A particle physics experiment could have directly observed dark energy

About 25 years ago, astrophysicists noticed something very interesting about the Universe. The fact that it was expanding had been known since the 1920s, thanks to the observation of Edwin Hubble. But thanks to the observations that astronomers made with the space observatory that bore his name (the The Hubble Space Telescope), they began to notice how the rate of cosmic expansion was accelerating!

This led to the theory that the Universe is filled with an invisible and mysterious force, known as dark energy (DE). Decades after its proposal, scientists are still trying to pin down this elusive force that accounts for about 70% of the Universe’s energy budget. According to a recent study by an international team of researchers, the XENON1T experiment may have already detected this elusive force, opening new possibilities for future ED research.

The research was led by Dr Sunny Vagnozzi, a researcher at the Kavli Institute of Cosmology (KICC) at the University of Cambridge, and Dr Luca Visinelli, a Fellowship for Innovation (FELLINI) researcher (who is maintained with the support of the Marie Bourse Sklodowska-Curie) at the National Institute of Nuclear Physics (INFN) in Frascati, Italy. They were joined by researchers from the Institute for Theoretical Physics (IPhT), the University of Cambridge and the University of Hawaii.

Both DM and DE are part of the Lambda Cold Dark Matter (LCDM) model of cosmology, which posits that the Universe is filled with cold, slow-moving (DM) particles that interact with normal matter via the force of gravity alone. Lambda represents DE, which accelerates the expansion of the Universe. Since they are only discerned by observing their effect on the large-scale structure of the Universe, conventional thought holds that none of the forces interact with normal matter via electromagnetism or the weak or strong nuclear force. .

However, some DM theories posit that there is some level of interaction with visible matter, which researchers are actively testing. However, instead of more test results, astrophysicists and cosmologists still don’t know how DE fits into the physical laws that govern the Universe. Candidates so far include a modification of Einstein’s general relativity (GR), the presence of a new field, or a cosmological constant (CC). As Dr. Visinelli told Universe Today via email:

“For this reason, dark energy is perhaps even more mysterious than dark matter. We are seeing the effects of dark energy across a number of observations, starting with the seminal work on 1A supernovae in as standard candles. Assuming that dark energy is indeed a field, the quanta associated with it would be extremely light and carry very little energy. This is the reason why very little work has been done on this type of searches.

Their work is based on new research that goes beyond the standard LCDM model of cosmology to consider that DE interacts with light by affecting its properties (i.e. polarization, color, direction ). However, these interactions could be subject to screening mechanisms that prevent local experiments from detecting them. In this model, it is predicted that dark energy quanta can be produced in the Sun.

The XENON1T detector, shown below. Credit: XENOX Collaboration.

As Dr. Vagnozzi explained, the possible connection between screening and dark energy first came to him when he was taking a shower one day:

“I remember it was June 20 and I was taking a shower and thinking about solar axions (without) explaining XENON, and I realized the obvious solution was screening, because that would stop production in the denser stars. The screening is usually associated with dark energy and/or altered gravity patterns, and it just clicked.

I immediately WhatsApped Luca and we started working on it immediately (and reached out to our other co-authors who are experts in dark energy/modified gravity patterns).

For the purposes of their study, the team led by Dr. Vagnozzi and Dr. Visinelli reviewed data published by the XENON Collaboration, a DM research team made up of 135 researchers from 22 institutions around the world. At the heart of their experiment is a 3,500 kg (7,715 lb) detector of ultra-radio-pure liquid xenon housed in a 10 m (32.8 ft) tank of water. Located at the INFN Laboratori Nazionali del Gran Sasso, XENON is also the most sensitive dark matter (DM) experiment ever performed.

In 2020, the Collaboration published the results of its experimental campaign (2016 to 2018), which showed an unexpected rate of electronic recoil events. According to the collaboration, this did not constitute a detection of DM but could be explained by a tiny residual quantity of tritium in the experiment, the existence of a new particle (such as the solar axion), or an unexplained property of the neutrinos.

The upper PMT network with all the electrical cables. Credit: XENON Dark Matter Project

However, for the purposes of their study, the team led by Vagnozzi and Visinelli speculated that this may have been the first direct detection of DE. Said Vagnozzi:

“In our model, dark energy has special properties: its mass term is related to the density of the environment, so denser materials would shield the effects of dark energy, while denser environments light such as intergalactic space would allow for long range dark energy.

“In this model called “chameleon”, dark energy quanta are produced in the region of the Sun in which the electromagnetic field is strongest, the tachocline, which is the region in which the transport of energy to the interior of the Sun goes from the radiative transition to the convection.The high energy density of the electromagnetic radiation in the region allows a strong coupling with the chameleon field and its production.

If true, this would mean that experiments around the world currently focused on dark matter research could also be devoted to the hunt for dark energy. To that end, Dr. Vagnozzi and Dr. Visinelli hope that this study will spark interest in particle models of ED and that the search for these elusive particles can be conducted in parallel with ongoing DM research. At the very least, these experiments will test theories about ED that go beyond the LCDM model, helping scientists narrow the list of candidates. Says Dr. Visinelli:

“Many other experiments designed for Dark Matter may also contain information about these chameleons, and we hope that the design of future setups for this research will be considered. An independent test using cosmological data crossed with the predictions of the chameleon model would also be necessary. As for us, we plan to refine our paper’s calculations using a solar model, study chameleon production in massive stars, and get in touch with experimentalists for updates.

Illustris simulation, showing the distribution of dark matter in 350 million by 300,000 light-years. Galaxies are represented by high density white dots (left) and normal baryonic matter (right). Credit: Markus Haider/Illustris

In a recent paper, Dr. Vagnozzi and Dr. Visinelli conducted a study to examine whether pure elastic scattering between dark energy and baryonic (aka normal) matter could leave a visible imprint in cosmological observations. They determined that this was not likely, at least when applied to observations sensitive to the linear evolution of cosmic structure, such as the cosmic microwave background (CMB) and large-scale structure clustering. at the linear level.

However, Dr. Vagnozzi also works with a Ph.D. student in Munich to extend this study and predict the implications of DE interaction with normal matter. Specifically, they want to examine the effect this would have on the nonlinear clustering of the large-scale structure of the Universe, as well as the structure of galaxies and galaxy clusters. Coupled with large-scale surveys, which will benefit from next-generation telescopes in the years to come, astronomers and cosmologists could be on the verge of shedding light on the “dark universe”!

Further reading: physical examination D

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