Bullet Cluster: A Challenge to LCDM Cosmology

TL;DR

This paper uses large cosmological simulations to show that the high-velocity collision observed in the Bullet Cluster is extremely unlikely under the standard LCDM model, challenging its validity.

Contribution

It provides the first quantitative analysis demonstrating the incompatibility of the Bullet Cluster's high infall velocity with LCDM predictions.

Findings

The probability of such high infall velocities in LCDM is less than 4×10^{-9}.

The Bullet Cluster's observed velocity is incompatible with LCDM unless a lower velocity scenario is found.

The study constrains the initial conditions needed to explain the Bullet Cluster within cosmological models.

Abstract

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.