27 March 2015
There’s nothing like darkish matter to fireside up a dialogue about astrophysics. We’ve got a number of proof to assist darkish matter, and we’ve decided a few of it’s bodily properties similar to the truth that it’s chilly reasonably than sizzling (ruling out issues like neutrinos). However as a result of we don’t know every little thing about darkish matter, and since we haven’t straight detected darkish matter particles in a lab, there are fixed accusations from most people that darkish matter is only a “patch” over our ignorance, or that astronomers have merely made up darkish matter so as to maintain the established order (and our jobs) alive. If we will’t see it, the argument goes, then we will’t be sure it exists. Besides we can see it, such because the blue smudges within the picture above.
The picture exhibits a false-color illustration of darkish matter in six totally different galaxy clusters. Simply as we will’t see infrared straight, however can detect it’s presence by way of CCD cameras, we will detect darkish matter by its gravitational results on background gentle. The photographs are a part of a latest work that analyzed the darkish matter distribution in 72 colliding galaxy clusters. The outcomes have additional outlined the character of darkish matter. For one factor, the distribution of normal matter and darkish matter are distinctly totally different in these clusters. A lot in order that the crew can validate the existence of darkish matter to 7.6σ. In different phrases, the possibility of those outcomes being a false constructive is
However the outcomes didn’t cease there. As a result of these are colliding galaxy clusters, the darkish matter from every area is colliding as properly. Since darkish matter is diffuse reasonably than clumped into stars, when darkish matter collides it’s going to work together, and the power of this interplay will be decided by the ensuing distribution. We measure the interplay power of particles as a cross part, and this work exhibits that the self-interaction cross part for darkish matter is lower than 0.47 cm2/g. That is fairly small, and it implies that not solely is darkish matter weakly interacting with common matter, it’s weakly interacting with itself. Mainly, it’s even darker than we thought.
This constraint is robust sufficient that it eliminates a number of the fashions attempting to increase the usual mannequin of particle physics. It additionally places into query a number of the claims of gamma rays being produced by colliding darkish matter. There are nonetheless issues to find out about darkish matter, and we’d nonetheless like to detect darkish matter straight, however the declare that darkish matter won’t exist is untenable at this level.