In a laboratory in the center of the University of California, Berkeley, near Oppenheimer Way, the street named for “the father of the atomic bomb”, a team of physicists fine-tunes a sophisticated apparatus in search of the elusive “chameleon particle”.
A buzz of anticipation fills the air as they prepare to launch an experiment that could reveal one of the Universe’s deepest mysteries: dark energy.
Assuming that The Lambda-CDM model of cosmology that’s right, dark energy represents nearly 70% of the total energy of the observable Universe and is the driving force behind its accelerated expansion. However, despite its great 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 of the Australian National University and Dr. Adam G. Riess of John Hopkins University.
Through observations of distant supernovae, researchers realized that the Universe was expanding at an ever-accelerating rate. This discovery earned 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 an enigma – perhaps the biggest in physics today,” the Nobel laureate said. proclamation reads the Royal Swedish Academy of Sciences. “What is known is that dark energy makes up about three quarters of the Universe. Therefore, the findings of the 2011 Nobel laureates in Physics have helped to uncover a Universe that is, to a large extent, unknown to science. And everything is possible again.”
Independent observations, including cosmic microwave background experiments and galaxy redshift surveys, have confirmed the existence of dark energy. However, twenty-six years after its initial discovery, the exact nature of dark energy remains “perhaps the greatest puzzle” 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 intriguing proposal is that dark energy is mediated by an exotic yet-to-be-discovered scalar particle that exerts a repulsive force depending on the density of the surrounding matter. This hypothetical particle, known as a “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, driving 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 influence of dark energy on the accelerating expansion of space.
Now, in the Holger Müller lab at UC Berkeley, physicists are breaking new ground to solve the mystery of dark energy. They have designed the most precise instruments to date, capable of measuring even the smallest gravitational anomalies.
The discovery of even small deviations in the accepted theory of gravity would be a massive discovery, providing evidence for the existence of the hypothetical chameleon particle.
In recent experiments, physicists designed a new instrument that combines an atom interferometer for precise gravity measurements with an optical lattice to hold atoms in place.
This arrangement allowed the researchers to immobilize free-falling atoms for significantly longer periods, increasing the accuracy of their measurements by a factor of five compared to previous experiments.
By immobilizing small clusters of cesium atoms inside a vertical vacuum chamber, researchers can separate each atom into a quantum state. Half of the atom is closer to the weight of tungsten in this state, allowing scientists to measure the phase difference between the two halves of the atomic wave function. This process enables them to calculate differences in gravitational pull with unprecedented precision.
In findings recently published in Naturethe researchers found that despite the revolutionary experimental design, the results failed to show any deviation from Newtonian gravity.
However, physicists hope that with the expected improvements in the precision 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 ability of this new technology to hold atoms for up to 70 seconds and potentially 10 times longer extends the possibilities of investigating gravity at the quantum level, explained Dr. Holger Müller, a 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. Muller in one RELEASE from UC 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 can hold our atoms 20 or 30 times longer than anyone else, because our sensitivity increases by the second or fourth power of the holding time, we can we were 400 to 800,000 times more likely 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 experiencing slightly different gravitational forces, allowing researchers to detect small differences in gravitational pull. This capability could eventually reveal the presence of the supposed chameleon particle or other unknown exotic phenomena related to dark energy.
In addition to its potential for detecting dark energy, the lattice atom interferometer designed by Muller’s team holds promise for various applications, including quantum sensing.
This technology is particularly sensitive to gravity and inertial effects, making it suitable for building advanced gyroscopes and accelerators. The optical lattice’s ability to hold atoms in place rigidly also makes it resilient to environmental imperfections or noise, which can allow precise measurements in challenging environments, such as at sea.
Since 2015, Dr. Muller has looked for evidence of chameleon particles using an atomic interferometer.
“Atomic interferometry is the art and science of using the quantum properties of a particle, namely the fact that it is both a particle and a wave. We split the wave up so that the particle takes two paths at the same time and then interferes [with] those at the end”, explained Dr. Müller. “The waves can either be in phase and add up, or the waves 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 atom interferometer and cesium atoms released into a vacuum chamber to mimic the emptiness 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 lab succeeded watching atoms are much longer, up to 20 seconds, adding an optical grid and tungsten weight to increase the effect of gravity.
In another more recent experiment, published in the June 2024 edition of Nature Physicspostdoctoral fellow Cristian Panda and Dr. Muller demonstrated the ability to extend the holding time of atoms from 20 seconds to an amazing 70 seconds.
The researchers achieved this remarkable feat by stabilizing a laser beam inside the resonant chamber of the atom lattice interferometer and adjusting the temperature to less than a millionth of a Kelvin above absolute zero.
Although the results so far have failed to show the existence of chameleon particles, the researchers say their repeated success in expanding the time to observe gravitational effects lays the groundwork for even more precise experiments.
Dr. Muller and his team are currently building a new atomic lattice interferometer with improved vibration control and lower temperatures. This next-generation instrument is predicted to produce results 100 times more accurate than their last 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 feels incredibly close. Ultimately, these advances at UC Berkeley represent an important step forward in unraveling one of the Universe’s greatest mysteries and the true nature of dark energy.
The researchers say that the successful demonstration of gravitational quantum entanglement would be a breakthrough comparable to the first demonstration of photon quantum entanglement by the late Dr. Stuart Freedman and Dr. John Clauser in 1972.
In 2022, Dr. Clauser was awarded the prize 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 retired 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 reached by email: tim@thedebrief.org or via encrypted email: LtTimMcMillan@protonmail.com