Spherical universe. Conceptual artwork of the universe as a spherical ball on a table. This represents a 'closed' universe, where the overall geometry of the universe forms the curved surface of a four-dimensional hypersphere on which the galaxies are seen. The geometry of the universe is determined by the amount and density of matter and energy. In a closed universe, the amount of matter is sufficient to halt the expansion of the universe and the expansion then reverses in a contraction towards a Big Crunch. The curvature is said to be positive and the volume is finite, but without boundaries. The implication of a spherical geometry is that, if you travel far enough, it is possible to return to your starting point.