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Dark within light
Is dark matter lurking at the centre of our bright sun? Yes, say two research groups who believe the elusive stuff is cooling the solar core.

The insight doesn't significantly affect the sun's overall temperature. Rather, a core chilled by dark matter would help explain the way heat is distributed and transported within the sun, a process that is poorly understood.

Dark matter doesn't interact with light and so is invisible. The only evidence for its existence is its gravitational effects on other objects, including galaxies. These effects suggest dark matter makes up about 80 per cent of the total mass of the universe.

The idea that it might lurk at the heart of the sun goes back to the 1980s, when astronomers found that the number of ghostly subatomic neutrinos leaving the sun was only about a third of what computer simulations suggested it should be. Dark matter could have explained the low yield because it would absorb energy, reducing the rate of the fusion reactions that produce neutrinos.

However, the problem was solved another way when it was found that neutrinos oscillate between three kinds, only one of which was being detected on Earth. As a result, the idea of solar dark matter was dropped.

Now it is being resurrected in the light of recent searches for dark matter, which have put limits on the mass of the particles that it is made of and shown that it interacts only very weakly with ordinary matter. These led Stephen West of Royal Holloway, University of London, and his colleagues to explore what would happen if particles that fell within these limits exist in the sun.

Their simulations show that gravity would pull such dark particles to the centre of the sun, where they would absorb heat. Some of these dark matter particles would then carry this heat from the core to the surface, decreasing the core temperature.

Similar, earlier work published this week by Mads Frandsen and Subir Sarkar of the University of Oxford also supports the idea that dark matter in the sun would cool the core (Physical Review Letters, DOI: link). Their calculations used a dark matter particle with a mass of 5 gigaelectronvolts - lighter than the one in West's simulations.

Frandsen points out that this would make the dark matter particle about five times as heavy as a proton or neutron - which is consistent with the observation that there seems to be around five times as much dark matter as ordinary matter in the universe. "This is a very interesting dark matter candidate because it gives us a way to understand the ratio of matter to dark matter," he says.

Sarkar and Frandsen say that their solar dark matter particle also resolves another problem. Heat energy travels in the sun by conduction and radiation around the core, and by convection nearer the surface, but the position of the so-called convective boundary between these regions is disputed.

Simulations based on the sun's composition suggest that the boundary is further out than is indicated by sound waves detected on the surface of the sun, which are affected by the position of this boundary. Sarkar and Frandsen say that including their proposed dark matter particle in the simulations would bring this boundary inwards, resulting in closer agreement between simulations and observation.

Not everyone is convinced. Joyce Guzik, West's collaborator at Los Alamos National Laboratory in New Mexico, points out that while there is a problem with current models of the sun, the difficulty is that these models already give a lower solar temperature than the one observed. Adding a chilling effect at the core only makes this discrepancy harder to resolve.

We may not have to wait long to find out whether there is dark matter in the sun. Both research groups agree that if there is core cooling, it should reduce the output of some kinds of solar neutrinos by around 10 per cent. It should be possible to check for this reduction when neutrino detectors in Canada and Italy become able to collect more sensitive data.

Dark stars might make black holes

The sun may not be the only star with a potentially dark heart. We could soon find out whether dark matter helped form the enormous stars that turned into the supermassive black holes at the centre of most galaxies.

The origin of such black holes is a mystery. One theory says they are the remnants of the universe's first stars, thought to have formed inside massive dark matter clouds. These stars may have had cores rich in dark matter particles of a type that would have annihilated one another in bursts of radiation. This extra power could have allowed these stars to grow larger than ordinary ones, resulting in the formation of supermassive black holes when the stars died. But it was not clear whether there was any hope of detecting dark stars.

Then in June, a study led by Katherine Freese of the University of Michigan in Ann Arbor indicated that dark stars could attain up to 10 million times the sun's mass (The Astrophysical Journal, DOI: link).

In a subsequent study, Erick Zackrisson of Stockholm University in Sweden and colleagues have worked out the apparent brightness of such stars. They conclude that they should be within sight of NASA's infrared James Webb Space Telescope, due to be launched in 2013.