Lindsey Bignell next to the ANU's 14UD particle accelerator at the Heavy Ion Accelerator Facility, which he uses to understand the response of the dark matter detector to different types of particles. Photo: Lannon Harley
Space frontier, down a gold mine
Scientists will build a research lab at the bottom of a gold mine in country Victoria in an effort to crack one of the Universe’s biggest mysteries. Will Wright reports.
An attempt to make one of the greatest discoveries about the Universe from the bottom of a gold mine more than a kilometre underground might seem counterintuitive to the average person, but not to ANU physicist Dr Lindsey Bignell.
He and his colleagues at ANU and other Australian universities are part of a global effort to detect dark matter, which scientists say is spread across the Universe and helps to hold galaxies together. The challenge is that dark matter is invisible.
“Dark matter constantly passes through you and me and the entire planet – it’s everywhere,” Bignell says.
“We know it’s there, but scientists still don’t have definitive proof that it exists. Finding that proof would be on par with recent Nobel Prize-winning discoveries of the Higgs Boson – a particle which helps give mass to elementary particles – and the detection of ripples in space and time known as gravitational waves. It would be enormously important.”
This begs the question, though – why go to the bottom of a gold mine in the Victorian town of Stawell to solve such a monumental mystery about space?
“Our dark matter lab will be a kilometre underground, which means that almost all high-energy particles from space bombarding the Earth’s surface will be blocked before they reach our detector,” Bignell says.
The new experiment, which is planned to start in late 2020, will operate two detectors: one in the southern hemisphere at Stawell and the other in the northern hemisphere, in Italy.
“By having a detector in each hemisphere we are better positioned to confirm a discovery, since dark matter should appear the same everywhere on Earth,” Bignell says.
Like many other scientific experiments, the team has chosen a long, technical name with an acronym that’s easy to remember: Sodium Iodide with Active Background Rejection Experiment – SABRE.
The SABRE team is growing special crystals (thallium-doped sodium iodide, for those in the know) designed to help measure some dark matter particles that pass through the Earth.
“Our detector will be set up in such a way that the crystals will light up when dark matter particles pass through them, which we will detect using extremely sensitive instruments that can measure the tiniest amount of light,” Bignell says.
“We need to ensure these crystals are as pure as possible, removing any radioactive contamination that could overwhelm the dark matter signal. To put this in perspective, your body per kilogram is about 10 million times more radioactive than our crystals.”
Our solar system is orbiting the centre of the Milky Way, which means that we’re constantly passing through a cloud of dark matter that envelops our galaxy.
As the Earth orbits the Sun, it moves faster into the dark matter cloud during part of the year and slower at other times.
“This shift in speed leads to a change in the rate that dark matter particles pass through the Earth and have a chance to interact in our detectors – we aim to detect that annual change.”
Achieving this feat would be impressive scientifically, but why should the average person care about proving that dark matter exists?
“Learning new things about the Universe is important for the same reason that creating music or making films is important,” Bignell says. “Our curiosity is part of what makes us special as humans.”
The search for dark matter is monumentally difficult, which forces scientists to work out problems that have never been solved before and push the limits of what is possible.
“We’re developing new technologies that could eventually find use in other areas and we’re attracting the best and brightest young scientists to Australia through our endeavours to answer this wicked science problem. Isn’t that worth getting excited about?”