Characterising galaxy cluster scaling relations as cosmic isotropy tracers using FLAMINGO simulations

¹ Leiden Observatory, Leiden University, P.O. Box 9513 2300 RA Leiden, The Netherlands
² SRON Netherlands Institute for Space Research, Niels Bohrweg 4, NL-2333 CA Leiden, The Netherlands
³ Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany
⁴ Lorentz Institute for Theoretical Physics, Leiden University, P.O. box 9506 2300 RA Leiden, The Netherlands

^⋆ Corresponding author: yujiehe@strw.leidenuniv.nl

Received: 1 April 2025
Accepted: 17 August 2025

Abstract

The standard cosmological model, ΛCDM, assumes isotropy on large cosmic scales. However, recent studies using galaxy cluster scaling relations have reported an apparent H₀ anisotropy at 5.4σ that could be attributed to large bulk flows extending beyond 500 Mpc, which is in disagreement with ΛCDM. To quantify the statistical tension of the observational galaxy cluster data used in past studies with ΛCDM, we utilised the isotropic (2.8 Gpc)³ run of the FLAMINGO (ΛCDM) simulations, the largest hydrodynamical cosmological simulation available to date. We created 1728 simulated lightcones and studied the apparent level of anisotropy traced by X-ray and thermal Sunyaev-Zeldovich scaling relations in the same cluster sample selection and methodology as in the past study. We find the probability of such apparent anisotropies randomly emerging in cluster scaling relations within a ΛCDM universe to be 0.12% (3.2σ). The discrepancy goes up to ∼3.6σ when modelled as a bulk flow at z <  0.1. We also find that statistical noise accounts for over 80% of the anisotropy amplitude in each lightcone, with large peculiar velocities contributing less than 20%. We also show that anisotropy amplitudes are highly sensitive to the intrinsic scatter in the scaling relations, with tighter relations providing stronger constraints. Nevertheless, the tension between the past results and ΛCDM persists, albeit at a lower significance than previously reported.