At the University of California, Berkeley’s downtown lab, just off Oppenheimer Way, a street named after “father of the atomic bomb‘, a team of physicists carefully tweaks an ingenious apparatus in pursuit of the elusive ‘chameleon particle’.
A hum of anticipation fills the air as they prepare for an experiment that could reveal one of the universe’s deepest mysteries: dark energy.
Assuming Lambda-CDM model of cosmology is correct, dark energy accounts for nearly 70% of the total energy of the observable universe and is the impetus behind its accelerated expansion. Yet despite its immense 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 Lawrence Berkeley National Laboratory, Dr. Brian P. Schmidt from the Australian National University and Dr. Adam G. Riess of John Hopkins University. .
Through the observation of distant supernovae, scientists have realized that the universe is expanding at an ever faster rate. This discovery earned the three scientists the 2011 Nobel Prize in Physics.
“The acceleration is thought to be driven by dark energy, but what that dark energy is remains a mystery—perhaps the greatest in physics today,” Nobel Prize notification reads the Royal Swedish Academy of Sciences. “Dark energy is known to make up about three-quarters of the universe. That’s why the findings of the 2011 Nobel laureates in physics helped reveal a universe largely unknown to science. And everything is possible again.”
Independent observations, including experiments with the cosmic microwave background and galaxy redshift surveys, have confirmed the existence of dark energy. Still, twenty-six years after its first discovery, the exact nature of dark energy remains “perhaps the greatest” mystery in physics.
Various theories have been proposed to explain its existence, including the possibility that dark energy could be the vacuum energy of the universe or a dynamic energy field called quintessence.
Another interesting suggestion is that dark energy is mediated by a yet-undiscovered 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 the fifth fundamental force of nature, much weaker than gravity.
In the void of space, the chameleon particle would exert a repulsive force over long distances, driving the accelerated expansion of the universe. However, on Earth, surrounded by matter, the particle’s range would be extremely limited. This would explain the anomalous impact of dark energy on the accelerated expansion of the universe.
Now, in Holger Müller’s lab at UC Berkeley, physicists have broken a new path to solving the mystery of dark energy. They designed the most accurate instruments to date, capable of measuring even the smallest gravitational anomalies.
The detection of even small deviations in the accepted theory of gravity would be a huge breakthrough, offering proof of the existence of the hypothetical chameleon particle.
In recent experiments, physicists have designed a new device that combines an atom interferometer for precise measurements of gravity with an optical grating that holds atoms in place.
This setup allowed the researchers to immobilize freely falling atoms for significantly longer periods of time, increasing the accuracy 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 scientists were able to separate each atom into a quantum state. Half of the atom is closer to the tungsten mass in this state, allowing scientists to measure the phase difference between the two halves of the atomic wave function. This process allows them to calculate differences in gravitational attraction with unprecedented precision.
In the findings just published in Naturethe researchers revealed that despite the revolutionary experimental design, the results showed no deviations from Newtonian gravity.
Nevertheless, the physicists hope that with the expected improvement in the accuracy of their new instrument, new and exciting possibilities will open up for testing theories about the nature of dark energy, including the existence of the chameleon particle.
The new technology’s ability to hold atoms for up to 70 seconds, and potentially 10 times longer, expands the possibilities of investigating gravity at the quantum level, explained Dr. Holger Müller, professor of physics at UC Berkeley and co-author of the study.
Previous experiments have well 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,” said Dr. Müller v release from UC Berkeley. “But no one has ever seen an experimental signature of it.
“It’s very hard to even know if 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 retention time, we might have a 400 to 800,000 times better chance to find experimental evidence that gravity is indeed quantum mechanical.”
This new experimental design can hold atoms in a quantum superposition of two states, each with slightly different gravitational forces, allowing researchers to detect minute differences in gravitational attraction. This capability could eventually reveal the presence of the hypothesized 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 inertial effects, making it suitable for building advanced gyroscopes and accelerometers. The optical lattice’s ability to hold atoms firmly in place also makes it immune to environmental imperfections or noise, which could enable precise measurements in challenging environments such as the sea.
Since 2015, Dr. Muller evidence of chameleon particles using an atomic interferometer.
“Atomic interferometry is the art and science of using 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 passes through two paths simultaneously and then we interfere [with] is at the end,” explained Dr. Müller. “Waves can either be in phase and add up, or they can be out of phase and cancel each other out.” The trick is that whether they are in phase or out of phase depends very sensitively on some quantity you might want to measure, such as acceleration, gravity, rotation, or fundamental constants.
In initial tests using an atomic interferometer and cesium atoms released into a vacuum chamber to mimic the emptiness of space, Dr. Muller and his colleagues observed the 10 to 20 milliseconds it took the atoms to reach above the heavy aluminum sphere.
In 2019, physicists in Muller’s laboratory could observe atoms for 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 issue Natural physicspostdoctoral researcher Cristian Panda and Dr. Muller demonstrated the ability to increase the dwell time of atoms from 20 seconds to an astonishing 70 seconds.
The researchers achieved this remarkable feat by stabilizing the laser beam in the resonance chamber of a grating atom interferometer and tuning the temperature to less than a millionth of a Kelvin above absolute zero.
Although the results have so far failed to prove the existence of the chameleon particle, the researchers say their repeated success in extending the time to observe gravitational effects lays the groundwork for even more precise experiments.
Dr. Muller and his team are currently building a new lattice atom interferometer with improved vibration control and lower temperatures. This next-generation instrument is expected to provide 100 times more accurate results than their recent experiments. This level of precision could be sensitive enough to finally reveal the quantum properties of gravity.
As researchers continue to push the boundaries, the potential discovery of dark energy seems tantalizingly close. Ultimately, these advances at UC Berkeley represent a significant step forward in unraveling one of the greatest mysteries of the universe and the true nature of dark energy.
Scientists say that a successful demonstration of gravitational quantum entanglement would be a breakthrough comparable to the first demonstration of photon quantum entanglement by the late Dr. by Stuart Freedman and Dr. by John Clauser in 1972.
In 2022, Dr. Clauser awarded Nobel Prize in Physics for his part in proving the existence of quantum entanglement, a phenomenon that Albert Einstein once famously described as “spooky action at a distance.”
Tim McMillan is a former law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the intelligence community, and psychology-related topics. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be contacted by email: tim@thedebrief.org or via encrypted email: LtTimMcMillan@protonmail.com