Περίληψη :
The isotropy of cosmic expansion and the dissipation of bulk flows at >150 Mpc scales is a crucial assumption of LCDM; as such, it should be scrutinized observationally. Multiwavelength scaling relations of galaxy clusters are a powerful tool to test the isotropy of the local Universe in numerous independent ways. In our past work, we used X-ray, infrared, and Sunyaev-Zeldovich cluster data to robustly detect an apparent 9% angular variation of the Hubble constant (H0) at a 5 sigma level. This anomaly is consistently found in independent cluster samples and multiwavelength scaling relations, and an exhaustive list of possible systematic biases cannot alleviate it. The apparent H0 anisotropy could be alternatively attributed to a 900 km/s cluster bulk flow extending up to 500 Mpc. Both of these scenarios strongly contradict LCDM in the local Universe. New scaling relations with higher precision and newly acquired X-ray and optical cluster data confirm the previous findings. Furthermore, we use the 1st eROSITA cluster catalog to test the angular variation of H0. Interestingly, eROSITA independently supports the existence of a cosmic anisotropy within 800 Mpc. We additionally use state-of-the-art cosmological, hydrodynamical simulations to confirm the rarity of the observed H0 variation within simulated LCDM universes. Finally, the future looks bright for this application. The vast amount of optical and X-ray cluster data coming from Euclid, DESI, and eROSITA in the near future will allow for an unprecedented characterization of cosmic isotropy using galaxy clusters from the local Universe up to z~1.5 scales.
