For example, one can form a linear combination of such terms ρ ρ = A a − − 3 + B a − − 4 + C a 0 where A is the density of "dust" (ordinary matter, w = 0) when a = 1 ; B is the density of radiation (w = ⁠ 1 / 3 ⁠) when a = 1 ; and C is the density of "dark energy" (w = −1). One then substitutes this into ( a ˙ ˙ a ) 2 = 8 π π G 3 ρ ρ − − k c 2 a 2 and solves for a as a function of time.