A new preprint asks, “How rare is the Bullet Cluster?”. In the paper, the authors use a numerical simulation of a universe with the current ΛCDM cosmology to see how many Bullet Cluster-like objects should exist in the universe. The simulation covers a volume of billions of cubic parsecs (a nontrivial fraction of the size of the visible universe) and contains dark matter “particles” with masses equivalent to tens of billions of suns. In this context, a “particle” is not a single dark matter particle but rather a clump of matter. Since matter is clumped together into these large “particles” in the simulation, the simulation will only be able to give information about what happens on scales much larger than a single “particle.”
The authors define a Bullet Cluster-like object via constraints, or cuts, on the properties of the sub-clusters. (Remember, the Bullet Cluster is the result of the collision of two large clusters and merger of some of the mass in the clusters.) These properties include the separation between sub-clusters, the sub-cluster masses, and the sub-cluster velocities. The search is restricted only to a limited range of redshifts. While the number of Bullet Cluster-like systems is quite dependent on the exact constraints, the authors find that it’s reasonable to expect around one such system given the constraints that they consider the most interesting. Thus, current knowledge of these systems in the real universe is consistent with the ΛCDM cosmology, but if too many more are discovered, their existence would potentially require cosmologists to reconsider their models.