Union3.1: Reducing Systematics in Supernova Cosmology with Self-consistent Measurements of Host Galaxy Properties for 2000 Type Ia Supernovae
arXiv:2601.19424
Abstract
Photometrically derived distances of Type Ia supernovae (SNe Ia) rely on a empirical correction based on host galaxy properties, e.g., global stellar mass. Unbiased cosmology inference therefore requires the self-consistent determination of host properties across the full range of redshifts probed, which we undertake here for approximately 2000 SNe in the Union3 compilation (now Union3.1). We use homogeneous, optical-infrared photometry from the DESI Legacy Imaging Surveys to infer global galaxy properties using the stellar population synthesis and SED-fitting code Prospector. We find that the host masses of SNe in Union3 were on average overestimated, while the opposite was true for SNe in Pantheon+. After correction, the two studies' average distance modulus estimated for low-redshift SNe, previously mag discrepant, come into 0.01 mag agreement. Updating the UNITY SN analysis, we find the uncertainties on all standardization parameters shrink to - their previous sizes. For flat-CDM, we find from SNe alone (a shift from Union3). We then combine with measurements of Baryon Acoustic Oscillations and the Cosmic Microwave Background exactly as done by DESI DR2 and find for flat CDM, and , corresponding to evidence against a cosmological constant (down from per DESI-DR2+Planck+Union3). Updating the DESI-DR2+Planck+SN combined probe analysis with the recent Dovekie recalibration of DES-SN5YR (B. Popovic et al. 2025) or the updated Pantheon+, we find (was before Dovekie) and (was before our correction to Pantheon+) evidence, respectively, against a cosmological constant--a significantly improved consistency between SN analyses.
26 pages and 12 figures (main body); 43 pages and 25 figures (total including Appendices)