A former South Dakota gold mine is the last place you might think to look to solve one of the universe’s biggest mysteries. Yet what lies buried in the Sanford Underground Research Facility, 1.47 km beneath the surface, could be our best chance of detecting the ghost of the galaxy: dark matter.
Deep within those old mine tunnels, accessible only by a shaft from the surface, is seven tonnes of liquid xenon, sitting perfectly still (figure 1).
This is the LUX-ZEPELIN (LZ) experiment. It’s looking for the tiny signatures that dark matter is predicted to leave in its wake as it passes through the Earth. To have any chance of success, LZ needs to be one of the most sensitive experiments on the planet.
“The centre of LZ, in terms of things happening, is the quietest place on Earth,” says Chamkaur Ghag, a physicist from University College London in the UK, and spokesperson for the LZ collaboration. “It is the environment in which to look for the rarest of interactions.”
For more than 50 years astronomers have puzzled over the nature of the extra gravitation first observed in galaxies by Vera Rubin, assisted by Kent Ford, who noticed stars orbiting galaxies under the influence of more gravity than could be accounted for by visible matter. (In the 1930s Fritz Zwicky had noticed a similar phenomenon in the movement of galaxies in the Coma Cluster.)
Most (though not all – see part one of this series “Cosmic combat: delving into the battle between dark matter and modified gravity“) scientists believe this extra mass to be dark matter. “We see these unusual gravitational effects, and the simplest explanation for that, and one that seems self-consistent so far, is that it’s dark matter,” says Richard Massey, an astrophysicist from Durham University in the UK.
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