An expression for the critical density is found by assuming Λ to be zero (as it is for all basic Friedmann universes) and setting the normalised spatial curvature, k, equal to zero. When the substitutions are applied to the first of the Friedmann equations we find: ρ ρ c = 3 H 2 8 π π G = 1.8788 × × 10 − − 26 h 2 k g m − − 3 = 2.7754 × × 10 11 h 2 M ⊙ ⊙ M p c − − 3 ,