On Larger $H_0$ Values in the CMB Dipole Direction

TL;DR

This study finds evidence that the Hubble constant $H_0$ varies across the sky, being larger in the direction of the CMB dipole, which could imply deviations from standard cosmological models.

Contribution

It provides observational evidence of anisotropic $H_0$ variations using quasars and gamma-ray bursts, suggesting potential new physics beyond FLRW cosmology.

Findings

$H_0$ is larger in the CMB dipole direction with >2σ significance in QSOs.

Combined data increases significance to ~3σ, indicating possible anisotropy.

Results are consistent with known cosmic dipole and galaxy cluster anisotropies.

Abstract

On the assumption that quasars (QSO) and gamma-ray bursts (GRB) represent \textit{standardisable candles}, we provide evidence that the Hubble constant $H_0$ adopts larger values in hemispheres aligned with the CMB dipole direction. If substantiated, this trend signals a departure from FLRW cosmology. In particular, QSOs show a definite trend, whereas our findings in GRBs are consistent with an isotropic Universe, but we show in a sample of GRBs calibrated with Type Ia supernovae (SN) that this conclusion may change as one focuses on GRBs more closely (mis)aligned with the CMB dipole direction. The statistical significance in QSOs alone is $\gtrsim 2 \sigma$ and when combined with similar trends in strong lensing, Type Ia SN and calibrated GRBs, this increases to $\sim 3 \sigma$. Our findings are consistent with reported discrepancies in the cosmic dipole and anisotropies in galaxy cluster scaling relations. The reported variations in $H_0$ across the sky suggest that Hubble tension may be a symptom of a deeper cosmological malaise.