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

Type Ia supernovae (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 supernova (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. In addition to finding evidence for two different light-curve-shape ( x 1 ) 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 Unified Nonlinear Inference for Type Ia cosmologY (UNITY) on the same (two-mode) simulations. For a flat ΛCDM cosmology, we find Ω m = 0.33 4 − 0.024 + 0.025 from SNe alone; for a flat w 0 – w a cosmology, we find w 0 = − 0.76 0 − 0.082 + 0.084 and w a = − 0.7 9 − 0.30 + 0.28 when including SNe, baryon acoustic oscillation, and cosmic microwave background. In the 2D w 0 – w a plane, UNITY1.8 decreases the tension with flat ΛCDM by 0.1 σ to ∼3 σ compared to single-mode UNITY.