The Bullet Cluster and Λ Cold Dark Matter (CDM)

The Bullet Cluster consists of two colliding clusters of galaxies (well strictly speaking, the name Bullet Cluster refers to the smaller sub-cluster, moving away from the larger one). Gravitational lensing* studies of the Bullet Cluster are claimed to provide the best evidence for the existence of particle dark matter. But the Bullet Cluster isn’t the incontrovertible evidence for particle dark matter that we have been told it is; it’s possible to explain the Bullet Cluster with models of modified gravity. Modifying gravity works by introducing additional fields that are coupled to gravity. There’s no reason that, in a dynamical system, these fields have to be focused at the same place where the normal matter is. Modified gravity should have a path dependence that leads to such a delocalisation as is observed in this, and other, cluster collisions. https://arxiv.org/abs/1003.0939 *A gravitational lens is a distribution of matter (such as a cluster of galaxies) between a distant light source and an observer, that is capable of bending the light from the source as the light travels towards the observer. This effect is known as gravitational lensing, and the amount of bending is one of the predictions of Albert Einstein's general theory of relativity.