麻豆传媒

One of the world鈥檚 leading dark matter detectors has wrapped up a nearly two-year-long search for the mysterious particles, without finding a single whiff. The results suggest that the days may be numbered for the dominant model of dark matter.

We鈥檝e known since the 1930s that without dark matter鈥榮 gravitational pull, galaxies would spin themselves apart. This mysterious substance, which does not emit light or interact with normal matter except through gravity, should make up around 85 per cent of the universe鈥檚 mass.

After ruling out ordinary matter that just doesn鈥檛 emit much light, theorists settled on some basic characteristics for their quarry: it should be made up of particles that have some mass and interact weakly with other matter. They called them 鈥渨eakly interacting massive particles鈥�, or WIMPs, and set about building detectors that could catch them.

What鈥檚 unknown is how often these particles bounce off each other 鈥� their scattering cross section 鈥� and their mass. They should also occasionally bump into normal matter. These rare collisions are what experiments like the (LUX) are designed to pick up, in order to determine WIMPs鈥� properties.

But today at the 聽in Sheffield, UK, the LUX team announced their final 20-month run, from October 2014 to May this year, ended without a single dark matter detection. That means LUX has ruled out a large number of possible cross sections and masses for WIMPs 鈥� to the point where some physicists argue it might be time to abandon the idea all together.

鈥淚 think we are getting to the point where the limits are excluding so much of the parameter space that we should rethink,鈥� says 聽at Harvard University. 鈥淧erhaps the dark matter is not WIMPs.鈥�

A decade left

聽at Brown University in Rhode Island, who works on LUX, says there is still much more room to explore. Plans are already underway for an that will be 70 times more sensitive.

鈥淭hat鈥檚 a lot of new models you can test,鈥� he says. 鈥淲e keep knocking over models, and right now any one of those could turn out to be the correct one.鈥�

Gaitskell says the experiment鈥檚 technology, which detects flashes in pools of liquid xenon when they are hit by a WIMP, is improving faster than Moore鈥檚 law 鈥� the rate at which the number of transistors on a computer chip increases. That鈥檚 a good聽sign for those hoping we can still catch WIMPs 鈥� the next generation of detectors should be exponentially better, not just a bit better.

鈥淭o avoid everybody dying of boredom and running out of money, you have to do it as fast as you can,鈥� he says. But that means in a decade or so, WIMPs will be out of hiding places. 鈥淥n a 15-year view you have to be ready to admit that, if we fail to see anything.鈥�

Another ding against WIMPs is the lack of any new particles at the Large Hadron Collider, which could offer hints at how dark matter interacts. It has spotted signs of something with a mass of 750 gigaelectronvolts, but as it is produced by two photons colliding, it probably isn鈥檛 related, says Loeb.

鈥淭hat鈥檚 the only surprise at the LHC,鈥� says Loeb. 鈥淚 wouldn鈥檛 connect it to the dark matter.鈥�

Now what?

So if dark matter isn鈥檛 WIMPs, what is it? There are no shortage of alternatives waiting in the wings. Light-weight particles called axions are one option, while tiny black holes left over from the big bang are another.

Some renegades want to do away with dark matter altogether. at the Weizmann Institute in Rehovot, Israel, has fought a 30-year battle to explain the need for extra matter in the universe by instead changing the way gravity works on galactic scales, a theory called modified Newtonian dynamics (MOND).

鈥淚 am certainly not surprised when I see negative reports coming from dark matter search enterprises,鈥� says Milgrom, but he would like the search to continue.

鈥淣ot finding dark matter at higher and higher sensitivity will only strengthen the case for MOND,鈥� he says. 鈥淣ot looking for it at all will not help, and may lead to a standoff.鈥�

WIMPs have endured in part because theorists are unwilling to give up on decades of research built up around them 鈥� it鈥檚 always possible to tweak your model and save it from the latest experimental data, says Loeb.

鈥淭he good news about physics is that experiments set the agenda,鈥� he says. 鈥淭heorists that have no connections with experiments miss being wrong, and that鈥檚 not physics in its actual sense.鈥�

Gaitskell is no stick-in-the-mud 鈥� he quit a previous experiment, the Cryogenic Dark Matter Search, when it seemed like it wouldn鈥檛 be able to deliver evidence for WIMPs. But he says it鈥檚 too soon to count WIMPs out altogether.

鈥淧eople aren鈥檛 always ready to say to themselves, the scientific question we鈥檙e choosing to answer could take an entire lifetime,鈥� he says. 鈥淚 don鈥檛 think at this stage there has been any build up in the science that says it鈥檚 not WIMPs. I think it鈥檚 more that people would rather talk about something new.鈥