Determine the origin of the larger scatter in the distant CSP SN Ia sample

Determine whether the larger scatter observed for the more distant Type Ia supernovae in the Carnegie Supernova Project (CSP) Hubble-flow sample arises from unaccounted-for errors (for example, K-correction errors that are not currently included) or from the nearby calibrator sample being too small to capture the true intrinsic dispersion, and quantify the impact of each effect on the inferred Hubble constant.

Background

To infer H0 the authors calibrate SNe Ia using distances from Cepheids, TRGB, and JAGB stars, and then fit the Hubble diagram with an MCMC framework. They found that the nearby calibrator sample exhibits smaller scatter than the more distant CSP Hubble-flow sample.

Allowing only a single intrinsic-scatter parameter produced an anomalously small posterior dispersion in the absolute magnitude parameter (P0) and hence in H0, so the analysis introduced an additional calibration-scatter term (σ_cal) to account for the larger dispersion in the calibrator-vs.-Hubble-flow samples. This increased the P0 scatter by a factor of five and doubled the H0 uncertainty.

The authors explicitly note that it is not clear whether the larger scatter in the distant sample is due to unaccounted-for effects (e.g., K-corrections) or simply the limited size of the nearby sample. Resolving this uncertainty is necessary to solidify the H0 determination.