Bullet Cluster

. It is made up of two galaxy clusters that are colliding, one moving through the other, about 3.7 billion light-years away in the constellation Carina. The major components of the cluster pair—stars, gas and the putative dark matter—behave differently during collision, allowing them to be studied separately. The stars of the galaxies, observable in visible light, were not greatly affected by the collision, and most passed right through, gravitationally slowed but not otherwise altered. The hot gas of the two colliding components, seen in X-rays, represents most of the baryonic, or "ordinary", matter in the cluster pair. The gases of the intracluster medium interact electromagnetically, causing the gases of both clusters to slow much more than the stars. The third component, the dark matter, was detected indirectly by the gravitational lensing of background objects, as calculated using the best available theory of gravity in general relativity. This provides support for the idea that most of the gravitation in the cluster pair is in the form of two regions of collisionless dark matter, which bypassed the gas regions during the collision. The Bullet Cluster is one of the hottest-known clusters of galaxies. It provides an observable constraint for cosmological models, which may diverge at temperatures beyond their predicted critical cluster temperature. The bow shock radiation output is equivalent to the energy of 10 typical quasars. However, subsequent work has found the collision to be consistent with LCDM simulations, with the previous discrepancy stemming from small simulations and the methodology of identifying pairs. Earlier work claiming the Bullet Cluster was inconsistent with standard cosmology was based on an erroneous estimate of the in-fall velocity based on the speed of the shock in the X-ray-emitting gas. == As evidence against modified gravity ==

and X-ray light, respectively, and also mapped the gravitational potential using gravitational lensing. As shown in the images on the right, the X-ray gas is in the center, while the galaxies are on the outskirts. During the collision, the X-ray gas interacted and slowed down, remaining in the center, while the galaxies largely passed by one another, as the distances between them were vast. The gravitational potential reveals two large concentrations centered on the galaxies, not on the X-ray gas, where most of the normal matter is located. In ΛCDM one would also expect the clusters to each have a dark matter halo that would pass through each other during the collision (assuming, as is conventional, that dark matter is collisionless). This expectation for the dark matter is a clear explanation for the offset between the peaks of the gravitational potential and the X-ray gas which was detected at a statistical significance of 8. It is this offset between the gravitational potential and normal matter that was claimed by Clowe et al. as "A Direct Empirical Proof of the Existence of Dark Matter" arguing that modified gravity theories fail to account for it. In MOND, one would expect the "missing mass" to be centred on regions which experience accelerations lower than a0, which, in the case of the Bullet Cluster, correspond to the areas containing the galaxies, not the X-ray gas. Nevertheless, MOND still fails to fully explain this cluster, as it does with all other galaxy clusters, due to the remaining mass residuals in several core regions of the Bullet Cluster. Mordehai Milgrom, the original proposer of MOND, has posted an online rebuttal of claims that the Bullet Cluster proves the existence of dark matter. He contends that the observed characteristics of the Bullet Cluster could just as well be caused by undetected standard matter. He has argued that all galaxy clusters could host cold dense hydrogen gas clouds of roughly equal to the mass of the visible baryons which could explain the failures of MOND in galaxy clusters. Such cold dense hydrogen clouds are unlikely to exist however due to feedback from AGNs which prevent hydrogen gas from cooling. There are other alternate theories of gravity like the MOG and Many-body gravity (MBG), which claim to be able to explain the bullet cluster's weak gravitational lensing. ==See also==