Here, Ω Ω r , Ω Ω m , and Ω Ω Λ Λ are normalized values of the present radiation energy density, matter density, and " dark energy density", respectively (the latter representing the cosmological constant), and Ω Ω k = 1 − − Ω Ω r − − Ω Ω m − − Ω Ω Λ Λ determines the curvature. The Hubble parameter at a given redshift is then H ( z ) = H 0 E ( z ) .