The shape of space

[HeavilyArmed]

I may not understand an answer here but I need to ask the question.

We have a hugely red shifted proto-galaxy 'A' in one direction, maybe 12+ billion ly distant and in exactly the opposite direction we have another, 'B', exactly the same. It's not possible that A + B are 24 billion ly distant. Does this mean that at any point in space you will be surrounded by galaxies in every direction out all the way back to time ~0? If that's true what does that say about the shape of space (it's space-time, isn't it?)?

[bunny]

[ QUOTE ]
I may not understand an answer here but I need to ask the question.

We have a hugely red shifted proto-galaxy 'A' in one direction, maybe 12+ billion ly distant and in exactly the opposite direction we have another, 'B', exactly the same. It's not possible that A + B are 24 billion ly distant.

[/ QUOTE ]
I think the problem is an assumption that there is an observer-invariant answer to "how far apart are they?". We would say Yes, they are 24 billion light years apart. They havent seen each other "yet" (again from our persepective).

[speedfreek]

I assumed the problem was because that would mean one galaxy would be moving away from the other at faster than the speed of light, which seems to violate the speed limit.

Well it seems it may be possible for A + B to = 24 billion light years. The expansion of space could be responsible. Einsteins limit on mass moving at the speed of light doesn't apply if it is space-time itself that is expanding, making the distances larger. We see what could be evidence of this expansion by measuring how much the wavelength of light from an object is stretched (this is termed cosmological redshift).

Our observable universe seems to consist of, as you say, galaxies in every direction all the way back in time. But the further away the objects are, the faster they seem to be moving away from us (locally, galaxies can attract and move towards each other due to their gravity, but on the large scales, everything seems to be moving apart).

Also, a galaxy we see 12 billion light years away was that far away 12 billion years ago. If space is expanding, and the expansion seems faster the further away we look, then that galaxy could be a lot further away by now. The most distant objects we see at around 13 billion light years away are currently estimated to be around 46 billion light years away now!

What does all this say about the shape of space-time? Well I don't think we can simply picture it as a 3d model. The problem is, to model like this assumes some sort of edge to the shape. To have an edge you have to have something for it to make an edge against. The problem is, we don't know what that is. Another dimension? No dimensions? It makes finding a shape for the universe very difficult.

Couple this with the idea that wherever you are you see galaxies all round, going back in time and expanding away from you, and it seems like space curves away somehow, maybe back on itself.

Also, we don't know if our observable universe is all there is. There maybe a larger scale universe out there beyond our observable limit that we cannot see yet but a viewer on the most distant objects from us could. [img]/images/graemlins/smile.gif[/img]

[ill rich]

i've always heard the best guess to the universe's shape is something like a sideways U.

[bunny]

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I assumed the problem was because that would mean one galaxy would be moving away from the other at faster than the speed of light, which seems to violate the speed limit.


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This arises from a similar confusion. We do indeed see the two separating at faster than the speed of light, but neither of them is moving faster than c with respect to us (and when they eventually see each other, they will also not see anything moving faster than light).