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Any-Broccoli-3911 t1_j8egw3r wrote

Reply to comment by Aseyhe in In the twin paradox, what happens if the travelling twin never U-turn to get back to earth? (explanation in the post) by PoufPoal

A spherical universe is one of the 3 solutions for a homogeneous and isotropic universe (flat and hyperbolic are the 2 other ones). There's no preferred frame in a spherical universe.

Still for the person who moves, the circumference of the universe is smaller. So he sees his twin on Earth crossing the universe in a shorter time than his twin sees him crossing the universe in the spaceship. So at the end of the travel, the spaceship twin will say that the total time passed is less than what the Earth twin will say.

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Aseyhe t1_j8ejbv8 wrote

I was referring to a toroidal universe, since its spacetime can be flat. Bringing in spacetime curvature complicates things unnecessarily!

However, the spherical universe does have a (local) preferred frame: the frame of a comoving observer.

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