Radius of curvature
The radius of curvature in our universe might be:
- positive; a positive curvature implies closed space, a universe with a definite, finite volume but with no boundary.
- negative; a negative curvature implies open space, an infinite universe.
- zero; the limiting case of zero curvature is `flat' Euclidean space with an infinite radius.
There are various types of curvature, and, in all but flat space, the amount of curvature has a wide range of possible values.
- Edwin Hubble (1937). The Observational Approach to Cosmology. Oxford University Press.