The bullet cluster is not a cosmological anomaly

The Bullet Cluster (1E0657-56) merger is of exceptional interest for testing the standard cold-dark-matter plus cosmological constant cosmological model, and for investigating the possible existence of a long- or short-range ''fifth-force'' in the dark sector and possible need for modifications of general relativity or even of Newtonian gravity. The most recent previous simulations of the Bullet Cluster merger required an initial infall velocity far in excess of what would be expected within the standard cosmological model, at least in the absence of additional forces or modifications to gravity. We have recently carried out much more detailed simulations than previously had been done, making pixel-by-pixel fits to 2D data-maps of the mass distribution and X-ray emission, allowing for triaxial initial configurations and including MHD and cooling. Here, we compare the initial conditions of the Bullet Cluster merger as found in our new simulations to the initial conditions in similar-mass merging clusters in the Horizon cosmological simulation. We conclude that our initial infall velocity, 2900 km/s at a separation of 2.5 Mpc, is consistent with ΛCDM, given the inferred main cluster mass of 2 × 10¹⁵ M_o. The initial concentration and shape found for the smaller (Bullet) cluster are typical for clusters of this mass range, but both quantities seem somewhat low for the larger (Main) cluster. Due to the paucity of examples of clusters with such a high mass in simulations, these features of the main cluster cannot presently be used to test ΛCDM.