Seekers of dark matter are getting very excited. But has this elusive particle, which holds the key to where the universe’s mass is hidden, really been discovered?by Philip Ball / June 25, 2020 / Leave a comment
The world of fundamental physics has been set abuzz with the announcement of a possible sighting of something unknown. The prominent XENON1T collaboration, which operates a detector buried beneath Gran Sasso mountain in central Italy, announced on 17th June that it had seen a signal above “known backgrounds,” meaning that it may have detected particles of unknown origin—perhaps with some of the characteristics expected of a hypothetical candidate particle for dark matter. To put it in the simplest terms, it is possible that the project has finally seen some of the elusive particles thought to constitute about 85 per cent of the matter in the universe.
If so, the implications are immense, and the discovery would certainly rank alongside Nobel-Prize-winning detections of hitherto unknown particles and phenomena, such as the gravitational waves that garnered the 2017 Nobel, or the Higgs boson seen in the Large Hadron Collider at Cern in Geneva in 2012. But no one is cracking open the champagne yet, and for good reason. If past experience is any guide, there is always the risk of a false alarm: the signal could have more mundane origins.
Hitting the right tone here is quite a challenge. The easy response would be a cynical one: we’ve been here before with mistaken “sightings” of dark matter, so it’s all a storm in a teacup. But you don’t need to go to the other extreme of credulous, breathless headlines declaring a revolution in physics. A degree of cautious excitement is perfectly warranted while the XENON1T scientists comb through the details, and await independent verification, to see if the result will stand up to scrutiny.
As I explained last year for Prospect, dark matter is so-called because it seems to be almost totally invisible, not interacting either with light or with the ordinary matter from which all atoms are made. Pretty much its sole effect on the visible universe is via gravity: because it has mass, it exerts a gravitational tug on ordinary matter. The main reason dark matter is thought to exist at all is that there seems to be an unexplained gravitational influence in galaxies that holds them together as they rotate: the movements of the visible stars and gas can’t by wholly accounted for by their own gravity alone. There also seems to be something unseen that…