Dark energy, the mysterious force that causes the universe to accelerate, may be responsible for unexpected results from the XENON1T experiment, below the Italian Apennine Mountains.
A new study led by researchers at the University of Cambridge and published in the journal physical review d, suggests that some of the unexplained results from the XENON1T experiment in Italy may be caused by dark energy, not the dark matter the experiment was designed to detect.
“It was surprising that this excess was in principle caused by dark energy rather than dark matter. When things click together like that, it’s really special.” – Sunny Vagnozzi
They built a physical model to help explain the results, which may have originated from dark energy particles produced in a region of the Sun with strong magnetic fields, although future experiments will be required to confirm this interpretation. The researchers say their study could be an important step toward direct detection of dark energy.
Everything that our eyes can see in the sky and in our everyday world – from tiny moons to huge galaxies, from ants to blue whales – makes up less than five percent of the universe. The rest is dark. About 27% is dark matter – the invisible force that holds galaxies and the cosmic web together – while 68% is dark energy, causing the universe to expand at an accelerating rate.
Dr Sunny Fagnozzi of the Kavli Institute of Cosmology in Cambridge said: ‘Although both components are invisible, we know a lot about dark matter, its existence having been suggested as early as the 1920s, while dark energy wasn’t discovered until 1998. “. The paper’s first author. “Large-scale experiments like XENON1T are designed to directly detect dark matter, by looking for signs of dark matter ‘collides’ with ordinary matter, but dark energy is more difficult.”
To discover dark energy, scientists generally look for gravitational interactions: the way gravity pulls on objects. And on larger scales, the gravitational effect of dark energy is disgusting, pulling things apart and making the expansion of the universe accelerate.
About a year ago, the XENON1T trial reported an unexpected, or increased, signal over the expected background. “These types of abuse are often risky, but they can also occasionally lead to fundamental discoveries,” said Dr. Luca Vecinelli, a researcher at the Frascati National Laboratories in Italy, who is a co-author of the study. “We discovered a model in which this signal can be attributed to dark energy, rather than the dark matter the experiment was originally designed to detect.”
At the time, the most common explanation for the increase was axions — hypothetically, very light particles — produced in the sun. However, this interpretation does not hold up to observations, because the amount of axes required to interpret the XENON1T signal would radically change the evolution of stars much heavier than the Sun, in contrast to what we observe.
We are far from fully understanding what dark energy is, but most physical models of dark energy will lead to the existence of the so-called fifth force. There are four fundamental forces in the universe, and anything that cannot be explained by one of these forces is sometimes referred to as the result of an unknown fifth force.
However, we do know that Einstein’s theory of gravity works very well in the local universe. Therefore, any fifth force associated with dark energy is undesirable and should be “hidden” or “examined” when it comes to small scales, and can only work on larger scales where Einstein’s theory of gravity fails to explain the acceleration of the universe. To hide the fifth force, many models of dark energy are equipped with so-called screening mechanisms, which dynamically hide the fifth force.
Vagnozzi and colleagues built a physical model, which used a type of screening mechanism known as chameleon aliens, to show that dark energy particles produced in the Sun’s strong magnetic fields could explain the excess of XENON1T.
“Sifting our chameleon stops the production of dark energy particles in very dense objects, avoiding the problems that solar axes have,” Vagnozzi said. “It also allows us to separate what happens in the very dense local universe from what happens on larger scales, where the density is very low.”
The researchers used their model to show what would happen in the detector if dark energy was produced in a particular region of the sun, called the tachocline, where magnetic fields are particularly strong.
“It was really surprising that this excess was caused in principle by dark energy rather than dark matter,” Fagnozzi said. “When you click things together like that, it’s really special.”
Their calculations suggest that experiments like XENON1T, designed to detect dark matter, could also be used to detect dark energy. However, the original increase has yet to be convincingly confirmed. “We first need to know that this wasn’t just a coincidence,” Vesinelli said. “If XENON1T actually sees something, you’d expect to see a similar excess again in future trials, but this time with a much stronger signal.”
If the excess is caused by dark energy, upcoming upgrades to the XENON1T experiment, as well as experiments pursuing similar goals such as LUX-Zeplin and PandaX-xT, mean that dark energy could be directly detected within the next decade.
Reference: “Direct Detection of Dark Energy: Excess XENON1T and Future Prospects” by Sunny Fagnozzi, Luca Vecinelli, Philip Brax, Ann Kristen Davis, and Jeremy Sachstein, Sep 15, 2021, physical review d.
DOI: 10.1103/ PhysRevD.104.063023