Cosmological implications of the anisotropy of ten galaxy cluster scaling relations

The hypothesis that the late Universe is isotropic and homogeneous is adopted by most cosmological studies. The expansion rate $H_0$ is thought to be spatially constant, while bulk flows are often presumed to be negligible compared to the Hubble expansion, even at local scales. Their effects on the redshift-distance conversion are hence usually ignored. Any deviation from this consensus can strongly bias the results of such studies and thus the importance of testing these assumptions cannot be understated. Scaling relations of galaxy clusters can be effectively used for that. In previous works, we observed strong anisotropies in cluster scaling relations, whose origins remain ambiguous. By measuring many different cluster properties, several scaling relations with different sensitivities can be built. Nearly independent tests of cosmic isotropy and bulk flows are then feasible. We make use of up to 570 clusters with measured properties at X-ray, microwave, and infrared wavelengths, to construct 10 different cluster scaling relations (five of them presented for the first time) and test the isotropy of the local Universe. Through rigorous tests, we ensure that our analysis is not prone to generally known systematic biases and X-ray absorption issues. By combining all available information, we detect an apparent $9\%$ spatial variation in the local $H_0$ between $(l,b)\sim ({280^{\circ}}^{+35^{\circ}}_{-35^{\circ}},{-15^{\circ}}^{+20^{\circ}}_{-20^{\circ}})$ and the rest of the sky. The observed anisotropy has a nearly dipole form. Using Monte Carlo simulations, we assess the statistical significance of the anisotropy to be $>5\sigma$. This result could also be attributed to a $\sim 900$ km/s bulk flow which seems to extend out to at least $\sim 500$ Mpc. These two effects are indistinguishable until more high$-z$ clusters are observed by future all-sky surveys, such as eROSITA.