Last year, XENON1T, the world's most sensitive dark matter detector, seemed to deliver a hit. Not of dark matter, but something else. Perhaps neutrinos, perhaps solar axions, perhaps radioactive pollution in the detector.
Now a different team of physicists has come up with a different answer. The signal could be consistent not with dark matter, but dark energy, they say. If this is indeed what caused the spike in XENON1T's detections, it represents an important milestone in the search for this mysterious force.
Dark energy, like dark matter, is unknown to us. Dark matter is the name we give to mass we can't detect directly. We infer its existence because there's more gravity in the Universe than we can account for by tallying the stuff we can detect – way more. Roughly 5 percent of the Universe is normal matter, like stars, black holes, planets, and us. Around 21 percent is dark matter.
The remaining 74 percent or so is dark energy. We haven't been able to directly detect it, either; instead, we infer its existence in the accelerating expansion of the Universe. Something is making the Universe spread faster than we can account for, and we call that something dark energy.
"Despite both components being invisible, we know a lot more about dark matter, since its existence was suggested as early as the 1920s, while dark energy wasn't discovered until 1998," said cosmologist Sunny Vagnozzi of Cambridge University's Kavli Institute for Cosmology in the UK.
"Large-scale experiments like XENON1T have been designed to directly detect dark matter, by searching for signs of dark matter 'hitting' ordinary matter, but dark energy is even more elusive."
XENON1T is a tank filled with 3.2 metric tons of ultra-pure liquid xenon and fitted with arrays of photomultiplier tubes. It's completely sealed and completely dark so researchers can detect the flash of electroluminescence as particles interact, resulting in a tiny shower of electrons from the xenon atoms in what's known as electron recoil.
Because the majority of these are produced by known particle interactions, we have a pretty solid idea of how many electron recoil events should be taking place as part of the general background noise. That number is around 232 ± 15 per year. Instead, XENON1T detected 285 events from February 2017 to February 2018.
The most likely explanation, scientists found, was a type of hypothetical particle called solar axions, first floated in the 1970s to resolve the question of why strong atomic forces follow something called charge-parity symmetry, when most models say they don't need to.
But there's a problem: If the Sun can produce axions, so should all stars. However, the observed heat loss in very hot stars places stringent limits on axion interactions with subatomic particles.
So, Vagnozzi and his team set out to test the possibility that dark energy was responsible for the excess. Now, dark energy may be a mystery, but most physical models of dark energy result in an unknown fifth force of nature, beyond electromagnetism, gravity, and two nuclear interactions.
Because the accelerated expansion of the Universe is only detectable on very large scales, and gravity works on local scales, any dark matter model that suggests a fifth force would also need to adequately explain why that force isn't obvious in our astronomical neighborhood.
Vagnozzi and his team developed a methodology based around a mechanism called chameleon screening, which avoids the mess of explaining why we don't see the fifth force by assuming it's too short-ranged in dense environments like ours.
"Our chameleon screening shuts down the production of dark energy particles in very dense objects, avoiding the problems faced by solar axions," Vagnozzi said.
"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."
Their results showed that dark energy particles from a strongly magnetic region of the Sun called the tachocline – between the radiative interior and the outer convective zone – could have produced the signal observed in the XENON1T data. This is preferred over the background-only explanation, with a confidence of 2.5 sigma.
It's still not quite as strong as the solar axions explanation, which had a confidence level of 3.5 sigma; or even neutrinos or radioactive pollution, which both had a confidence level of 3.2 sigma.
It does present an alternative solution, one without the thorny problems associated with the others. As the researchers wrote in their paper, it "raises the tantalizing possibility that XENON1T may have achieved the first direct detection of dark energy."
That is, of course, if the signal was real. We need another detection before we can be sure of that, and with XENON1T currently undergoing upgrades, we have a little while to wait.
Even if the signal doesn't show up in the next observing run, however, the paper has laid the groundwork for thinking outside the box when detection is finally confirmed.
"It was really surprising that this excess could in principle have been caused by dark energy rather than dark matter," Vagnozzi said. "When things click together like that, it's really special."
The research has been published in Physical Review D.
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