The XENON1T Time Projection Chamber TPC in a clean room.Photo: XENON1T / Purdue University
A team of physicists at the University of Cambridge suspects that dark energy may have muddled results from the XENON1T experiment, a series of underground vats of xenon that are being used to search for dark matter.
Dark matter and dark energy are two of the most discussed quandaries of contemporary physics. The two darks are placeholder names for mysterious somethings that seem to be affecting the behavior of the universe and the stuff in it. Dark matter refers to the seemingly invisible mass that only makes itself known through its gravitational effects. Dark energy refers to the as-yet unexplained reason for the universe’s accelerating expansion. Dark matter is thought to make up about 27% of the universe, while dark energy is 68%, according to NASA.
Physicists have some ideas to explain dark matter: axions, WIMPs, SIMPs, and primordial black holes, to name a few. But dark energy is a lot more enigmatic, and now a group of researchers working on XENON1T data says an unexpected excess of activity could be due to that unknown force, rather than any dark matter candidate. The team’s research was published this week in Physical Review D.
The XENON1T experiment, buried below Italy’s Apennine Mountains, is set up to be as far away from any noise as possible. It consists of vats of liquid xenon that will light up if interacted with by a passing particle. As previously reported by Gizmodo, in June 2020 the XENON1T team reported that the project was seeing more interactions than it ought to be under the Standard Model of physics, meaning that it could be detecting theorized subatomic particles like axions—or something could be screwy with the experiment.
“These sorts of excesses are often flukes, but once in a while they can also lead to fundamental discoveries,” said Luca Visinelli, a researcher at Frascati National Laboratories in Italy and a co-author of the study, in a University of Cambridge release. “We explored a model in which this signal could be attributable to dark energy, rather than the dark matter the experiment was originally devised to detect.”
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“We first need to know that this wasn’t simply a fluke,” Visinelli added. “If XENON1T actually saw something, you’d expect to see a similar excess again in future experiments, but this time with a much stronger signal.”
Despite constituting so much of the universe, dark energy has not yet been identified. Many models suggest that there may be some fifth force besides the known four known fundamental forces in the universe, one that is hidden until you get to some of the largest-scale phenomena, like the universe’s ever-faster expansion.
Axions shooting out of the Sun seemed a possible explanation for the excess signal, but there were holes in that idea, as it would require a re-think of what we know about stars. “Even our Sun would not agree with the best theoretical models and experiments as well as it does now,” one researcher told Gizmodo last year.
Part of the problem with looking for dark energy are “chameleon particles” (also known as solar axions or solar chameleons), so-called for their theorized ability to vary in mass based on the amount of matter around them. That would make the particles’ mass larger when passing through a dense object like Earth and would make their force on surrounding masses smaller, as New Atlas explained in 2019. The recent research team built a model that uses chameleon screening to probe how dark energy behaves on scales well beyond that of the dense local universe.
“Our chameleon screening shuts down the production of dark energy particles in very dense objects, avoiding the problems faced by solar axions,” said lead author Sunny Vagnozzi, a cosmologist at Cambridge’s Kavli Institute for Cosmology, in a university release. “It also allows us to decouple what happens in the local very dense Universe from what happens on the largest scales, where the density is extremely low.”
The model allowed the team to understand how XENON1T would behave if the dark energy were produced in a magnetically strong region of the Sun. Their calculations indicated that dark energy could be detected with XENON1T.
Since the excess was first discovered, the XENON1T team “tried in any way to destroy it,” as one researcher told The New York Times. The signal’s obstinacy is as perplexing as it is thrilling.
“The authors propose an exciting and interesting possibility to expand the scope of the dark matter detection experiments towards the direct detection of dark energy,” Zara Bagdasarian, a physicist at UC Berkeley who was unaffiliated with the recent paper, told Gizmodo in an email. “The case study of XENON1T excess is definitely not conclusive, and we have to wait for more data from more experiments to test the validity of the solar chameleons idea.”
The next generation of XENON1T, called XENONnT, is slated to have its first experimental runs later this year. Upgrades to the experiment will hopefully seal out any noise and help physicists home in on what exactly is messing with the subterranean detector.
More: What Is Dark Matter and Why Hasn’t Anyone Found It Yet?
