Probing cosmic anisotropy with galaxy clusters and supernovae

TL;DR

This study investigates directional variations in the Hubble constant using galaxy clusters and supernovae, finding a consistent ~2σ deviation from isotropy aligned with the CMB dipole, across various models and calibrations.

Contribution

It introduces a combined analysis using galaxy clusters and supernovae with different calibrations to probe cosmic anisotropy in H_0.

Findings

H_0 variations are consistent across calibration methods.

Approximately 2σ deviation from isotropy detected.

Maximum ΔH_0 aligns with the CMB dipole direction.

Abstract

Using $\Lambda$CDM and Pad\'e-(2,1) cosmography, we study directional variations in the Hubble constant, $H_0$, using galaxy cluster and Type Ia Supernovae (from Pantheon Plus) by the hemisphere decomposition method. Since there is a degeneracy between $H_0$ and absolute magnitude $M_B$ for Supernovae, Cepheid host calibration is usually required to constrain $H_0$. Hence, in this work in order to complement the Cepheid host calibration in Supernovae, we also use calibrations based on galaxy cluster scaling relations. We find that there is a $\lesssim 1\sigma$ difference in $H_0$ variations when using galaxy clusters as calibrators compared to Cepheids highlighting that the variations in $H_0$ are robust across different calibration methods. Across all combinations of models and data sets used, we obtain a consistent deviation $\sim 2\sigma$ from isotropy. In nearly all cases, we notice that the maximum $\Delta H_0$ aligns with the CMB dipole direction.