Day 808

Friday after Christmas.

1412.7719
How are is the Bullet Cluster (in a $\Lambda$CDM universe)?
Kraljic, Sarkar

Consider the expected number of Bullet-like systems on the sky up to a specified redshift, which allows for direct comparison with observations.  Using a Hubble volume N-body simulation with high resolution, investigate how the number of such systems depends on the masses of the halo pairs, their separations, and collision angle.  This enables extraction of an approximate formula for the expected number of halo-halo collisions given specific collisional parameters.  Use extreme value statistics to analyze the tail of the pairwise velocity distribution and demonstrate that it is fatter than the previously assumed Gaussian form.  Estimate that the number of DM halo pairs as or more extreme than the Bullet cluster is 1.3+2.0-0.6 up to z=0.3.  The discovery of more such systems would thus be a challenge to the standard cosmology.