While a perfectly flat universe would extend out to infinity , a flat universe with a multiply-connected topology would have finite size. If we could somehow determine whether one or more dimensions are wrapped in on themselves, then we would know that the universe is finite in that dimension. We could then use those observations to measure the total volume of the universe.
But how would a multiply-connected universe reveal itself?
The team found that a multiply-connected universe about three to four times larger than our observable bubble best matched the CMB data. While this result technically means that you could travel in one direction and end up back where you started, you wouldn't be able to actually accomplish that in reality. We live in an expanding universe , and at large scales the universe is expanding at a rate that is faster than the speed of light, so you could never catch up and complete the loop.
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