Physicists have worked out that most matter in the Universe must be made up of "dark matter" we can't see, otherwise galaxies wouldn't have enough gravitational pull to hold themselves together. So far, the most likely candidates for dark matter are WIMPs, or weakly interacting massive particles, that have 50 to 100 times the mass of a proton. The trouble is, no one has ever detected one.
That may be because WIMPS, by definition, tend to pass straight through ordinary matter. Or dark matter might be made of something else. In 1999, Edward Kolb of the Enrico Fermi Institute in Chicago and his colleagues proposed another candidate particle that would have been created just after the big bang. The tremendous energies would have created particles 10 billion times the mass of WIMPs, so the researchers named these behemoths Wimpzillas.
"Although they have a weird name, Wimp-zillas are among the most reasonable of current speculative ideas in the field," says Angela Olinto, also of the Enrico Fermico Institute. And if Wimpzillas exist, they would explain another puzzle: where do ultra-high energy cosmic rays come from? These beams of particles bombard Earth from space. Their energies are too high for them to have travelled from a distant source, so they must have been created close by, but astrophysicists have no idea what in our neighbourhood could have caused them.
The annihilation or decay of Wimpzillas would create such high-energy particles says Kolb, along with Pasquale Blasi of the Arcetri Astrophysical Observatory in Florence, Italy, and Rainer Dick of the University of Saskatchewan in Canada, in a paper to be published in the journal Astroparticle Physics. As this superheavy dark matter would exist in our Galaxy, the particles could easily reach Earth.
While WIMPs have been impossible to detect, finding their heavier counterparts should be easier. The researchers have calculated that if ultra-high-energy cosmic rays are made by Wimpzillas, they should be composed mainly of gamma rays, rather than protons or atomic nuclei. And it should soon be possible to check this.
The next generation of cosmic-ray detectors such as the Pierre Auger Observatory in Argentina and the Extreme Universe Space Observatory, which will be deployed on the International Space Station, will be able to peer into the centre of our Galaxy. If Blasi and his colleagues are right, they expect to see gamma rays streaming out from the centre of the Galaxy, as Wimpzillas should be there in abundance. "It could revolutionise our understanding of basic physics in this century," says Dick.
Author: Anil Ananthaswamy
New Scientist issue: 1st June 2002
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