Bullet Cluster: A Challenge to LCDM Cosmology
  To quantify how rare the bullet-cluster-like high-velocity merging systems
are in the standard LCDM cosmology, we use a large-volume 27 (Gpc/h)^3 MICE
simulation to calculate the distribution of infall velocities of subclusters
around massive main clusters. The infall-velocity distribution is given at
(1-3)R_{200} of the main cluster (where R_{200} is similar to the virial
radius), and thus it gives the distribution of realistic initial velocities of
subclusters just before collision. These velocities can be compared with the
initial velocities used by the non-cosmological hydrodynamical simulations of
1E0657-56 in the literature. The latest parameter search carried out recently
by Mastropietro and Burkert showed that the initial velocity of 3000 km/s at
about 2R_{200} is required to explain the observed shock velocity, X-ray
brightness ratio of the main and subcluster, and displacement of the X-ray
peaks from the mass peaks. We show that such a high infall velocity at 2R_{200}
is incompatible with the prediction of a LCDM model: the probability of finding
3000 km/s in (2-3)R_{200} is between 3.3X10^{-11} and 3.6X10^{-9}. It is
concluded that the existence of 1E0657-56 is incompatible with the prediction
of a LCDM model, unless a lower infall velocity solution for 1E0657-56 with <
1800 km/s at 2R_{200} is found.
