freyar
Extradimensional Explorer
Could Sigil (a small torus-shaped universe with gravity pulling toward the "walls" of the universe) exist with those physical laws? Can it be modeled mathematically?
Generally speaking, general relativity allows any sort of universe you want, but the trick is that you have to figure out what sort of matter/energy distribution would give that shape to the universe. I'm not sure that a toroidal universe like Sigil (with a boundary) is something that can easily be made with normal matter, but we've talked a lot about wormholes and warp drive spacetimes that require exotic matter/energy anyway.