14 DAY TRIAL // If you think a black hole is the most energetic event in the Universe, you might want to think again, after you see the surprising X-ray images of the galaxy cluster crashing into one another, images captured by the orbiting X-ray telescopes XXM-Newton and Chandra.
The Bullet Cluster is a supermassive cluster resulting from the collision of two smaller clusters, now revealing not only the biggest collision in the Universe, but also a lot faster than previously thought. It's relatively easy to find individual galaxies colliding, since they discard trails of hot gas that stretch across space, providing bright indicators of the crash.
However, it's much harder to observe entire clusters colliding, when each may contain from ten to thousands of galaxies. This particular event is thought to be the proof for the existence of black matter, as the resulting gases slowed more than the galaxies and measurements showed that large amounts of mass that should have fallen behind with the gases continued with the galaxies.
Seen in X-rays, the gases interacted electromagnetically, and, without the help of dark matter, their velocity shouldn't have changed more than the stars of the two colliding clusters. In the Bullet Cluster, the collision is happening across our line of sight, so we can clearly see the two clusters, one passing through the other, like an arrow through an apple.
The speed of the collision has been measured by Renato Dupke and colleagues from the University of Michigan, Ann Arbor, to be of over 3300 km/s, which is puzzling, because earlier computer models concluded that such a high speed could never be reached.
"There is now a growing body of evidence that these high collision velocities are possible," says Dupke, who lets cosmologists explain how this could happen. It is generally believed that this kind of large-scale collisions are rare in the deep space, ranging from less than one in a thousand clusters to one in a hundred.