DARK matter – the mysterious stuff that is the majority of the matter of the universe – is annihilating in the heart of our galaxy. That’s the controversial explanation for an unexpected excess of microwaves coming from the Milky Way’s centre.
NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) has built up a full-sky map of the microwave radiation left over from the big bang. To analyse this afterglow, astronomer Douglas Finkbeiner of Princeton University first took the microwave radiation expected from within our galaxy into account. While doing so, he noticed a surplus of microwaves coming from the galaxy’s centre.
The microwaves are typical of those generated when high-energy electrons and positrons spiral around magnetic fields, to produce what is known as synchrotron radiation. Finkbeiner was pondering the source of such particles when cosmologist Amber Miller of Columbia University in New York mentioned by chance that physicists were theorising that high-energy particles would be produced during the annihilation of dark-matter particles called neutralinos.
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And since gravity dictates that dark matter should be densest at the centre of our galaxy, annihilations would most likely occur there. Miller said that theorists were seeking experimental evidence of the resultant synchrotron radiation from the Milky Way’s core. Finkbeiner slowly warmed to the idea. “If it exists,” he told Miller, “I may have found it.”
Finkbeiner did some quick-and-dirty calculations based on the expected distribution of dark matter in the galaxy, the estimated mass of the neutralino and the likelihood of the particles annihilating. “I did the calculations on the back of an envelope,” he says. He found that the annihilations could account for the excess of microwaves, and more detailed calculations confirmed the possibility ().
Other evidence supports this theory, says Finkbeiner. NASA’s Compton Gamma Ray Observatory has seen an excess of gamma rays coming from the galactic centre. Some physicists have suggested that this is starlight which has had an energy boost from the very same high-energy electrons that are needed to explain the microwaves. “When you get two separate lines of argument pointing to something it gets important,” says Finkbeiner. “It’s one of the biggest surprises of my career.”
Daniel Hooper of the University of Oxford – a member of a team that claimed in 2003 that the annihilation of dark matter particles lighter than neutralinos could explain the excess of some, but not all gamma rays – is cautious about Finkbeiner’s work. “I think it is very interesting, but I’m not yet completely convinced about the details,” Hooper says.
WMAP project scientist Charles Bennett of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, is more sceptical. “If true, it would certainly be a great WMAP discovery,” he says. However, Bennett thinks the synchrotron emission could simply be due to high-energy electrons generated by supernova explosions.
Conclusive evidence could come from the Gamma Ray Large Area Space Telescope, due for launch in 2006. “Rare dark matter annihilations will result in two gamma-ray photons of a very precise energy,” says Finkbeiner. “If we’re really lucky, we’ll see a sharp peak in the gamma-ray spectrum.”