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chriscurtis
2007-Oct-19, 10:21 PM
If the sun was older than the radius of the universe and you could see the sun in every line of sight from here, what would it look like? Would the sun be smeared out across the whole sky? What would be the relationship between the size of the universe and what you could see?

chriscurtis
2007-Oct-19, 10:22 PM
Sorry, that's... if the sun was older than the length of time it takes light to travel across the universe.

01101001
2007-Oct-19, 10:33 PM
If the sun was older than the radius of the universe and you could see the sun in every line of sight from here, what would it look like? Would the sun be smeared out across the whole sky? What would be the relationship between the size of the universe and what you could see?

I don't understand. Your hypothetical contains the phrase "you could see the sun in every line of sight from here", and that seems to be the answer you seek.

It's kind of like: "If my grandmother were a trolley, and she had blue wheels, what color would her wheels be?"

Then, the hypothetical doesn't ring true. That the sun had an age near (I can't imagine: beyond) the age of the universe, doesn't have any implications for where it would be seen currently. It would just be old and still close, covering around half a degree in the daytime sky -- it wouldn't appear wherever you looked.

speedfreek
2007-Oct-19, 11:16 PM
Certain models of the universe predict that, if the whole universe was smaller than our observable universe and therefore light had the time to propagate, and if the universe was positively curved, an observer might end up seeing the same region of space when looking in different directions (although they might be looking at different epochs in different directions and may not recognise them for what they are). A straight line might actually be a section of a giant loop.

One group - Key, Cornish, Spergel and Starkman, set out to test these models using matching circle analysis of the WMAP data, but have, so far, found no evidence for these models and claim to have ruled out the Poincaré dodecahedral space (soccer ball model) which might throw up repeated lines of sight in 12 directions. They claim to find that the universe is within 2% of being flat and conclude that if space is positively curved, that curvature has a radius larger than our observable universe.

They seem to have decided that the whole universe is indeed larger than the observable universe, but I am unclear as to how this rules out the Poincaré model - surely they can only rule it out for our observable universe, but the topology could still apply to the whole thing? :confused: What I mean is that by analysing the WMAP data, they are looking at the CMBR picture of our observable universe only.

http://arxiv.org/abs/astro-ph/0604616 Extending the WMAP Bound on the Size of the Universe

Even if they were incorrect, and that model was not ruled out, we would not see multiple images of the sun filling the sky, but might see incredibly dim repeated images of the sun of the same angular size as the sun, with a very high cosmological redshift. And different topologies would cause different patterns of repetition.

chriscurtis
2007-Oct-20, 01:27 AM
Thanks speedfreek.