SOHO/NASA
We don’t yet know what dark matter is made of, but the sun might help us find out. If dark matter particles are extremely light, they could bounce off atomic nuclei within the sun and gain enough energy in the process that we could detect them.
Chris Kouvaris at the University of Southern Denmark in Odense and his colleagues calculate that particles of this “sub-GeV” dark matter could be accelerated to speeds in excess of 600 kilometres per second in this way.
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Dark matter permeates the cosmos, so if these sub-GeV particles exist some of them should be hitting the sun all the time. As they bounce around within the sun, some would gain enough speed to escape towards Earth.
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Faster particles are easier to detect because they have more energy, so this solar boost could be the key to making dark matter visible to us.
But the sub-GeV dark matter particles will need to interact with the normal matter in our detectors. For this to happen, they will need a helper particle to mediate that interaction. For example, weakly interacting massive particles (WIMPs), another leading contender for dark matter, interact with ordinary matter by exchanging subatomic particles called W and Z bosons.
“There are basically two particles that are needed: you need the dark matter particles, and you need some other particle for them to interact through,” says Daniel McKinsey at the University of California, Berkeley. “Some of my more sceptical colleagues would say that it’s like believing in two tooth fairies. I say, if you believe in one tooth fairy, why not two?”
McKinsey says this idea is still a “big deal”, because it gives us an entirely new way to look for dark matter after decades of fruitless searching.
Need for speed
The upper speed limit for particles to remain inside our galaxy is 544 kilometres per second. If they go any faster than that, gravity won’t be able to keep them within the Milky Way.
“If it has a larger velocity than the escape velocity of the galaxy, then it’ll go out of the galaxy and we’ll never be able to detect it,” says Kouvaris. Unless, that is, Earth gets in the way – a prospect that is much more likely if the particles are accelerated to such high speeds by our sun.
Current detectors aren’t sensitive enough to see any of these particles that happen to hit our planet after getting a boost from the sun. But upcoming instruments like the upgraded CRESST-III detector in Italy may be able to detect them.
Dark matter makes up over 80 per cent of the universe’s mass, so if the new detectors do find these particles, Kouvaris and McKinsey agree, it’ll be the biggest discovery in modern physics. And if they don’t, researchers will simply have to find new ways to look.
Reference: arxiv.org/abs/1709.06573
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