Assuming the universe is curved and that is it possible to travel in a straight line and return to your starting point, it would make sense that at a younger age when the universe was smaller that light from an object sent in opposite directions would eventually reach the opposite "pole". Do we see any possibilities of this? Seeing the same object from opposite sides in opposite sides of the sky? Such object would have to be very old, I know.
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2$\begingroup$ The problem is that the early universe was opaque which essentially serves as a wall we can't see through. So you could see the photons emitted in one direction, but not both. $\endgroup$– Greg MillerSep 16, 2022 at 15:02
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1$\begingroup$ Even if the curvature is positive the curvature is so small that light hasn't had time to circumnavigate the universe yet. I have some info about that here. And expansion makes that even worse. But people attempting to show that the curvature is positive have looked for such patterns. $\endgroup$– PM 2RingSep 16, 2022 at 20:15
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$\begingroup$ OTOH, we often see multiple images due to gravitational lensing. Eg, with the Twin Quasar, the first identified gravitationally lensed object, there's a 417 ± 3-day time lag between the two images. $\endgroup$– PM 2RingSep 16, 2022 at 20:30
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We can't, because the universe isn't curved enough to allow light to travel far enough since the big bang
If the universe was more highly curved, this might be possible. And the objects that appeared "twice" might not be very distant - or only one of the images would be distant. It was even speculated that it might be possible to see the milky way and Andromeda as a distant pair of galaxies.
There were also searches of the cosmic microwave background to investigate if parts of it could be different views of the same pattern. These turned up negative.
So while "yes" if the universe had sufficient positive curvature, it might be possible to see the same object twice, it turns out that the curvature of the universe is very close to zero, so we don't.
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1$\begingroup$ Could you add some references for the papers you mention? I'd like to have a look at them $\endgroup$– PrallaxSep 19, 2022 at 6:39
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$\begingroup$ The cosmic microwave pattern was a colloquium lecture I attended at university... long ago. $\endgroup$– James KSep 19, 2022 at 7:20
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$\begingroup$ You say "the universe isn't curved enough", implying the universe has some curvature. But that's not true: the universe has zero curvature despite the intense desire of many astrophysicists to prove we live in some kind of bubble. $\endgroup$ Sep 19, 2022 at 9:58
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1$\begingroup$ It might have zero curvature, its not known for sure is the curvature is zero, positive or negative.... $\endgroup$– James KSep 19, 2022 at 10:00
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$\begingroup$ Wishing I had someone nearby that knows this well enough to discuss some ideas I have on the subject of a "closed" flat universe. $\endgroup$ Sep 21, 2022 at 17:17