Matter accounts for 31.7% of the mass-energy content of the universe, and 84.5% of the matter is dark matter. In other words, what we can measure today (ordinary matter) accounts for only a tiny fraction of the universe’s mass-energy content. For years, scientists have been on the lookout for the elusive dark matter particles, as well as signs of dark energy. Efforts so far have been to no avail. Despite the setbacks, we know a thing for sure: dark matter exists. If it’s there, we’ll eventually find a way to detect it, but what if we’ve gone about this the wrong way? US physicists suggest a different approach: instead of looking for dark matter particles, we should be looking for evidence of their collision – dark radiation.

The Large Underground Xenon (LUX) Dark Matter Detector  is located 4,850 ft (about 1 mile) underground at the Sanford Underground Laboratory in the Homestake Mine in Lead, South Dakota. The LUX experiment needs to be operated underground in order to reduce signal background caused by high-energy cosmic rays at the Earth's surface. Construction workers turned this former gold mine into a state-of-the-art underground research facility. The tunnel to the right, shown at the time of construction, provides access to the cavern that houses the LUX experiment. (Image Credit: Matt Kapust, Sanford Underground Research Facility)

The Large Underground Xenon (LUX) Dark Matter Detector is located 4,850 ft (about 1 mile) underground at the Sanford Underground Laboratory in the Homestake Mine in Lead, South Dakota. The LUX experiment needs to be operated underground in order to reduce signal background caused by high-energy cosmic rays at the Earth’s surface. Construction workers turned this former gold mine into a state-of-the-art underground research facility. The tunnel to the right, shown at the time of construction, provides access to the cavern that houses the LUX experiment. (Image Credit: Matt Kapust, Sanford Underground Research Facility)

Scientists use several methods to listen for dark matter particles. At CERN, physicists have attempted to slam matter together at huge energies in hope they might produce dark matter particles. This is a more pro-active approach. The other is more passive and involves “waiting it out”. For instance, detectors are placed in space on satellites or on Earth in caves where there’s little noise. The idea is that, maybe, one day a dark matter particle will hit one of these detectors. Ian Shoemaker, a physicist at Penn State, argues that might never happen. One reason is that dark matter isn’t very dense in this part of the Universe, according to studies which measure the influence of dark matter (remember, we can only infer dark matter activity based on the effects it has on ordinary matter).

“If we add another way of looking for dark matter — the way, we suggest — then we will increase our chances of detecting dark matter in our underground cavities,” says Shoemaker.

Dark radiation – dark matter’s love child

What Shoemaker and fellows at the Los Alamos National Laboratory, USA suggest is scanning for dark radiation instead. The researchers believe that when two dark matter particles meet, they should behave like ordinary particles in the sense that these should collide and annihilate creating radiation in the process. “Underground detection experiments may be able to detect the signals created by dark radiation,” Shoemaker says.

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It might just work, and considering this alternate route doesn’t involve deploying new hardware, but just tweaking existent detectors, there’s no reason why we shouldn’t try it. How this could work was discussed at large in a paper published in Physical Review Letters.

Looking for telltale signs of dark matter particles isn’t exactly a new idea. Some satellites are tuned to look for signs of dark matter interaction in places like the center of our galaxy, the Milky Way, and the Sun may also be such an area.

“It makes sense to look for dark radiation in certain places in space, where we expect it to be very dense — a lot denser than on Earth,” explains Shoemaker, adding:

“If there is an abundance of dark matter in these areas, then we would expect it to annihilate and create radiation.”

However, these satellite experiments might be looking for the wrong signals. For instance, physicists assume that when dark matter particles annihilate, the interaction will produce photons, “but if dark matter annihilates into dark radiation then these satellite-based experiments are hopeless,” Shoemaker says.

While there’s no immediate practical use for the discovery of dark matter, discovering such particles could be the greatest scientific achievement in history.

“There is no way of predicting what we can do with dark matter, if we detect it. But it might revolutionize our world. When scientists discovered quantum mechanics, it was considered a curiosity. Today quantum mechanics plays an important role in computers,” Shoemaker says.

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