Since the constant G is known and the expansion rate H can be measured by observing the speed at which distant galaxies are receding from us, ρ ρ c can be determined. Its value is currently around 10 kg m. The ratio of the actual density to this critical value is called Ω, and its difference from 1 determines the geometry of the universe: Ω > 1 corresponds to a greater than critical density, ρ ρ > ρ ρ c , and hence a closed universe. Ω < 1 gives a low density open universe, and Ω equal to exactly 1 gives a flat universe.