where the variables refer to Friedmann–Einstein universe, where D is defined in the equations, c is the speed of light, ℓ ℓ is the scale factor, P is the radius of the universe (measured in light years) and P 0 its maximal value, ρ ρ is the mean density of matter, t is time and the age of the universe (last line, measured in years), and κ κ is the Einstein's gravitational constant.