Testing the mapping between redshift and cosmic scale factor

Understand

The canonical redshift-scale factor relation, 1/a=1+z, is a key element in the standard LambdaCDM model of the big bang cosmology.

• Despite its fundamental role, this relation has not yet undergone any observational tests since Lemaitre and Hubble established the expansion of the Universe.
• It is strictly based on the assumption of the Friedmann-Lemaitre-Robertson-Walker metric describing a locally homogeneous and isotropic universe and that photons move on null geodesics of the metric.
• Thus any violation of this assumption, within general relativity or modified gravity, can yield a different mapping between the model redshift z=1/a-1 and the actually observed redshift z_obs, i.e.