Have we detected dark energy? Scientists say it's a possibility

Date:
September 15, 2021
Source:
University of Cambridge
Summary:
Dark energy, the mysterious force that causes the universe to
accelerate, may have been responsible for unexpected results from
the XENON1T experiment, deep below Italy's Apennine Mountains.



FULL STORY ==========================================================================
Dark energy, the mysterious force that causes the universe to accelerate,
may have been responsible for unexpected results from the XENON1T
experiment, deep below Italy's Apennine Mountains.


==========================================================================
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.

========================================================================== Story Source: Materials provided by University_of_Cambridge. The original
text of this story is licensed under a Creative_Commons_License. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Sunny Vagnozzi, Luca Visinelli, Philippe Brax, Anne-Christine Davis,
Jeremy Sakstein. Direct detection of dark energy: The XENON1T
excess and future prospects. Physical Review D, 2021; 104 (6) DOI:
10.1103/ PhysRevD.104.063023 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210915135120.htm

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