Most of the Universe is missing. By observing the way that galaxies spin, astronomers have calculated that there must be five times as much invisible ‘dark matter’ as ordinary stuff in the cosmos.
For three decades, attempts have been made to find dark matter particles, but with zero success. Now, a group of astrophysicists from Europe and the US think that the answer to detecting the elusive stuff may lie in the deep past, beneath our feet.
So far, most attempts to directly detect dark matter have focused on hypothetical particles known as WIMPs (‘weakly interacting massive particles’). Detectors have been built to look for the energy released when these particles hit the nuclei of other atoms, but with no collisions so far detected, it might be that WIMPs don’t exist. Or could it just be that we need a more sensitive detector?
The new idea is to look for the tracks of WIMPs in ancient rocks, detecting dark matter over geological timescales. WIMPs colliding with the atomic nuclei in certain rock minerals could, in theory, create tiny changes to the minerals’ crystal structure.
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“The interaction with dark matter would make the [atomic] nucleus move some tens to hundreds of nanometres through the crystal, leaving a damage track,” says Sebastian Baum at Stockholm University, who is taking part in the research.
The rocks being studied could be as much as one billion years old, with WIMPs potentially leaving tracks all that time. The researchers think that their method could reach a sensitivity of around 100 times that of the best existing detectors.
“We plan to dig up rocks from 10 kilometres below the surface of the Earth via ultra-deep boreholes that already exist,” says Dr Katherine Freese at the University of Michigan, who is also taking part in the study.
“The point of going deep underground is that the dark matter can penetrate that far down, whereas other particles, especially cosmic rays, get stuck closer to the surface.” So this’ll help to reduce the amount of tracks from other potential sources.
Once the samples have been obtained, the plan is to use imaging techniques such as helium-ion beam microscopy to look for the tracks, which may be 1,000 times shorter than the width of a human hair.
If all goes well with funding, the researchers hope to have their first results by 2025. If they don’t discover WIMPs, it’ll be back to the drawing board – again – for dark matter theorists. But if they do, it’ll solve one of the biggest mysteries in science.
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