Well, every object is separated from you by a spacelike interval. If some distant object starts accelerating quickly enough, it may become forever inaccessible.
Also, an object distant enough on way-bigger-than-galaxy-superclaster scale can have Hubble speed more than c relative to us.
Also, an object distant enough on way-bigger-than-galaxy-superclaster scale can have Hubble speed more than c relative to us.
Are you sure about this? I don’t understand relativity much, but I would suspect this to be another case of “by adding speeds classically, it would be greater than c, but by applying proper relativistic calculation it turns out to be always less than c”.
Proper relativistic velocity arithmetics you mention is about special relativity theory—i.e. local flat-space case. Hubble runaway speed is supposed to be about global ongoing space distortion, i.e. it is strictly about general relativity. As far as I know, it is actually measured based on impulse change in photons, but it can be theoretically defined using time needed for a lightspeed round-trip.
When this relative speed is small, everything is fine; if I understand correctly, if Hubble constant is constant in the long term and there are large enough distances in the universe, it would take ray of light exponential time (not linear) to cross distances above some threshold.
In the inflationary model of early universe, there is some strange phase where distances grow faster than light could cover them—it is possible as it is not motion of matter in space, but change of the stucture of space. http://en.wikipedia.org/wiki/Inflationary_model
Well, every object is separated from you by a spacelike interval. If some distant object starts accelerating quickly enough, it may become forever inaccessible.
Also, an object distant enough on way-bigger-than-galaxy-superclaster scale can have Hubble speed more than c relative to us.
Are you sure about this? I don’t understand relativity much, but I would suspect this to be another case of “by adding speeds classically, it would be greater than c, but by applying proper relativistic calculation it turns out to be always less than c”.
It looks like it is even weirder.
Proper relativistic velocity arithmetics you mention is about special relativity theory—i.e. local flat-space case. Hubble runaway speed is supposed to be about global ongoing space distortion, i.e. it is strictly about general relativity. As far as I know, it is actually measured based on impulse change in photons, but it can be theoretically defined using time needed for a lightspeed round-trip.
When this relative speed is small, everything is fine; if I understand correctly, if Hubble constant is constant in the long term and there are large enough distances in the universe, it would take ray of light exponential time (not linear) to cross distances above some threshold.
In the inflationary model of early universe, there is some strange phase where distances grow faster than light could cover them—it is possible as it is not motion of matter in space, but change of the stucture of space. http://en.wikipedia.org/wiki/Inflationary_model