Galaxy clusters as probes of cosmic isotropy

Abstract: Scaling relations of galaxy clusters are a powerful probe of cosmic isotropy in the late Universe. Owing to their strong cosmological dependence, galaxy cluster scaling relations can obtain tight constraints on the spatial variation of cosmological parameters, such as the Hubble constant ($H_0$), and detect large-scale bulk flow motions. Such tests are crucial to scrutinise the validity of $\Lambda$CDM in the local Universe and determine at what cosmic scales (if any) extra-galactic objects converge to isotropy within the Cosmic Microwave Background rest frame. This review describes the methodology for conducting cosmic isotropy tests with cluster scaling relations and examines possible systematic biases. We also discuss the results of past studies that reported statistically significant observed anisotropies in the local Universe. Finally, we explore the future potential of cluster scaling relations as a cosmic isotropy probe given the large amount of multi-wavelength cluster data expected in the near future.