CosmoCoffee

[Molly Swanson]

This paper is an interesting counterpoint to Lee and Komatsu 1003.0939 regarding the likelihood of an object like the Bullet Cluster existing in a LCDM universe.

Lee and Komatsu make the claim that the bullet is moving too fast: based on N-body simulations, they find that the estimated initial relative velocity of 3000 km/s is very rare - none of the bullet-like clusters in the N-body simulations (with a volume comparable to the Hubble volume) have such high velocities. They estimate the probability of finding a system with 3000 km/s relative velocity is 3.3x10⁻¹¹ to 3.6x10^−9.

Here Forero-Romero et al take a different approach: they use hydrodynamical simulations with gas and use the projected separation between the gas and dark matter as their main indicator rather than the relative velocity. In contrast to Lee and Komatsu, they find that 1−2% of the simulated clusters show a DM-gas displacement as large as the Bullet Cluster, from which they conclude that bullet-cluster-like objects can easily be found in an LCDM universe.

They are measuring somewhat different things, so they are not necessarily inconsistent with each other, despite coming to the opposite conclusions. Forero-Romero et al are working with a smaller simulation volume so they need to extrapolate to higher masses, but the claim of Lee and Komatsu depends on the estimate of 3000 km/s for the initial velocity, which is not directly measurable. The 3000 km/s number comes from the best fit model to non-cosmological hydrodynamic simulations of 0711.0967 - if the structure of the Bullet Cluster can be explained with a lower infall velocity, the disagreement with LCDM would no longer be valid.

So, which of these is more convincing? Do we need to worry about the Bullet Cluster disproving LCDM?