In the current model of cosmology, if the density of our universe is equal to 1, or the critical density, then our universe is flat. If it is less than 1, it is negatively curved like a saddle. And, if it is greater than 1, it is positively curved like the surface of a sphere. According to general relativity, the curvature of space determines how light travels. So these various geometries predict different results in the size and pattern of hot and cold spots in the cosmic microwave background. To calculate the geometry, the WMAP team measures the observed angular size of the hot and cold spots of the CMB, and compares these to the predictions. The team found that the characteristic observed size of the spots is one degree, which is consistent with a flat universe.