The sun could be a net for dark matter, a new study suggests. If dark matter happens to take a certain specific form, it could build up in our nearest star and alter how heat moves inside it in a way that would be observable from Earth.

Dark matter is the mysterious stuff that makes up about 83 percent of the matter in the universe, but doesn't interact with electromagnetic forces. Although the universe contains five times as much dark matter as normal matter, dark matter is completely invisible both to human eyes and every kind of telescope ever devised. Physicists only know it's there because of its gravitational effect on normal matter. Dark matter keeps galaxies spinning quickly without flying apart and is responsible for much of the large-scale structure in the universe.

Current dark matter detectors are looking for WIMPs, or weakly interacting massive particles, that connect only with the weak nuclear force and gravity. Based on the most widely accepted theories, most experiments are tuned to look for a particle that is about 100 times more massive than a proton. The chief suspect is also its own antiparticle: Whenever a WIMP meets another WIMP, they annihilate each other.

"This is something that has always worried me," said astroparticle physicist Subir Sarkar of the University of Oxford. If equal amounts of matter and antimatter were created in the big bang, the particles should have completely wiped each other out by now. "Obviously that did not happen, we are here to prove it," he said. "So something created an asymmetry of matter over antimatter," letting a little bit of matter survive after all the antimatter was gone.

Whatever made regular matter beat out regular antimatter could have worked on dark matter as well, Sarkar suggests. If dark matter evolved similarly to regular matter, it would have to be much lighter than current experiments expect, only about 5 times the mass of a proton. That's a suggestive number, Sarkar says.

"If it were five times heavier, it would get five times the abundance. That's what dark matter is," he said. "That's the simplest explanation for dark matter in my view."

The trouble is, these light particles are much more difficult to detect with current experiments. In a paper in the July 2 Physical Review Letters, Sarkar and Oxford colleague Mads Frandsen suggest another way to find light dark matter: Look to the sun.

Because lightweight dark matter particles wouldn't vaporize each other when they meet, the sun should collect the particles the way snowballs collect more snow.

"The sun has been whizzing around the galaxy for 5 billion years, sweeping up all the dark matter as it goes," Sarkar said.

The buildup of dark matter could solve a pressing problem in solar physics, called the solar composition problem. Sensitive observations of waves on the sun's surface have revealed that the sun has a much easier time transporting heat from its interior to its surface than standard models predict it should.

Dark matter particles that interact only with each other could make up the difference. Photons and particles of regular matter bounce off each other on their way from the sun's interior to its surface, so light and heat can take billions of years to escape. But because dark matter particles ignore all the regular matter inside the sun, they have less stuff in their way and can transport heat more efficiently.

"When we do the calculation, to our amazement, it turns out this is true," Sarkar said. "They can transport enough heat to solve the solar composition problem."

Next, Sarkar and Frandsen calculated how being full of dark matter would affect the number of neutrinos the sun gives off. They found that the neutrino flux would change by a few percent. That's not much, Sarkar said, but it's just enough to be detected by two different neutrino experiments – one in Italy called Borexino and one in Canada called SNO+ – that are soon to get under way.

"It's a speculative idea, but it's testable," Sarkar said. "And the tools to test it are coming on line pretty fast. We don't have to wait 20 years."

The idea of lightweight dark matter influencing the sun is "not too much of a stretch, in my opinion," said physicist Dan Hooper of Fermilab in Illinois. "I look at their numbers, and they're very plausible to me."

Some puzzling results from dark matter detectors hint that these lightweight particles could have already been detected. Earlier this year, a germanium hockey puck in a mine in Minnesota called the Coherent Germanium Neutrino Technology (CoGeNT) detected a signal from a particle about 7 times the mass of the proton, though they're not sure yet whether it's dark matter. Another detector in Italy called DAMA has reported similar results.

"There's an increasingly compelling body of evidence accumulating" that dark matter is just a few times as massive as a proton, Hooper said. "The jury is still out, but if this is really what's going on, we should be able to know it with some confidence in the next year or so."

Update: Regular matter makes up 5 percent of the energy density of the universe, and dark matter makes up 25 percent (five times more than regular matter). The remaining 70 percent is dark energy.

Image: NASA/Solar Dynamics Observatory

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