Here H is the Hubble parameter, a measure of the rate at which the universe is expanding. ρ ρ is the total density of mass and energy in the universe, a is the scale factor (essentially the 'size' of the universe), and k is the curvature parameter — that is, a measure of how curved spacetime is. A positive, zero or negative value of k corresponds to a respectively closed, flat or open universe. The constants G and c are Newton's gravitational constant and the speed of light, respectively.