According to a new study published in the journal Physical Review D, some unexpected results from the XENON1T experiment, which was designed to study dark matter, may have been generated by dark energy rather than dark matter.
They built a physical model to help explain the data, which could have come from dark energy particles produced in a region of the Sun with strong magnetic fields, however further research is needed to corroborate this theory. According to the researchers, their findings could represent a significant step toward the direct detection of dark energy.
Everything visible to our eyes in the sky and in our everyday life – from tiny moons to gigantic galaxies, from ants to blue whales – accounts for less than 5 percent of the universe. The rest is in the dark. About 27 percent is dark matter, the unseen force that holds galaxies and the cosmic web together, and 68 percent is dark energy, which causes the universe to expand at a faster rate.
“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,” says Dr Sunny Vagnozzi – the paper’s first author.
“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.”
Generally, scientists hunt for gravitational interactions to find dark energy: the way gravity pulls particles around. And on the biggest scales, dark energy’s gravitational influence is repulsive, drawing objects apart and accelerating the expansion of the Universe.
About a year ago, the XENON1T experiment reported an unexpected signal, or excess, over the expected background.
“These sorts of excesses are often flukes, but once in a while they can also lead to fundamental discoveries,” says Dr Luca Visinelli – co-author of the study.
“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.”
At the time, the most commonly accepted explanation for the excess was that it was caused by axions — imaginary, incredibly light particles created by the Sun. This interpretation, however, does not fit the data, as the amount of axions required to explain the XENON1T signal would significantly modify the development of stars far heavier than the Sun, contradicting what we observe.
We are still far from understanding dark energy completely, but the majority of scientific models for dark energy imply the existence of a so-called fifth force. The cosmos is composed of four fundamental forces, and everything that cannot be explained by one of them is occasionally referred to as the outcome of an unknown fifth force.
We do know, however, that Einstein’s theory of gravity works exceptionally well in the local universe. Thus, any fifth force connected with dark energy is undesirable and must be ‘hidden’ or ‘screened’ on small scales, operating only on the biggest sizes when Einstein’s theory of gravity fails to account for the acceleration of the Universe. To conceal the fifth force, several dark energy models incorporate so-called screening systems that dynamically conceal the fifth force.
Vagnozzi and his co-authors developed a physical model using chameleon screening to demonstrate that dark energy particles created in the Sun’s strong magnetic fields might account for the XENON1T excess.
“Our chameleon screening shuts down the production of dark energy particles in very dense objects, avoiding the problems faced by solar axions,” adds Vagnozzi.
“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 authors used their model to demonstrate what would happen in the detector if the dark energy was generated in a region of the Sun called the tachocline, which has extremely high magnetic fields.
“It was really surprising that this excess could in principle have been caused by dark energy rather than dark matter,” said Vagnozzi. “When things click together like that, it’s really special.”
According to their findings, experiments such as XENON1T that are meant to detect dark matter might also be used to detect dark energy. However, the initial excess must be persuasively established.
“We first need to know that this wasn’t simply a fluke,” said Visinelli. “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.”
If the excess was caused by dark energy, future enhancements to the XENON1T experiment, as well as comparable experiments such as LUX-Zeplin and PandaX-xT, suggest that direct detection of dark energy may be possible within the next decade.
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