How rare is the Bullet Cluster (in a $\Lambda$CDM universe)?

TL;DR

This paper assesses how rare the Bullet Cluster is within the standard b1cdm cosmology by estimating the expected number of similar systems, considering their velocities, masses, and collision parameters, and finds such systems are very rare.

Contribution

It introduces a new method to estimate the expected number of Bullet-like systems using N-body simulations and extreme value statistics, improving upon previous probability-based approaches.

Findings

Estimated number of similar systems up to redshift 0.3 is about 1.3, with large uncertainty.

Requiring observed collision conditions reduces expected number to 0.1.

Discovery of more such systems could challenge the b1cdm cosmological model.

Abstract

The Bullet Cluster (1E0657-56) is well-known as providing visual evidence of dark matter but it is potentially incompatible with the standard $\Lambda$CDM cosmology due to the high relative velocity of the two colliding clusters. Previous studies have focussed on the probability of such a high relative velocity amongst selected candidate systems. This notion of `probability' is however difficult to interpret and can lead to paradoxical results. Instead, we 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 we investigate how the number of such systems depends on the masses of the halo pairs, their separation, and collisional angle. This enables us to extract an approximate formula for the expected number of halo-halo collisions given specific collisional parameters. We use extreme value statistics to analyse the tail of the pairwise velocity distribution and demonstrate that it is fatter than the previously assumed Gaussian form. We estimate that the number of dark matter halo pairs as or more extreme than 1E0657-56 in mass, separation and relative velocity is $1.3^{+2.0}_{-0.6}$ up to redshift $z=0.3$. However requiring the halos to have collided and passed through each other as is observed decreases this number to only 0.1. The discovery of more such systems would thus indeed present a challenge to the standard cosmology.