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Last year, XENON1T, the world’s most sensitive dark matter detector, appeared to be a big hit. No dark matter, but something else. Maybe neutrinos, maybe solar axions, maybe radioactive pollution in the detector.
Now another team of physicists have come up with a different answer. The signal could be consistent not with dark matter, but with dark energy, they say. If this is what caused the peak in XENON1T detections, it represents an important step 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 that we cannot detect directly. We infer its existence because there is more gravity in the Universe than we can explain by counting the things we can detect – much more. About 5% of the Universe is normal matter, like stars, black holes, planets, and us. About 21% is dark matter.
The remaining 74 percent is dark energy. We were also unable to detect it directly; instead, we infer its existence in the accelerating expansion of the Universe. Something is causing the Universe to spread faster than we can explain, and we call it something dark energy.
“Although the two components are invisible, we know a lot more about dark matter, since its existence was suggested as early as the 1920s, while dark energy was not discovered until 1998,” said the cosmologist Sunny Vagnozzi of the Kavli Institute for Cosmology at the University of Cambridge in Great Britain.
“Large-scale experiments like XENON1T have been designed to detect dark matter directly, looking for signs of dark matter” hitting “ordinary matter, but dark energy is even more elusive.”
XENON1T is a tank filled with 3.2 tonnes of ultra-pure liquid xenon and equipped with arrays of photomultiplier tubes. It’s completely sealed and completely dark So researchers can detect the flash of electroluminescence as the particles interact, resulting in a tiny shower of electrons from the xenon atoms in what is known as the electron recoil.
Since the majority of them are produced by known particle interactions, we have a pretty good idea of how many electron recoil events are expected to take place as part of the general background noise. This number is approximately 232 ± 15 per year. Instead, XENON1T detected 285 events from February 2017 to February 2018.
The most likely explanation, scientists said, was a hypothetical type of particle called solar axions, which first floated in the 1970s to resolve the question of why strong atomic forces follow what is called the charge-parity symmetry, while most models say they don’t need.
But there is a problem: if the Sun can produce axions, all stars should. However, the heat loss observed in very hot stars places strict limits on the interactions of axions 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 a fifth force unknown to nature, beyond electromagnetism, gravity, and two nuclear interactions.
Since the accelerated expansion of the Universe is only detectable at a very large scale and gravity operates at local scales, any dark matter model suggesting a fifth force should also adequately explain why this force n is not evident in our astronomical neighborhood.
Vagnozzi and his team have developed a methodology based on a mechanism called chameleon screening, which avoids explaining why we don’t see the fifth force by assuming it’s too short in dense environments like ours.
“Our chameleon screening stops the production of dark energy particles in very dense objects, avoiding the problems encountered by solar axions,” said Vagnozzi.
“It also allows us to decouple what is happening in the very dense local Universe from what is happening at larger 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 to the substantive explanation only, with a confidence of 2.5 sigma.
This is still not as strong as the explanation of solar axions, which had a confidence level of 3.5 sigma; or neutrinos or radioactive pollution, both of which had a confidence level of 3.2 sigma.
It presents an alternative solution, one without the thorny issues associated with the others. As the researchers wrote in their paper, this “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 are sure, and with XENON1T currently being upgraded, we have a little bit of time to wait.
Even if the signal does not appear in the next round of sightings, the article laid the groundwork for thinking outside the box when detection is finally confirmed.
“It was really surprising that this excess could in principle be caused by dark energy rather than dark matter,” Vagnozzi said. “When things fit together like that, it’s really special.”
The research was published in Physical examination D.
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