On the impact of the supernova subsamples in reducing the Hubble tension

Abstract

The persistent 4-6$\ddot{I}ƒ$ difference between early- and late-time Hubble constant ($H_0$) measurements, known as the "Hubble tension", is a major problem in modern cosmology. We study how differences in colour ($c$), stretch ($x_1$), and host galaxy properties-stellar mass ($M$) and specific star formation rate (sSFR)-between calibration and Hubble Flow (HF) Type Ia supernova (SN Ia) samples used by SH0ES affect SN luminosity standardization and $H_0$ estimates. We generate subsamples from both, estimating $H_0$, $M_B$, $\hat{I}\pm$, ${\hat{I}}^2$, $\hat{I}{”}_{host}$, and $\ddot{I}{ƒ}_{int}$. We use Kolmogorov-Smirnov to assess the consistency between subsamples and reveal how parameter estimates change as sample matching improves. The calibration sample is not fully representative of the HF sample, especially in $M$ and sSFR. Improving sample consistency leads to changes in $H_0$, $M_B$ and $\ddot{I}{ƒ}_{int}$, though overall values remain broadly stable. Better-matched subsamples tend to yield a mass step consistent with zero within 1$\ddot{I}ƒ$. By disentangling SN subpopulations, we find persistent differences in $H_0$ ($\sim$2-3$\ddot{I}ƒ$) and $M_B$ ($\sim 2\ddot{I}ƒ$) between low- and high-stretch SNe: $H_0=75.27\pm 1.18$ km s${}^{-1}$ Mpc${}^{-1}$ for low-stretch and $H_0=71.25\pm 1.59$ km s${}^{-1}$ Mpc${}^{-1}$ for high-stretch, resulting in Hubble tensions of 6.07$\ddot{I}ƒ$ and 2.52$\ddot{I}ƒ$. These differences suggest SNe Ia subpopulations with varying dust and intrinsic colour not captured by a single ${\hat{I}}^2$, impacting cosmology. Estimating a single $H_0$ for both subpopulations yields $H_0=73.78\pm 2.17$ km s${}^{-1}$ Mpc${}^{-1}$, with a much larger uncertainty that lowers the Hubble tension from $5.87\ddot{I}ƒ$ to $\sim 2.86\ddot{I}ƒ$. Our results suggest that the mass step may arise from an over-correction of more than one SN subpopulation associated to different environments.

53 pages, 21 figures, 23 tables. Typos corrected. Appendices E, F, and G added