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.
