Flat universe. Computer artwork showing the geometrical curvature of a 'flat' (planar) universe. The galaxies are shown on a grid, which represents the overall geometry of the universe. The curvature of a universe is determined by the amount and density of matter and energy. In an open universe, the amount of matter is insufficient to prevent the universe expanding forever. In a closed universe, there is enough matter to halt the expansion, followed by a contraction to a Big Crunch. In a flat universe, the amount of matter has a critical value between these two extremes. It is a matter of debate as to which of these models is a correct representation of the universe.