In previous equations, a, ρ, and p are functions of time. ⁠ k / a ⁠ is the spatial curvature in any time-slice of the universe; it is equal to one-sixth of the spatial Ricci curvature scalar R since R = 6 c 2 a 2 ( a ¨ ¨ a + a ˙ ˙ 2 + k c 2 ) in the Friedmann model. H ≡ ⁠ ȧ / a ⁠ is the Hubble parameter.