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ZombieCupcake22 t1_j6mid7f wrote

If the universe is spatially flat and can still be finite.

The surface of the torus (donut) can also be spatially flat, but it is finite. So you have two possibilities for a flat Universe: one infinite, like a plane, and one finite, like a torus, where both are flat.

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Chromotron t1_j6mioqq wrote

Be careful, technically a torus is not flat at most points when drawn as a donut. One needs to use the "flat torus" which can only be properly embedded ("drawn") in 4 dimensions.

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The_A4_Paper t1_j6miz2h wrote

To add to this though, Not all tori are flat. Donut isn't flat it's negative on the inside and positive on the outside. When we specifically talk about tori that are flat, we usually refer to them as "flat torus"

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