I don’t think so. Any events in the past now will still be in the past in a billion years. The past light-cone can only get bigger. (I think you might be misunderstanding my use of the word “past”. I’m using the relativistic definition: the set of all events from which one can reach present-day Earth while travelling slower than lightspeed.)
Without getting mathematical: there are galaxies moving away from us faster than the speed of light (and moreover every galaxy outside the Local Group is accelerating away from us). In the future these galaxies will not be visible to Earth-based observers. Similarly the CMB will be more redshifted and hence contain less information. So if you’re using a meaning of “event” such that every Planck volume of space produces an event every Planck time regardless of whether there are any atoms there or not, then yes, that number can only go up. But if you’re talking about actually interesting things to observe, then it’s certainly going down.
There are galaxies moving away from us faster than the speed of light (and moreover every galaxy outside the Local Group is accelerating away from us). In the future these galaxies will not be visible to Earth-based observers.
If they’re moving away from us faster than the speed of light, they’re not observable now either. As for currently observable galaxies, the event horizon between what we can observe and what we cannot is receding at lightspeed, relativity does not allow us to observe anything break the light barrier, therefore nothing observable can outrace the event horizon and become hidden from us. Black holes notwithstanding because Stephen Hawking is still working on that one.
Similarly the CMB will be more redshifted and hence contain less information.
I don’t think redshifting destroys information. Unless you mean that the information will be hidden by noise from within our own galaxy, which is perfectly true.
Anyway, I accept that the amount of data one can gather from current observations may go down over time. But, over a long enough time period, the amount of data one can gather from current and past observations will go up, because there is more past to choose from. Of course, even over millennia it will only go up by an insignificant amount, so we should be careful with the data we have.
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.
I don’t think so. Any events in the past now will still be in the past in a billion years. The past light-cone can only get bigger. (I think you might be misunderstanding my use of the word “past”. I’m using the relativistic definition: the set of all events from which one can reach present-day Earth while travelling slower than lightspeed.)
Without getting mathematical: there are galaxies moving away from us faster than the speed of light (and moreover every galaxy outside the Local Group is accelerating away from us). In the future these galaxies will not be visible to Earth-based observers. Similarly the CMB will be more redshifted and hence contain less information. So if you’re using a meaning of “event” such that every Planck volume of space produces an event every Planck time regardless of whether there are any atoms there or not, then yes, that number can only go up. But if you’re talking about actually interesting things to observe, then it’s certainly going down.
If they’re moving away from us faster than the speed of light, they’re not observable now either. As for currently observable galaxies, the event horizon between what we can observe and what we cannot is receding at lightspeed, relativity does not allow us to observe anything break the light barrier, therefore nothing observable can outrace the event horizon and become hidden from us. Black holes notwithstanding because Stephen Hawking is still working on that one.
I don’t think redshifting destroys information. Unless you mean that the information will be hidden by noise from within our own galaxy, which is perfectly true.
Anyway, I accept that the amount of data one can gather from current observations may go down over time. But, over a long enough time period, the amount of data one can gather from current and past observations will go up, because there is more past to choose from. Of course, even over millennia it will only go up by an insignificant amount, so we should be careful with the data we have.
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.