Speaker
Description
The isotropy of cosmic expansion and the dissipation of bulk flows on large scales are fundamental assumptions of the $\Lambda$CDM paradigm. However, evidence from independent cosmological probes for large-scale cosmic dipoles and anomalous bulk flows continues to mount. These emerging tensions, therefore, require careful and independent scrutiny.
Galaxy clusters constitute powerful and largely independent probes of potential expansion anisotropies. In previous work, we exploited multiwavelength scaling relations of galaxy clusters to detect a $9\%$ dipole variation in the local Hubble constant ($H_0$) at the $4\sigma$ significance level. This apparent anisotropy can be interpreted as a coherent bulk flow of $\sim800$ km s$^{-1}$ extending to $\sim500$ Mpc, well beyond the depth currently accessible to peculiar-velocity galaxy surveys. Either interpretation would pose a significant challenge to $\Lambda$CDM in the local Universe.
In this talk, I will present new results based on X-ray, optical, and SZ observations of more than 3500 newly discovered galaxy clusters from eROSITA. Using numerous different scaling relations and a hierarchical Bayesian analysis, we detect a ~7\% H_0 dipole at a >4$\sigma$ level, in agreement, and fully independently, with cluster past studies. The anisotropy signal is further strengthened ($\sim 5\sigma$) when complementing our analysis with new X-ray and optical cluster data from other cluster samples, which independently support the presence of an expansion dipole anomaly or a $\sim 1000$ km/s cluster bulk flow extending to 600 Mpc. Our analysis localizes the signal predominantly to the nearby Universe ($z<0.2$), favoring a local origin rather than a purely global anisotropy.
If confirmed, this anomaly could have important implications for local $H_0$ determinations and the Hubble tension. I will conclude by highlighting the strong discovery potential of upcoming surveys (Euclid, LSST, DESI, 4MOST) for precision tests of cosmic isotropy with galaxy clusters.
| Other topic / keywords: | Cosmic dipole, anisotropy, bulk flows |
|---|