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

TL;DR

This study investigates how differences in supernova samples affect the measurement of the Hubble constant and explores the existence of distinct supernova subpopulations that influence cosmological inferences.

Contribution

It reveals that sample mismatches and supernova subpopulations significantly impact H0 estimates and the observed Hubble tension, highlighting the importance of accounting for these factors.

Findings

Sample matching improves parameter stability.

Distinct supernova subpopulations show different H0 values.

Accounting for subpopulations reduces Hubble tension from 6σ to ~3σ.

Abstract

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