It's also possible to have a flat universe (no curvature), where a straight line can return to the same point. An example is a torus (donut shape). You can see

It's also possible to have a flat universe (no curvature), where a straight line can return to the same point. An example is a torus (donut shape). You can see this by noticing that a torus can be projected onto a flat sheet (or created from one) without messing with any relative scales or angles, while the surface of a sphere cannot. (For a sphere, you need Mercator or some other projection which does not conserve an honest representation of relative sizes - e.g. Greenland vs. Africa in Mercator.)