THE nature of dark matter and the source of ultra-high energy cosmic rays are two of the biggest unsolved mysteries in physics. A mammoth instrument nearing completion in Argentina could soon tell us whether the two questions are linked.
According to standard physics, cosmic rays with energies greater than about 5 × 1019 electronvolts collide with photons of the cosmic microwave background left over from the big bang and so lose energy over long distances. This puts a limit on how energetic they are when they reach Earth, called the Greisen-Zatsepin-Kuzmin (GZK) cut-off.
So, when Japan’s Akeno Giant Air Shower Array (AGASA) experiment announced in 2004 that, after 15 years of running, it had detected 11 cosmic rays with energies greater than the GZK cut-off, cosmic ray physics was thrown into turmoil. To put it into perspective, cosmic rays with energies of about 1020 eV should reach Earth at the rate of one particle per square kilometre per century. AGASA should have just seen one or two particles, not 11.
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These findings could be explained if the ultra-high energy cosmic rays were coming from within our galaxy. They wouldn’t have had time to interact with the cosmic microwave background, and so would not be subject to the GZK cut-off. However, no one has discovered astrophysical processes in our galaxy capable of producing these particles. This led to exotic explanations for what AGASA saw. One was that the ultra-high energy cosmic rays were being produced by the decay of super-heavy dark matter. Such particles could have been created during inflation – when the universe expanded exponentially just after the big bang.
“Ultra-high energy cosmic rays could be produced by super-heavy dark matter in the centre of the galaxy”
This theory received a knock when the HiRes experiment in Salt Lake City, Utah, did not find ultra-high energy cosmic rays in the numbers seen by AGASA (New Scientist, 16 October 2006, p 14). This week preliminary results from the cosmic ray observatory (see Photos) – a 3000-square-kilometre detector being built in western Argentina – were presented at the International Cosmic Ray conference in Merida, Mexico. The results also suggest that super-heavy dark matter particles cannot be the main source of ultra-high energy cosmic rays.
However, Roberto Aloisio and Francesco Tortorici of the Gran Sasso National Laboratory in Assergi, Italy, think that super-heavy dark matter particles in our galaxy could yet account for some, if not all, of the ultra-high energy cosmic rays arriving on Earth, and that the Auger observatory should be able to confirm or rule out this theory within the next five years ().
The key to testing the idea is that super-heavy dark matter particles would decay to produce cosmic rays in the form of photons. These neutral photons would not be deflected by our galaxy’s magnetic field and would thus point back to their source, whereas cosmic rays produced by astrophysical processes such as black holes are expected to be charged particles and hence would be deflected and not point back to their source.
“We know that the dark matter is accumulated in the halo of the galaxy, with a huge density in proximity of the galactic centre,” says Aloisio. So, if super-heavy dark matter particles exist, they would gather in greater numbers in the galactic centre, and we should see more cosmic ray photons coming from there than from other directions in the Milky Way. “This is a very important signature and it can be tested by the Auger observatory,” says Aloisio. “And this can be achieved with four to five years of statistics.”
Pasquale Blasi of the Astrophysics Observatory of Arcetri in Italy, who is a member of the team running the Auger experiment, agrees. “The idea is certainly feasible,” he says. “Auger is accumulating a lot of data very fast, and in a few years it would be certainly possible to impose tight limits [on the super-heavy dark matter particles].”
Aloisio says that if Auger does find more ultra-high energy cosmic rays coming from the galactic centre than from other regions, the number of events could tells us what fraction of dark matter is made of super-heavy particles.
