TWO mysterious streams of radiation are emanating from the heart of our galaxy, and dark matter could explain them both. The question is: what kind of dark matter?
In 2003, data from the Wilkinson Microwave Anisotropy Probe (WMAP) revealed a 鈥渉aze鈥� of unexplained microwave radiation at the centre of the Milky Way. A year earlier, the satellite INTEGRAL found an unusual stream of gamma rays coming from the same region.
Various theories have attempted to explain these anomalies. Now two groups have come up with different explanations of how dark matter, the stuff thought to make up most of the universe鈥檚 mass, could be behind them both.
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Michael Forbes at the University of Washington in Seattle and Ariel Zhitnitsky of the University of British Columbia in Vancouver, Canada, have come up with the idea that hypothetical dark-matter 鈥渘uggets鈥�, concentrated at the galactic centre, are responsible. They envisage nuggets comprising a dense core of quarks 鈥� or their antimatter equivalents 鈥� surrounded by a skin of antimatter positrons.
Electrons colliding with the nuggets would annihilate with positrons in the skin, emitting gamma rays at 511 kiloelectronvolts (keV) 鈥� the same energy as the observed gamma radiation (). This annihilation would also heat up the nuggets, causing them to emit microwaves.
Dan Hooper of Fermilab in Batavia, Illinois, likes the idea because the nuggets are not made of exotic particles. The idea could also explain why there seems to be much more matter than antimatter in the universe 鈥� the 鈥渓ost鈥� antimatter could be locked up in the nuggets, he says.
鈥淒ark-matter nuggets could explain why there seems to be much more matter than antimatter in the universe鈥�
Not everyone is persuaded. One objection is that contact with regular matter would probably have destroyed the nuggets long ago. 鈥淚 think it鈥檚 very difficult to get the theory right for them to have survived since the early universe,鈥� says G. Scott Watson at the University of Michigan, Ann Arbor.
Neal Weiner of New York University and colleagues also believe they can explain both the microwave haze and the gamma rays 鈥� but using a different form of dark matter. Their idea builds on previous work predicting that possible dark matter particles called WIMPs would release high-energy electrons and positrons during collisions. These particles would largely end up being spun around in the galactic magnetic field, emitting microwaves in the process. Those particles that do mutually annihilate would produce gamma rays, though not at the observed 511 keV energy.
Weiner reckons that gamma rays of the right energy will be produced if the WIMPs are able to go into an excited state, allowing them to briefly store the energy from bumping into each other. This state would then decay, releasing electrons and positrons which in turn annihilate to emit 511 keV gamma rays ().
鈥淚t鈥檚 a long shot,鈥� says Hooper, but also an intriguing possibility. Weiner says that his version of dark matter could show up in the Large Hadron Collider when it is switched on later this year.
Cosmology 鈥� Keep up with the latest ideas in our .
