Union Through UNITY: Cosmology with 2,000 SNe Using a Unified Bayesian Framework

TL;DR

The paper addresses the challenge of combining diverse Type Ia supernova datasets to constrain cosmology with controlled systematics. It introduces Union3, a large, calibrated SN Ia compilation, and the UNITY1.5 Bayesian framework to jointly model standardization, selection effects, and unexplained dispersion on a unified distance scale, while employing a blinded analysis. Key findings include updated SN-only and SN+BAO+CMB constraints showing mild tension with LambdaCDM (1.7–2.6 sigma) and possible evidence for thawing dark energy (w0 > -1, wa < 0). The authors also release SN distances, light-curve fits, and the UNITY1.5 toolkit to enable community-driven improvements as SN samples grow.

Abstract

Type Ia supernovae (SNe Ia) were instrumental in establishing the acceleration of the universe's expansion. By virtue of their combination of distance reach, precision, and prevalence, they continue to provide key cosmological constraints, complementing other cosmological probes. Individual SN surveys cover only over about a factor of two in redshift, so compilations of multiple SN datasets are strongly beneficial. We assemble an up-to-date "Union" compilation of 2087 cosmologically useful SNe Ia from 24 datasets ("Union3"). We take care to put all SNe on the same distance scale and update the light-curve fitting with SALT3 to use the full rest-frame optical. Over the next few years, the number of cosmologically useful SNe Ia will increase by more than a factor of ten, and keeping systematic uncertainties subdominant will be more challenging than ever. We discuss the importance of treating outliers, selection effects, light-curve shape and color populations and standardization relations, unexplained dispersion, and heterogeneous observations simultaneously. We present an updated Bayesian framework, called UNITY1.5 (Unified Nonlinear Inference for Type-Ia cosmologY), that incorporates significant improvements in our ability to model selection effects, standardization, and systematic uncertainties compared to earlier analyses. As an analysis byproduct, we also recover the posterior of the SN-only peculiar-velocity field, although we do not interpret it in this work. We compute updated cosmological constraints with Union3 and UNITY1.5, finding weak 1.7--2.6 sigma tension with LambdaCDM and possible evidence for thawing dark energy (w0 > -1, wa < 0). We release our SN distances, light-curve fits, and UNITY1.5 framework to the community.

Figures (2)

• Figure 1: Residuals from a linear color transformation between Bessell2012$UBVRI$ filters and the same filters shifted by 100Å. Each panel shows $UBVRI$ from left to right. We synthesize magnitudes for the stars (plotted with red stars) using the Pickles1998 stellar library and magnitudes for the SNe Ia (plotted with blue dots) using the hsiao07 SN Ia template from $-10$ to +15 days, redshifted from $z=0.01$ to 0.10. We subtract the linear color terms from the synthesized magnitude differences, as shown at the top of each panel. Mean offsets of a few hundredths of a magnitude are visible between stars and SNe, and the stars are generally more tightly clustered around the linear relations.
• Figure 2: The results of a SALT3 validation test to see if different rest-frame wavelength ranges give consistent distance moduli. The left panel shows the distance-modulus difference for each SDSS, SNLS, or Swope SN between rest-frame $UB$ and $UBVR$ as a function of the minimum rest-frame wavelength. The right panel shows the same quantity divided by the sensitivity of the distance modulus to the rest-frame $U$. Neither case shows a significant trend. The midpoint values shown are the intercept values at the location where the slope with wavelength and the intercept are uncorrelated. These values should be similar to the robust mean over all data.