Bullets in the Dark

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Science  25 Mar 2011:
Vol. 331, Issue 6024, pp. 1493-1495
DOI: 10.1126/science.331.6024.1493-d

Dark matter, amounting to 85% of the matter of the universe, interacts with ordinary matter through gravity and possibly through the weak nuclear force. Massey et al. propose a statistical method that could help constrain the extent to which dark matter and ordinary matter interact via the weak force. When two clusters of galaxies collide, as happened in the Bullet cluster, the gas composing most of the ordinary matter in the clusters is slowed down by drag forces, temporarily separating from the dark matter, which passes unimpeded through the collision. The degree of separation reflects the degree of interaction of dark matter, but constraints are limited by the rarity of cluster collisions. Numerical simulations of cosmic structure formation tell us that clusters are built gradually from smaller pieces of infalling matter. The new method treats these pieces as bullets, whose ordinary matter is expected to drag behind the dark matter. Although individual separations between ordinary and dark matter may not be significant, averaging through a large number of clusters could provide constraints that are tighter than those provided by the collision in the Bullet cluster.

Mon. Not. R. Astron. Soc. 10.1111/j.1365-2966.2011.18246.x (2011).

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