The first Friedmann equation is often seen in terms of the present values of the density parameters, that is H 2 H 0 2 = Ω Ω 0 , R a − − 4 + Ω Ω 0 , M a − − 3 + Ω Ω 0 , k a − − 2 + Ω Ω 0 , Λ Λ . Here Ω 0,R is the radiation density today (when a = 1), Ω 0,M is the matter (dark plus baryonic) density today, Ω 0, k = 1 − Ω 0 is the "spatial curvature density" today, and Ω 0,Λ is the cosmological constant or vacuum density today.