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> The universe is the same in 3D.

This would mean the universe has positive curvature. Experimental evidence points towards the universe being flat (zero curvature), though there is some margin for error that could go either way (positive or negative curvature).

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> This would mean the universe has positive curvature.

There are several flat compact manifolds, they're just not simply connected.

In particular, almost none of them are isotropic. The assumption that the universe is isotropic is part of the cosmological principle and the foundation of modern cosmology. It's a very natural assumption to make, so the vast majority of cosmologists are quite comfortable with it, but at the end of the day it's just an assumption that could be wrong.

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