Relative difference between expressions of the luminosity distance. The solid line shows the relative difference between the luminosity distance d_L(z) in Eq. (3) and the theoretical luminosity distance in the ΛCDM model in Eq. (6) for Ω_M = 0.308 and H₀ = 67.8 km s^-1 Mpc^-1 (Planck Collaboration XIII 2016). The dashed line shows the relative difference between the luminosity distance d_L(y), with y ≡ z/ (1 + z), in Cattoën & Visser (2008) and the theoretical luminosity distance in the ΛCDM model. The inset plot shows the corresponding differences in μ, given by Eq. (4), which are indicated by the corresponding line styles.