In a lab at the University of California, Berkeley, just off Oppenheimer Way, the street named after the “father of the atomic bomb”, a team of physicists meticulously adjusts a sophisticated device in search of the elusive “chameleon particle”.
A buzz of anticipation fills the air as they prepare to embark on an experiment that could unlock one of the deepest mysteries of the universe: dark energy.
Assuming that Lambda-CDM model of cosmology is correct, dark energy makes up nearly 70% of the total energy in the observable Universe and is driving its accelerated expansion. Yet despite its vast influence, this mysterious force remains shrouded in mystery.
The first direct evidence of dark energy was discovered in 1998 by two teams of scientists led by Dr. Saul Perlmutter of the Lawrence Berkeley National Laboratory, Dr. Brian P. Schmidt of the Australian National University, and Dr. Adam G. Riess from John Hopkins University. .
Thanks to the observation of distant supernovae, researchers understood that the Universe was expanding at an increasingly rapid rate. This revelation earned the three scientists the Nobel Prize in Physics in 2011.
“The acceleration is thought to be caused by dark energy, but what this dark energy is remains a puzzle – perhaps the greatest in physics today,” the Nobel laureate said. announcement The Royal Swedish Academy of Sciences states: “What is known is that dark energy makes up about three-quarters of the Universe. Therefore, the discoveries of the 2011 Nobel Prize winners in physics have helped to unveil a Universe that is, to a large extent, unknown to science. And everything is possible again. »
Independent observations, including experiments on the cosmic microwave background and studies of the redshift of galaxies, have confirmed the existence of dark energy. Yet twenty-six years after its initial discovery, the exact nature of dark energy remains “perhaps the greatest” enigma in physics.
Various theories have been proposed to explain its existence, including the possibility that dark energy could be the energy of the vacuum of space or a dynamic energy field called quintessence.
Another interesting proposition is that dark energy would be transmitted by a yet-to-be-discovered exotic scalar particle that exerts a repulsive force depending on the density of the surrounding matter. This hypothetical particle, known as the “chameleon particle” or “symmetron,” would represent a fifth fundamental force of nature, much weaker than gravity.
In the vacuum of space, a chameleon particle would exert a repulsive force over long distances, causing the accelerated expansion of the Universe. However, the particle’s range would be extremely limited on Earth, surrounded by matter. This would explain the anomalous impact of dark energy on the accelerated expansion of space.
Today, at UC Berkeley’s Holger Müller Laboratory, physicists are breaking new ground to solve the mystery of dark energy. They’ve designed the most precise instruments yet, capable of measuring even the smallest gravitational anomalies.
Detecting even minor deviations in the accepted theory of gravity would be a major breakthrough, providing proof of the existence of the hypothetical chameleon particle.
In recent experiments, physicists designed a new instrument that combines an atom interferometer for precise measurements of gravity with an optical lattice to hold atoms in place.
This setup allowed the researchers to hold free-falling atoms for much longer periods, improving the precision of their measurements by a factor of five compared to previous experiments.
By immobilizing small clusters of cesium atoms in a vertical vacuum chamber, the researchers could split each atom into a quantum state. In this state, half of the atom is closer to a weight of tungsten, allowing the scientists to measure the phase difference between the two halves of the atomic wave function. This process allows them to calculate differences in gravitational pull with unprecedented precision.
In the results just published in NatureThe researchers revealed that despite the revolutionary experimental design, the results showed no deviation from Newtonian gravity.
Still, physicists hope that with expected improvements in the precision of their new instrument, exciting new possibilities will open up for testing theories about the nature of dark energy, including the existence of the chameleon particle.
The ability of this new technology to hold atoms for up to 70 seconds and potentially 10 times longer expands the possibilities for studying gravity at the quantum level, explained Dr. Holger Müller, professor of physics at UC Berkeley and co-author of the study.
Previous experiments have clearly established the quantum nature of three of the four forces of nature: electromagnetism and the strong and weak forces. However, the quantum nature of gravity has never been verified.
