Here formula_2 is the Hubble parameter, a measure of the rate at which the universe is expanding. formula_3 is the total density of mass and energy in the universe, formula_4 is the scale factor (essentially the 'size' of the universe), and formula_5 is the curvature parameter — that is, a measure of how curved spacetime is. A positive, zero or negative value of formula_5 corresponds to a respectively closed, flat or open universe. The constants formula_7 and formula_8 are Newton's gravitational constant and the speed of light, respectively.