Based on the assumption that quasars (QSOs) and gamma-ray bursts (GRBs) represent standardizable candles, we provide evidence that the Hubble constant $H_0$ adopts larger values in hemispheres aligned with the cosmic microwave background (CMB) dipole direction. If substantiated, this trend signals a departure from Friedmann-Lemaître-Robertson-Walker 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 (SNe) 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 SNe, 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.

• Received 22 October 2021
• Accepted 14 April 2022

DOI:https://doi.org/10.1103/PhysRevD.105.103510