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

Abstract

The persistent 4-6$σ$ 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 $H_{0}$, $M_{B}$, $α$, $β$, $Δ_{host}$, and $σ_{int}$. 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 $σ_{int}$, though overall values remain broadly stable. Better-matched subsamples tend to yield a mass step consistent with zero within 1$σ$. By disentangling SN subpopulations, we find persistent differences in $H_{0}$ ($\sim$2-3$σ$) and $M_{B}$ ($\sim2σ$) 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$σ$ and 2.52$σ$. These differences suggest SNe Ia subpopulations with varying dust and intrinsic colour not captured by a single $β$, 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 $5.87σ$ to $\sim2.86σ$. Our results suggest that the mass step may arise from an over-correction of more than one SN subpopulation associated to different environments.