object A would not be going fast enough to outrace the expansion of space, and would never reach B. So B would never obtain any information about A if they are flying apart faster than light.
But because the expansion of the Universe is accelerating, the apparent receding velocity caused by the expansion is increasing, and, for any object distant enough, will at some point become greater than c, causing the object to disappear beyond the cosmological horizon.
This, obviously, assuming that the current theories are correct in this respect.
I think that was the point, but since the expansion is accelerating this was not always the case.
A and B are retreating faster than light now (in our reference frame), so the light they are emitting now will not reach each other.
However, the A and B are far apart, say 5 billion light years. 5 billion years ago A and B were receding more slowly—perhaps half the speed of light, so the light emitted 5 billion years ago from A is now reaching B. Hence, B currently sees light from A.
Five billion years in the future this will not be the case. Sometime in the next 5 billion years B will observe A to redshift all the way to zero and wink out.
Agreed. Thanks.