Same star in many places in the sky

[-Job-]

Consider the possibility that the universe isn't infinite exactly, but that it just wraps around (like the surface of a sphere) giving the impression of being infinitely large. If this is the case then some light emitted by a star, which is not absorved, may have the chance of going around the universe a couple of times. This would mean that some of the stars we see in the sky could actually be the same one at different stages.
Is there any data that suggests that this is or is not the case?

 

[Chronos]

Here is one such paper:

http://www.arxiv.org/abs/astro-ph/0310233
Constraining the Topology of the Universe
Authors: Neil J. Cornish, David N. Spergel, Glenn D. Starkman, Eiichiro Komatsu

The first year data from the Wilkinson Microwave Anisotropy Probe are used to place stringent constraints on the topology of the Universe. We search for pairs of circles on the sky with similar temperature patterns along each circle. We restrict the search to back-to-back circle pairs, and to nearly back-to-back circle pairs, as this covers the majority of the topologies that one might hope to detect in a nearly flat universe. We do not find any matched circles with radius greater than 25 degrees. For a wide class of models, the non-detection rules out the possibility that we live in a universe with topology scale smaller than 24 Gpc.

There are other discussions you will find if you google on Poincare dodecahedrons.

 

[-Job-]

Thanks for the link, it's an interesting article, a little out of my league, but interesting.

 

[SpaceTiger]

Thanks for the link, it's an interesting article, a little out of my league, but interesting.

The basic result of that paper is that the universe can only have the kind of "wrap-around" properties you describe if its scale is greater than ~24 Gpc (or if it's in the subset of topologies they don't consider). These analyses are ongoing, so this idea should be tested more precisely in the coming years. However, universes with "wrap-around" scales >> 24 Gpc would be impossible to detect with the CMB because the light would not yet have had the chance to wrap around.

 

[-Job-]

Right, that makes sense.

 

[Chronos]

Agreed, ST. That was pretty much the whole point, was it not? Alternative topologies were largely ruled out by this study... the more important result, IMO. Perhaps I read more into that paper than intended. I anticipated more discussion about higher dimensions would emerge in response to that paper. That has not really happened... which is the biggest surprise to me.

BTW, nice catch on the round trip thing ST! Assuming our Hubble bubble is still expanding, the 'hall of mirrors' effect will never reach us.

 

[russ_watters]

So...since none of the stars we can see are any further than a couple of hundred light years...no.

 

[SpaceTiger]

So...since none of the stars we can see are any further than a couple of hundred light years...no.

We can resolve individual (and very bright) stars in the Andromeda galaxy (~million light years). We can see the light from stars (though not individually resolved) out to billions of light years. If the universe had a small topological scale and these "wraparound" properties, multiple images of galaxies (which can be seen by starlight) might be possible.

Of course, with the CMB tests, we're not looking at starlight at all, so you're right that it doesn't directly answer the question. However, if the CMB tests give a null result (and they seem to), then the topological scale of a hypothetical finite universe would have to be large enough that we wouldn't see multiple images of galaxies either.

 

[Chronos]

Russ, I greatly respect your posts. They are usually outstanding. But, I don't see your point in this case. Crap... ST gave a better answer. Question for ST: does this result rule out any extra dimensions, in your opinion?

 

[-Job-]

Are these observations of the CMB taking into account a possible expansion of the universe or does this not alter the predictions made?
Does it make sense to have an infinite universe expanding? If it's infinite, where is it expanding to?

 

[russ_watters]

Sorry - I was thinking naked eye stars (kinda implied by the OP). Yeah, that misses the point since we can see galaxies from much, much further.

 

[Chronos]

The paper by Corning, et. al., does not rely on a preferred model of the universe [e.g., expanding universe.], it merely looks for the wrap-around effect.