Buried in a vast cave under 1,400m of rock beneath Italy’s Gran Sasso mountain range, the world’s largest and most sensitive dark matter detector is getting ready to spring to life.
Dubbed XENON1T, it’s the latest addition to the fleet of machines hunting for a mysterious yet elusive substance said to make up most of our universe. Researchers hope that it will start taking data this autumn.
Scientists suggest that there is only four per cent of matter that we can see, such as stars, planets and intergalactic gas; the rest is dark – invisible and thus far undetectable.
Approximately 73 per cent of the universe is believed to consist of so-called dark energy - a force that causes the universe to keep expanding and stop it from collapsing back in on itself. The remaining 23 per cent (which in fact comprises a weighty 85 per cent of the universe’s mass) is thought to be dark matter, whose gravity, researchers think, keeps the galaxies from flinging themselves apart as they rotate.
“Dark matter is the scaffolding that keeps together all structures in the Universe,” says Gianfranco Bertone, a physicist at the University of Amsterdam in the Netherlands. Without it, life as we know it may not have had the chance to evolve.
Scientists have been on the hunt for the mysterious substance ever since the Swiss astronomer Fritz Zwicky first used the term in the 1930s. Having observed the Coma galaxy cluster, he noted that its speed of revolution, which depends on the weight and position of the objects inside, implied the cluster had much more mass than he could see.
However, although there has been a considerable amount of indirect observational evidence for dark matter, we still have no idea what it is made of. XENON1T may provide some clues.
Bombardment by WIMPs
While we don’t know what they are, scientists think that millions of dark matter particles constantly pass through us undetected. The hope is that very rarely these particles do bump in to normal matter. It is these erratic instances that researchers are aiming to spot.
To improve our chances of registering the moment Weakly Interacting Massive Particles, or WIMPs – a leading dark-matter contender – interact with ordinary matter inside a detector, it has to be big; the bigger the better.
This image from the Hubble Space Telescope shows that a huge ring of dark matter likely exists surrounding the centre of CL0024+17 galaxy cluster [Credit: Nasa]
With one ton of frigid liquid xenon, 1T is the biggest yet. “There are strong theoretical reasons to believe that the dark matter mass and interaction strength with ordinary matter might be such that - while all previous experiments have been unable to directly detect dark matter - XENON1T may, for the first time in history, have the sensitivity to detect it,” says Ben Safdi, a physicist at the Massachusetts Institute of Technology in Cambridge, US.
If a WIMP happened to bump into a xenon atom, it would produce a tiny flash of light and a charge signal, which would be picked up by 248 ultra-sensitive photosensors inside XENON1T. The dark matter signals are predicted to have certain distributions in time and energy- the so-called signatures.
“If we have evidence for such a signature, and we can exclude that it comes from background noise, we might have detected dark matter particles,” says Laura Baudis, a physicist at the University of Zurich, one of the scientists working on the project.
To block out other particles that could interfere with the results, the detector is hidden under solid rock.
Why do we need another detector? After all, quite a few of them are dotted around the globe. The current leader in sensitivity is the Large Underground Xenon (LUX) at the Sanford Underground Research Facility in Lead, South Dakota. It uses 120 kg of liquid xenon, but will soon be dwarfed by the next-generation 7-ton LUX-ZEPLIN experiment, which is expected to start operating in 2016.
There are also other detectors, direct and indirect. A whole army of gamma-ray, neutrino and optical telescopes as well as cosmic ray observatories are searching for dark matter, alongside the attempts to create WIMPs inside the Large Hadron Collider at CERN. “Each of these approaches offers a different set of strengths and weaknesses and we get a much more complete picture by studying the data from them all together,” says physicist Dan Hooper at Fermilab in Batavia, Illinois.
While optical telescopes are good at spotting tell-tale indirect signs of dark matter, such as stars and gas being knocked off track by the gravity of an invisible material, such observations have so far not explained how dark matter fits in with the other particles in the universe.
What if the new mega-detector yields nothing? “Even a negative result from XENON1T would be very useful, in closing the door to a specific type of dark matter particle,” says Juan Collar, physicist at the University of Chicago.
“Dark matter is the most common ‘stuff’ in the universe,” says Richard Massey, a cosmologist at Durham University in the UK. “We're made from the odd stuff out. It's a bit embarrassing that we don't know what everything else is.”
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