# cosmological model

A cosmological model is a description of the universe in terms of its spacetime configuration. Assuming that matter, overall, is distributed evenly and
the same in all directions, there are three basic possibilities. (1) A **closed
universe** is positively curved like the surface of a ball and is
finite in extent; such a universe grows to a certain size before collapsing
again. (2) An **open universe** is negatively curved like a
saddle and is infinite in extent. (3) A **flat universe** has
zero curvature and is infinite in extent. The factor determining which kind
of spacetime we actually live in is the average density of matter in the
Universe. Depending on whether the average density is greater than, less
than or equal to the so-called critical
density, the cosmos is closed, open, or flat, respectively. Current
observations suggest a value tantalizingly close to that needed for spacetime
to be flat.