standard candles or rods measure $H(z)$ up to a multiplicative constant, related to the unknown absolute measure of the source luminosity or proper length. For instance, the flux of supernovae Ia (SNIa) with absolute luminosity $L$ are known only up to the constant $LH_{0}^{2}$; only ratios of fluxes at different redshifts are independent of the absolute normalization. The same is true of baryon acoustic oscillations: they measure only the ratio of the sound horizon at last scattering and the Hubble radius $H^{-1}(z)$ . We can therefore say that, without additional assumptions, background cosmological observations estimate $D(z)$ up to an overall constant as well as the dimensionless Hubble function $E(z)\equiv H(z)/H_{0}$. Notice, however, that real-time redshift-drift observations can estimate the absolute value of $H(z)$, while local measurements of the expansion measure $H_{0}$.

Combining $D(z)$ with $H(z)$, we can also estimate the present curvature parameter $\Omega_{k0}$. We can therefore determine the evolution of the combined matter and dark energy content, $1-\Omega_{k}$, at all times. If we assume that there are only two components of the cosmic fluid then we have only one free parameter, $\Omega_{\text{m}0}$. In fact, we can write $$\Omega_{x}=1-\Omega_{k}-\Omega_{\text{m}}=1-\frac{1}{E^{2}}\left(\Omega_{k0}a^{-2}+\Omega_{\text{m}0}a^{-3}\right)\,.$$ Therefore, we conclude that from background observables we can reconstruct both 