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The twin paradox of special relativity results from failing to recognize the fundamental asymmetry between the two twins: One and only one twin changes direction and thus undergoes acceleration. What happens if the universe is closed, so that the twin in the rocket can return to Earth without changing direction? The twin paradox is reformulated in a closed universe (a cylinder) and then resolved by showing that there is still a (much more subtle!) asymmetry between the two twins.
Tevian Dray (Sat,) studied this question.