Flat, closed and open universes. Artwork showing the geometrical curvatures of a 'flat' (left), 'closed' (centre) and 'open' (right) universe. The galaxies are shown on a curved grid, which represents the overall geometry of the universe. The universes have planar, spherical and saddle curvatures respectively. 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.