The geometry of the universe—which dictates whether the universe will expand forever or collapse back into a single point—depends on the density of the universe. Astronomers usually compare any density they are measuring to the critical density, which corresponds to the flat Euclidean space familiar from high school geometry class. If the density of the universe, denoted by , is equal to the critical density ( = 1), space is flat and the universe will expand slower and slower throughout time. If the density is higher than the critical density ( > 1), space will be positively curved like the surface of a sphere, and the universe will eventually collapse in on itself. If the density is lower than the critical density ( < 1), space will be negatively curved like the surface of a saddle. Evidence from the cosmic microwave background shows that our universe has the geometry of flat space. (Unit: 11)