Deep underground, scientists are closing in on one of the most elusive targets of modern science: dark matter. In subterranean laboratories in the US and Italy, they have set up huge vats of liquid xenon and lined them with highly sensitive detectors in the hope of spotting subatomic collisions that will reveal the presence of this elusive material.
However, researchers acknowledge that the current generation of detectors are reaching the limit of their effectiveness and warn that if they fail to detect dark matter with these types of machines, they could be forced to completely reappraise their understanding of the cosmos.
“Dark matter accounts for around 85% of all the universe’s mass but we have not been able to detect it so far – despite building more and more powerful detectors,” said physicist Professor Chamkaur Ghag of University College London. “We are now getting close to the limits of our detectors and if they do not find dark matter in the next few years, we may have to accept there is something very wrong with the way we think about the universe and about gravity.”
The hunt for dark matter began last century when astronomers found that galaxies appeared to be rotating too quickly to remain stable. Observations indicated they must have masses 10 times greater than their visible contents – stars, planets and dust clouds – otherwise they would tear themselves apart.
The missing material generating the extra gravity needed to hold galaxies together was dubbed “dark matter”. Astronomers initially thought it could be made up of stars too small or dim to be seen from Earth or by other candidates – such as neutron stars. However, new generations of powerful telescopes showed these were not viable possibilities.
So scientists turned from the astronomically large to the incredibly small to explain the universe’s missing mass. Vast numbers of undetected particles form invisible halos around galaxies and boost their gravitational fields, they argued. These hypothetical particles are called wimps – weakly interacting massive particles – and for two decades researchers have strived to detect them.
These efforts have involved building detectors deep underground where they are shielded from subatomic particles – triggered by cosmic rays hitting the upper atmosphere that constantly shower down on Earth and which would trigger streams of false positive readings on their instruments.
“The expectation has been that a wimp will strike a xenon nucleus and the resulting flash of light will be spotted by a detector and so reveal the presence of a dark matter wimp,” said Ghag. “Despite years of effort, we have yet to see a single flash like that, however. We need greater sensitivity.”
Now researchers are pinning their hopes on the two most sensitive wimp-hunters ever designed. One, built below Italy’s Gran Sasso mountains, is known as XENONnT. The other, Lux-Zeplin, has been constructed in an old South Dakota gold mine. Both devices have been filled with several tonnes of xenon – much more than has been put in any previous device – and that should increase chances of a nucleus being struck by a wimp.
Ghag, a member of the Lux-Zeplin team, said: “Both devices are now being put through operational tests, and in a few months those trials will be completed. We may find we have detected dark matter over that period – which would be very good news. If not, both devices will be run without interruption for several years. Essentially, the more xenon we have in our machines and the longer we run our detectors, the better our prospects of collisions occurring and dark matter revealing its presence.”
However, it is now accepted there is a prospect that this will not happen and dark matter could remain elusive. As Mariangela Lisanti, a physicist at Princeton University in New Jersey, stated in the journal Science recently: “The wimp hypothesis will face its real reckoning after these next-generation detectors run.”
If Lux-Zeplin and XENONnT fail to find Wimps, the two teams of scientists will have one final chance to use current technology to find them – by joining forces to create one final super-large detector that would contain tens of tonnes of xenon, a rare and expensive gas to isolate, and which would be run for several years.
And if that last-chance detector fails to find dark matter, scientists would be stumped. Making their machines even more sensitive would result in them being swamped by signals triggered by another type of subatomic particle, the neutrino, which rain down on the Earth in their trillions every second. Other approaches would have to be taken.
“It could be that in looking for wimps, we’re looking for our keys under the street lamp,” added Ghag. “Dark matter could be a lot weirder than we have assumed so far. It could be made of tiny black holes. Or it could be made of something that’s a million times lighter than a wimp and detecting that will be very hard. So we will have to be a lot more sophisticated in our attempts at detection.”
Such efforts to find a form of matter that can scarcely interact with normal matter may seem unnecessary. But if it were not for dark matter’s pervasive gravitational influence, galaxies and stars and planets would not have held together in the early universe and life as we know it would not have evolved. Hence scientists continuing efforts to discover its true nature.
