Last year, physicists reported that an experimental dark matter detector picked up a strange signal that could hint at new physics, with several suspects highlighted. Now, Cambridge scientists have proposed an answer that wasn’t considered at the time – the experiment may have picked up the first direct detection of dark energy, the mysterious force that’s accelerating the expansion of the universe.
Although it’s thought to outnumber regular matter five to one, dark matter remains elusive. It doesn’t interact with light and seems to mostly make itself known through gravitational influence on cosmic scales, like stars, galaxies and galaxy clusters. But once in a while, a dark matter particle might bump into a regular matter particle in a way that we could detect, with the right equipment.
XENON1T was one version of that equipment. Running in Italy between 2016 and 2018, the experiment was essentially a big tank full of liquid xenon, kept deep underground. The idea was that if a dark matter particle zipped through the tank, it would excite the xenon atoms to produce a flash of light and free electrons, which a suite of sensors can detect.
But it’s not quite as cut-and-dried as it may sound. Other known particles could have the same effect. Putting the experiment underground helps reduce this noise, but not all of it. So the scientists calculate an expected background level of events, then check if the actual detections are higher than that.
And sure enough, it was. Last year, the scientists reported a “surprising excess of events” – 53 to be exact – over the expected background of 232. Something strange seemed to be happening, but was it dark matter?
The leading candidate at the time was a hypothetical elementary particle called a solar axion. As the name suggests, these would be produced by the Sun, and while they themselves aren’t thought to be a dark matter candidate, other types of axions are, so finding evidence of any of them would be a major step.
But after further investigation, the Cambridge team says that far too many solar axions would be needed to produce the observed signal. Instead, they suggest a different culprit – a force carrier particle for dark energy. That’s the name given to the repellent force that seems to be causing the expansion of the universe to accelerate, and one model for it involves what are called “chameleon” particles.
Essentially, these particles are predicted to have different masses and influence based on how much matter is around them. So in high density areas, such as Earth, their mass is large but their force is only exerted over a tiny distance. Out in interstellar space, however, where there’s next to no matter, the chameleons would have smaller masses but their influence reaches much further away. This kind of switching would explain the odd observation that dark energy doesn’t seem to have any effect locally but a strong effect on galactic scales.
That hypothesis might sound a little too convenient to be true, but that’s what they’re for – ideas that can be tested to find evidence for or against them. And, the Cambridge team argues, we may have found evidence for chameleons as dark energy carriers in the excess of events in XENON1T.
The researchers modeled what would happen if chameleon particles produced by the Sun, in a strongly magnetic region called the tachocline, passed through the XENON1T detector. And sure enough, the signal looked an awful lot like the one that was observed.
“It was really surprising that this excess could in principle have been caused by dark energy rather than dark matter,” says Dr. Sunny Vagnozzi, first author of the study. “When things click together like that, it’s really special.”
Of course, the case is far from closed. The excess events haven’t been properly confirmed yet, but advanced versions of the experiment may be able to verify the results.
“We first need to know that this wasn’t simply a fluke,” says Dr. Luca Visinelli, co-author of the study. “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.”
The research was published in the journal Physical Review D.
Source: Cambridge University
