Date: Apr 09, 2014 Source: NewScientist (
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Enzyme ice cubes and nano-explosives may be our best bets for making direct contact with dark matter, the stuff that reputedly makes up most of the matter in the universe.
The strongest indirect sign of dark matter so far comes from the centre of the Milky Way. A gamma-ray halo seems to be all that is left from collisions between hypothetical particles called WIMPs. These weakly interacting massive particles are the leading candidates for dark matter.
But if the gamma-ray signal is real, the WIMP creating it is much heavier than ones tentatively seen in existing detectors. These experiments sit deep underground, where they are shielded from most background radiation that can create false signals. But a clear sign has not yet emerged, making it difficult to reconcile the potential strikes with indirect clues from space.
WIMP grenade
Last week, Andrzej Drukier of Biotraces in Herndon, Virginia, and his colleagues posted designs online for cheaper, simpler WIMP detectors. The team argue that, because of the way they operate, these experiments would produce less ambiguous strikes.
In the first design, they suggest embedding nanoscale grains of metal in an oxide mineral. When a WIMP strikes the metal, it heats it up enough to make it react with the oxide and spark what's called a thermite reaction, just like a tiny hand grenade going off.
"They won't hurt you. They're like sparklers, or balloons popping," says team member Katherine Freese at the University of Michigan in Ann Arbor. Setting up an array of cells could help tell WIMPs apart from more ordinary particles. A weak WIMP would only be able to make a single cell explode, while background radiation would be strong enough to trigger several nearby cells.
Drukier was still concerned about what might happen should they ever want to scale up the design. "If you had a tonne of explosives, is it too risky or not?" he says. In theory, the detector should not trigger enough mini explosions to spark a dangerous cascade. "But if we are wrong, it will be seriously wrong."
Crackle and pop
So in their second paper, the team propose a detector that involves enzymes, the molecules that help drive chemical reactions in living creatures. Enzymes are similar to explosives in that they produce excess heat and energy, but the enzyme itself breaks up when it gets too hot, so the reaction can turn itself off.
Drukier and his team suggest freezing a block of peroxide and water containing enzymes and metal nanoparticles. Nothing happens as long as the block remains frozen. But if a WIMP scatters off the metal, its energy will melt the ice. When the enzymes meet liquid peroxide, they split it up and make gas. The resulting bubble cracks the ice, making a noise that microphones can pick up. The sonic data can pinpoint the direction and energy of the WIMP strike, so researchers can match the particle more closely to indirect whiffs of dark matter.
Right now both ideas are just theory, and Drukier has no immediate plans to build the dark matter devices. He notes, however, that a similar idea could be used to find hidden weapons of mass destruction by detecting the neutrons they emit.
"I don't think I will get money for dark matter, but I'm almost sure I can get money for homeland security," he says.
