Is dark matter cooling off Sun's core?
London: Dark matter is cooling off the core of our sun, according to two research groups.
While the insight doesn''t significantly affect the sun''s overall temperature, a cool core would shed new light on the way heat is distributed and transported within the sun.
Dark matter doesn''t interact with light and so is invisible. The only evidence of its existence is its gravitational effects on other objects, including galaxies. These effects indicate dark matter constitutes nearly 80 per cent of the total mass of the universe.
The idea that it might lurk at the heart of the sun is almost two decades old, 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. Consequently, the idea of solar dark matter was dropped.
Now it is being resurrected in the wake 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.
Led by Stephen West of Royal Holloway, University of London, researchers explored 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. Their calculations used a dark matter particle with a mass of 5 gigaelectronvolts - lighter than the one in West''s simulations.
According to Frandsen, 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," The News Scientist quoted him, as saying.
Sarkar and Frandsen believe 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 claim that including their proposed dark matter particle in the simulations would bring this boundary inwards, resulting in closer agreement between simulations and observation.
West''s study has been published on the pre-print server arXiv.org, while Frandsen and Sarkar''s work appears in Physical Review Letters.