text: The universe may be compact in some dimensions and not in others, similar to how a cuboid is longer in one dimension than the others. Scientists test these models by looking for novel implications – phenomena not yet observed but necessary if the model is accurate. For instance, a small closed universe would produce multiple images of the same object in the sky, though not necessarily of the same age. As of 2023 current observational evidence suggests that the observable universe is spatially flat with an unknown global structure.
  Curvature of the universe
  The curvature is a quantity describing how the geometry of a space differs locally from flat space. The curvature of any locally isotropic space (and hence of a locally isotropic universe) falls into one of the three following cases:
  Zero curvature (flat)a drawn triangle's angles add up to 180° and the Pythagorean theorem holds; such 3-dimensional space is locally modeled by Euclidean space .
  Positive curvaturea drawn triangle's angles add up to more than 180°; such 3-dimensional space is locally modeled by a region of a 3-sphere .
  Negative curvaturea drawn triangle's angles add up to less than 180°; such 3-dimensional space is locally modeled by a region of a hyperbolic space .