Imagine you've got two guns pointed at one another from a great distance away. If the guns are filled with bird shot, most of the little pellets are going to completely miss one another, simply passing through the space separating them, with only the occasional pellet bouncing off of another. If the guns are instead filled with liquid foam, the foam will stick together when the guns are fired at each other. And finally, if all you had was something immaterial, like a gun that fired neutrinos, whatever you fired would simply travel through space, unperturbed by anything it encountered.
Well, when the Universe takes two clusters of galaxies and collides them, it's doing all three of these things at once. The individual stars act like the bird shot, mostly passing through everything undisturbed, as they're far too small to collide with any sort of substantial frequency. The neutral gas acts like liquid foam, slowing down, dissipating energy, and -- in response -- heating up and emitting X-rays. And finally, we even have a form of matter that makes up most of the mass, dark matter, which doesn't interact with the stars, or the gas, or itself. The only way we know of its existence is through its gravity.
Image credit: X-ray: NASA/CXC/M.Markevitch et al. Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et... [+]
The above image, of the Bullet Cluster, was the first colliding cluster of galaxies ever discovered. The starlight is shown as imaged by Hubble, the neutral gas emits X-rays, which are shown in pink as imaged by the Chandra X-ray telescope, and the dark matter makes itself known through its gravitation, mapped out in blue. As you can see, while the gas -- which far outmasses the stars -- lags behind the stars in the galaxies, the dark matter doesn't at all, as it doesn't slow down in the slightest.
A team of astronomers, led by Richard Massey at the University of Durham, has been studying colliding galaxies, groups and clusters for years now, looking to map out the background galaxies from these cosmic trainwrecks. If you can see how that background light is distorted (or not), you can determine whether the dark matter in these collisions was at all slowed down by interacting either with the gas or with itself, or whether it's completely collision-free. For the first seventy-two groups and clusters that they looked at, they saw no slowdown at all.
Image credit: NASA, ESA, D. Harvey (École Polytechnique Fédérale de Lausanne, Switzerland) and R.... [+]
But then something funny happened. They looked at another one: the galaxy cluster Abell 3827. And while all the prior groups and clustered showed evidence that the dark matter had passed completely through the other clusters and galaxies, with no slowdown or separation at all, this new cluster showed something different. For the first time, the dark matter in one of the colliding objects appeared to be separated from the stars!
Now, there are three possibilities here:
- either this cluster collision is unique, perhaps occurring on slower timescales and so allowing for a very small frictional force to build up slowly,
- or there are normal, non-dark-matter-related astrophysical effects that affect these small groups of galaxies (there are only four here) rather than large clusters,
- or this is a measurement error, and not a real effect.
The third possibility, in particular, is one we really need to rule out. The galaxies we're looking at are an estimated 1.4 billion light years away, while the separation between the normal and dark matter appears to be only 5,000 light years, or less than one part in 100,000. The "statistical significance" of this result is only about 3.3σ, which means there's around a 0.1% chance this could be a fluke; we typically demand a 5σ result before we confidently state we've got a new discovery.
Image credit: R. Massey et al., "The behaviour of dark matter associated with 4 bright cluster... [+]
But the possibility that we've discovered the very first evidence that dark matter interacts with itself simply can't be ignored. If it does -- even if it's in a novel, unexpected way -- it could finally provide the evidence we've been seeking for so long that leads us to understand exactly what this, the most dominant form of matter in the Universe, is actually made of.
