Observations outside of our Hubble Volume

[Sorcerer]

Ok, if we put a telescope for sake of argument say 10 light years away from the earth and set it to transmit it's data to us. Could we then see outside of hubble volume by 10 light years in that horizons direction and receive the images 10 years later?

[md65536]

Ok, if we put a telescope for sake of argument say 10 light years away from the earth and set it to transmit it's data to us. Could we then see outside of hubble volume by 10 light years in that horizons direction and receive the images 10 years later?

No, because any light that reaches the telescope and is then sent to Earth, could instead skip the telescope and reach Earth at least as fast as a signal from the telescope would reach us. A path on a geodesic from an object to the Earth should be no longer than a path from the object to the telescope plus one from the telescope to Earth.

 

I don't know if inflation can change that...

 

 

[Sorcerer]

Right I see that. BUT the Hubble volume is where the expansion of space adds up over a long distance so that light past this point is receeding faster than it's speed and therefore unobservable. Shouldn't a telescope 10 light years away be able to observe a different hubble volume, shifted by 10 light years?

 

IE we are removing 10 light years from the arithmetic series that leads to faster than light expansion away from us.

 

Please correct me if I'm wrong there.

[md65536]

IE we are removing 10 light years from the arithmetic series that leads to faster than light expansion away from us.

This is a bit over my head.

 

I would think the answer is still no. I think that either you'd assume that the telescope remains fixed at 10 LY away, and the expansion measured there would be the same as the expansion measured here?... Or, the expansion of space also applies to the space between you and the telescope, so that any effects of expansion "removed" from what the telescope observes, would be added back into what the telescope transmits to you.

 

 

 

Imagine two photons traveling toward you, close enough to each other that they follow an essentially identical geodesic, except that one happens to be relayed momentarily through the telescope. What could cause one of the photons to be observable, but the other not? If there's an answer I don't know it.

 

Probably not related to what you're talking about: Expansion allows that what is now observable will become unobservable. So it's possible that the telescope can make observations of some remote object, and delay transmitting it to you, and then later you may be able to "see" from the telescope some currently unobservable objects. However, you should only be able to see what was previously observable to you. If the telescope delayed an image of an object by a billion years, it would show the object as it was a billion years ago, just as it was when it was directly observable by you a billion years ago. Those would be observations of a different Hubble volume than what you see now. There would certainly be some form of delay (if not gigayears, then perhaps nano seconds).

[michel123456]

The Hubble sphere changes over time.

see Hubble limit from wiki.

The boundary of the Hubble volume is known as the "Hubble limit". Per Hubble's law, objects at the Hubble limit have an average comoving speed of c relative to an observer on the Earth. This is significant, because, in a universe in which the Hubble parameter was constant, light emitted at the present time by objects outside the Hubble limit could never be seen by an observer on the Earth. However, the Hubble "constant" is not constant. In a decelerating Friedmann universe, the Hubble sphere expands faster than the Universe and its boundary overtakes light emitted by receding galaxies. In an accelerating universe, the Hubble sphere expands more slowly than the Universe, and bodies move out of the Hubble sphere.[1] So the Hubble limit need not define the cosmological event horizon (that is, the boundary separating events visible at some time or other and those that are never visible[4]), because (depending upon the cosmological model) light emitted at earlier times by objects outside the Hubble sphere still may eventually arrive inside the sphere and be seen by us.[2] If, as is inferred from current observations, the expansion of the universe is in fact accelerating,[5] then at a later time, some objects within the Hubble limit no longer will be observed (by us) as they are today.

Bolded mine.

It corresponds to md65536's post.

 

The point is that the telescope must interact by some way with us by sending its pictures, and the max. theoretical speed to do that is the Speed Of Light. So, even if we could have sent this telescope with a spaceship traveling at SOL 10 LY away and take the pictures back at SOL, the pictures would show something that is still in Earth's past light-cone which means it was theoretically observable from Earth 10 years ago. There is no physical way to gather information from outside our past light-cone.

 

What would change is the parallax

 

Note:

I have never seen anywhere any observational evidence corroborating the bolded part, that "at a later time, some objects within the Hubble limit no longer will be observed (by us) as they are today." Are objects vanishing from our telescopes over time?

[Sorcerer]

Thanks martin/md65536 I fully understand that now. Was looking for a way to refute that to a friend.

 

""I have never seen anywhere any observational evidence corroborating the bolded part, that "at a later time, some objects within the Hubble limit no longer will be observed (by us) as they are today." Are objects vanishing from our telescopes over time? ""

 

and a very good question, u would think so wouldn't u, but since we've been observing for a limited time with telescopes that can veiw these places I guess we wouldn't have much evidence yet.

 

If I put it this way for argument would I be correct? :

 

Our hubble volume is increasing as our light cone expands, however the ammount of universe we observe proportional to its total remains the same, because the universe expands at the same rate. Thus our hubble volume remains the same fraction of the observable universe as it was just after inflation.

 

Or should I rephrase that to account for dark energy?

Edited December 31, 2011 by Sorcerer

[md65536]

""I have never seen anywhere any observational evidence corroborating the bolded part, that "at a later time, some objects within the Hubble limit no longer will be observed (by us) as they are today." Are objects vanishing from our telescopes over time? ""

[...]

Our hubble volume is increasing as our light cone expands, however the ammount of universe we observe proportional to its total remains the same, because the universe expands at the same rate. Thus our hubble volume remains the same fraction of the observable universe as it was just after inflation.

 

I defer to the experts if they're around. Otherwise...

 

Light cones don't expand. A light cone is essentially "a fixed moment in time as observed by a given observer". As time passes, we see the universe basically via different light cones.

Later light cones should allow the observation of light that's had more time to reach us, compared to light cones of events earlier in the lifetime of the universe. However, I don't think light cones tell you what you want to know here... See this previous related conversation: http://www.scienceforums.net/topic/61382-what-we-see/page__view__findpost__p__640081 [linked to a relevant post].

 

I guess it hasn't been decided whether expansion is accelerating, in which case: yes, objects would vanish from our telescopes over time.

As far as I know the evidence seems to indicate accelerated expansion, and vanishing objects is a predicted consequence, but I don't think it's been directly observed.

With smooth expansion, objects would be receding at approaching the speed of light before they disappeared at any greater rates of recession.

These objects would be so far away that they'd be dim and hard to see, and their light would be redshifted to wavelengths approaching infinity, which means no energy.

So rather than seeing an object "pop out of existence" in a telescope, you'd essentially see it get redder and darker and farther until it fades away to nothing.