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 \Lambda CDM , given the inferred main cluster mass of 2 \times 10 ^ { 15 } M _ { \odot } .
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 can not presently be used to test \Lambda CDM .