Banana Split: Improved Cosmological Constraints with Two Light-Curve-Shape and Color Populations Using Union3.1+UNITY1.8

TL;DR

The paper addresses potential biases in Type Ia supernova cosmology arising from assuming a single SN Ia population by introducing a two-mode, light-curve-shape and color population framework within UNITY. It extends the UNITY Bayesian hierarchical approach to Union3.1, incorporating latent SN parameters and population-specific color and shape distributions, and applies UNITY1.8 to obtain tightened cosmological constraints. Key findings include evidence for distinct light-curve-shape and color populations with host-galaxy mass and redshift dependencies, a residual host-mass luminosity step for unreddened SNe consistent with zero, and significantly reduced parameter uncertainties when using two modes; cosmological results include $\Omega_m\approx0.334$, $w_0\approx-0.760$, and $w_a\approx-0.79$ in appropriate combinations, with the inclusion of SNe increasing tension with flat $\Lambda$CDM from $2.1\sigma$ to $2.6\sigma$. These results bolster the importance of population- and host-aware SN standardization for robust dark energy inferences and highlight the potential impact on interpreting cosmological datasets.

Abstract

SNe Ia have been used to provide key constraints on the equation-of-state parameter of dark energy. They are generally standardized under the assumption that they belong to a single population, with luminosities standardized in a continuous (roughly linear) fashion using the observed light-curve timescale. We update the Union3+UNITY1.5 SN cosmology analysis in light of increasing evidence for at least two core populations of SNe Ia and apply this "UNITY1.8" model to the updated "Union3.1" compilation (Hoyt et al. 2026). In addition to finding evidence for two different light-curve-shape (x1) distributions, we also find that the color distributions are different, that the light-curve-shape/magnitude standardization relations are different, and that these populations have different distributions across host-galaxy stellar mass and redshift. Importantly, we find that the residual host-mass luminosity step found in prior SN Ia cosmology analyses is now consistent with zero for unreddened SNe. We report a significantly tightened constraint on the split in the red-color standardization between SNe in low- and high-mass galaxies. We find that the estimated uncertainties shrink on cosmological parameters when fitting the same SNe assuming two modes versus one mode. We confirm similar trends in simulated data when running both versions of UNITY on the same (two-mode) simulations. For a flat LambdaCDM cosmology, we find Om = 0.334+0.025-0.024 from SNe alone; for a flat w0-wa cosmology, we find w0 = -0.760+0.084-0.082 and wa = -0.79+0.28-0.30 when including SNe, BAO, and CMB. In the 2D w0-wa plane, adding SNe to BAO and compressed CMB increases the tension with flat LambdaCDM from 2.1 sigma to 2.6 sigma.