Free Astronomy Magazine July-August 2015

SPACE CHRONICLES T he distribution of dark matter in the cluster is shown with blue contour lines. The dark mat- ter clump for the galaxy at the left is significantly displaced from the position of the galaxy itself, possibly implying dark matter-dark matter interactions of an unknown nature are occur- ing. [NASA, ESO/R. Massey] the location of their dark matter too loose to draw statistically significant conclusions.) Lead author Richard Massey at Dur- ham University, explains: “We used to think that dark matter just sits around, minding its own business, except for its gravitational pull. But if dark matter were being slowed down during this collision, it could be the first evidence for rich physics in the dark sector — the hidden Uni- verse all around us.” The researchers note that more in- vestigation will be needed into oth- er effects that could also produce a lag. Similar observations of more gal- axies, and computer simulations of galaxy collisions will need to be made. Team member Liliya Williams of the University of Minnesota adds: “We itself, even very slightly, through forces other than gravity. Dark mat- ter has never before been observed interacting in any way other than through the force of gravity. (Com- puter simulations show that the ex- tra friction from the collision would make the dark matter slow down. The nature of that interaction is un- known; it could be caused by well- known effects or some exotic un- known force. All that can be said at this point is that it is not gravity. All four galaxies might have been sepa- rated from their dark matter. But we happen to have a very good mea- surement from only one galaxy, be- cause it is by chance aligned so well with the background, gravitation- ally lensed object. With the other three galaxies, the lensed images are further away, so the constraints on know that dark matter exists be- cause of the way that it interacts gravitationally, helping to shape the Universe, but we still know embar- rassingly little about what dark mat- ter actually is. Our observation suggests that dark matter might interact with forces other than gravity, meaning we could rule out some key theories about what dark matter might be.” This result follows on from a recent result from the team which observ- ed 72 collisions between galaxy clus- ters [3] and found that dark matter interacts very little with itself. The new work however concerns the motion of individual galaxies, rather than clusters of galaxies. Researchers say that the collision between these galaxies could have lasted longer than the collisions observed in the previous study — allowing the ef- fects of even a tiny frictional force to build up over time and create a mea- surable lag. (The main uncertainty in the result is the timespan for the col- lision: the friction that slowed the dark matter could have been a very weak force acting over about a bil- lion years, or a relatively stronger force acting for “only” 100 million years.) Taken together, the two results bracket the behaviour of dark mat- ter for the first time. Dark matter in- teracts more than this, but less than that. Massey added: “We are finally homing in on dark matter from above and below — squeezing our knowledge from two directions.” n