On the use of the local prior on the absolute magnitude of Type Ia supernovae in cosmological inference

A dark-energy, which behaves as the cosmological constant until a sudden phantom transition at very low redshift (z < 0.1), seems to solve the >4 sigma disagreement between the local and high-redshift determinations of the Hubble constant, while maintaining the phenomenological success of the A cold dark matter model with respect to the other observables. Here, we show that such a hockey-stick dark energy cannot solve the H-0 crisis. The basic reason is that the supernova absolute magnitude M-B that is used to derive the local H-0 constraint is not compatible with the M-B that is necessary to fit supernova, baryon acoustic oscillation, and cosmic microwave background data, and this disagreement is not solved by a sudden phantom transition at very low redshift. We make use of this example to show why it is preferable to adopt in the statistical analyses the prior on M-B as an alternative to the prior on H-0. The three reasons are: (i) one avoids potential double counting of low-redshift supernovae, (ii) one avoids assuming the validity of cosmography, in particular, fixing the deceleration parameter to the standard model value q(0) = -0.55, (iii) one includes in the analysis the fact that M-B is constrained by local calibration, an information which would otherwise be neglected in the analysis, biasing both model selection and parameter constraints. We provide the priors on M-B relative to the recent Pantheon and DES-SN3YR supernova catalogs. We also provide a Gaussian joint prior on H-0 and q(0) that generalizes the prior on H-0 by Supernova H0 for the Equation of State.

Automated summary

The paper discusses the impact of dark energy on the Hubble constant, arguing that hockey-stick dark energy cannot resolve the H-0 crisis. It suggests using a prior on M-B instead of H-0 in statistical analyses to avoid potential issues and emphasizes the importance of considering dark energy in such analyses.