“Most theorists probably agree that gravity is quantum,” Dr. Muller said in a paper. release by the University of Berkeley. “But no one has ever seen an experimental signature of this.”
“It’s very difficult to know whether gravity is quantum, but if we could hold our atoms 20 or 30 times longer than anyone else, because our sensitivity increases with the second or fourth power of the holding time, we could have a holding time of 400,000 to 800,000 times.” There is a many times greater chance of finding experimental proof that gravity is indeed quantum mechanics.
This new experimental design keeps atoms in a quantum superposition of two states, each subject to slightly different gravitational forces, allowing researchers to detect minute differences in gravitational attraction. This ability could potentially reveal the presence of the hypothetical chameleon particles or other unknown exotic phenomena related to dark energy.
In addition to its potential for discovering dark energy, the lattice atom interferometer designed by Muller’s team holds promise for a variety of applications, including quantum sensing.
This technology is particularly sensitive to gravity and inertia effects, making it suitable for building advanced gyroscopes and accelerometers. The optical lattice’s ability to hold atoms rigidly also makes it resistant to environmental imperfections or noise, which could enable precise measurements in harsh environments, such as at sea.
Since 2015, Dr. Muller has been searching for evidence of chameleon particles using an atom interferometer.
“Atom interferometry is the art and science of exploiting the quantum properties of a particle, that is, the fact that it is both a particle and a wave. We split the wave so that the particle follows two paths at the same time, and then we interfere with them at the end,” explains Dr. Müller. “Waves can either be in phase and add together, or they can be out of phase and cancel each other out. The problem is that whether they are in phase or out of phase depends very sensitively on some quantity that you want to measure, such as acceleration, gravity, rotation, or fundamental constants.”
In initial tests using an atomic interferometer and cesium atoms launched into a vacuum chamber to mimic the vacuum of space, Dr. Muller and his colleagues were able to observe the 10 to 20 milliseconds it took for the atoms to rise above a heavy aluminum sphere.
In 2019, physicists at the Muller laboratory could observe atoms much longer, up to 20 seconds, by adding an optical grating and a tungsten weight to increase the effect of gravity.
In another more recent experiment, published in the June 2024 edition of Physics of naturePostdoctoral researcher Cristian Panda and Dr. Muller demonstrated the ability to extend atom retention time from 20 seconds to an astonishing 70 seconds.
The researchers achieved this remarkable feat by stabilizing a laser beam in the resonance chamber of the lattice atom interferometer and changing the temperature to less than a millionth of a Kelvin above absolute zero.
Although the results have not yet succeeded in demonstrating the existence of the chameleon particle, the researchers say their repeated success in extending the time to observe gravitational effects lays the foundation for even more precise experiments.
Dr. Muller and his team are currently building a new lattice atom interferometer with better vibration control and lower temperatures. This next-generation instrument is expected to produce results 100 times more precise than their recent experiments. This level of precision could be sensitive enough to finally detect the quantum properties of gravity.
As researchers continue to push the boundaries, the potential discovery of dark energy appears ever closer. Ultimately, these advances at UC Berkeley represent a significant step forward in solving one of the greatest mysteries of the Universe and the true nature of dark energy.
The researchers say the successful demonstration of gravitational quantum entanglement would be a breakthrough comparable to the first demonstration of quantum entanglement of photons by the late Dr Stuart Freedman and Dr John Clauser in 1972.
In 2022, Dr. Clauser received the award Nobel Prize in Physics for his role in proving the existence of quantum entanglement, a phenomenon that Albert Einstein once described as “spooky action at a distance.”
Tim McMillan is a former law enforcement officer, investigative journalist, and co-founder of The Debrief. His writing focuses on defense, national security, the intelligence community, and psychology-related topics. You can follow Tim on Twitter: @LtTimMcMillan. You can contact Tim by email: tim@thedebrief.org or by encrypted email: LtTimMcMillan@protonmail.com