A new study, led by researchers at the University of Cambridge and reported
in the journal Physical Review D, suggests that some unexplained results
from the XENON1T experiment in Italy may have been caused by dark energy,
and not the dark matter the experiment was designed to detect.
They constructed 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 explanation. The researchers say their study could be an
important step toward the direct detection of dark energy.
Everything our eyes can see in the skies and in our everyday world—from tiny
moons to massive 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 holding galaxies and the cosmic web together—while 68% is
dark energy, which causes the universe to expand at an accelerated 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," said Dr. Sunny Vagnozzi from
Cambridge's Kavli Institute for Cosmology, 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."
To detect dark energy, scientists generally look for gravitational
interactions: the way gravity pulls objects around. And on the largest
scales, the gravitational effect of dark energy is repulsive, pulling things
away from each other and making the Universe's expansion accelerate.
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,"
said Dr. Luca Visinelli, a researcher at Frascati National Laboratories in
Italy, a 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 popular explanation for the excess were
axions—hypothetical, extremely light particles—produced in the Sun. However,
this explanation does not stand up to observations, since the amount of
axions that would be required to explain the XENON1T signal would
drastically alter the evolution of stars much heavier than the Sun, in
conflict with what we observe.
We are far from fully understanding what dark energy is, but most physical
models for dark energy would lead to the existence of a so-called fifth
force. There are four fundamental forces in the universe, and anything that
can't be explained by one of these forces is sometimes referred to as the
result of an unknown fifth force.
However, we know that Einstein's theory of gravity works extremely well in
the local universe. Therefore, any fifth force associated to dark energy is
unwanted and must be 'hidden' or 'screened' when it comes to small scales,
and can only operate on the largest scales where Einstein's theory of
gravity fails to explain the acceleration of the Universe. To hide the fifth
force, many models for dark energy are equipped with so-called screening
mechanisms, which dynamically hide the fifth force.
Vagnozzi and his co-authors constructed a physical model, which used a type
of screening mechanism known as chameleon screening, to show that dark
energy particles produced in the Sun's strong magnetic fields could explain
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," said
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 researchers used their model to show what would happen in the detector
if the dark energy was produced in a particular region of the Sun, called
the tachocline, where the magnetic fields are particularly strong.
"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."
Their calculations suggest that experiments like XENON1T, which are designed
to detect dark matter, could also be used to detect dark energy. However,
the original excess still needs to be convincingly confirmed. "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 the result of dark energy, upcoming upgrades to the
XENON1T experiment, as well as experiments pursuing similar goals such as
LUX-Zeplin and PandaX-xT, mean that it could be possible to directly detect
dark energy within the next decade.
Reference:
Sunny Vagnozzi et al, Direct detection of dark energy: The XENON1T excess
and future prospects, Physical Review D (2021).
DOI: 10.1103/PhysRevD.104.063023
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