Dark energy makes up roughly 70 percent of the universe, but it remains one of the greatest mysteries of physics that scientists have been trying to answer for decades.
Dark energy is speeding up the expansion of the universe and shaping the fate of the cosmos. It mainly reveals itself indirectly through observations of cosmic expansion, so researchers have only been able to confirm its existence by studying how galaxies move and how the universe has expanded. Those measurements tell scientists how much dark energy there is, but not what it is.
A new study from Associate Professor of Physics Alfredo Gurrola, Professor of Physics Robert Scherrer, and Ph.D. candidate Oem Trivedi proposes a new way to understand its physical nature. In their paper, Probing the Sound Speed of Dark Energy with a Lunar Laser Interferometer, the team suggests using a high-precision measurement tool called a laser interferometer on the moon to “listen” to dark energy by measuring its sound speed, or how fast tiny disturbances in spacetime travel.
The lunar laser interferometer would transform the way we understand dark energy. Because of its sensitivity in measuring the tiny effects of those disturbances, the tool would allow researchers to test competing theories of what comprises dark energy in a way current telescopes cannot. Instead of just being able to quantify dark energy, scientists would be able to characterize its behavior, opening the door to possible new technological breakthroughs and furthering our understanding of the universe.
“At the most immediate level, this is fundamental discovery science: learning the nature of dark energy would rewrite textbooks and reshape our understanding of space, time, and the basic laws of physics,” Gurrola said. “That kind of breakthrough can seed decades of new technologies and ideas, as was the case for GPS and semiconductors. Beyond practical spinoffs, major physics endeavors like probing dark energy on the moon inspire public interest in science, spur international collaboration, and create high-skill jobs in engineering, optics, and space science.”
In a second complementary paper, Probing Dark Energy on the Moon, Gurrola, Scherrer, and Trivedi expand upon their work and highlight how a potential measurement of the sound speed of dark energy on the moon would provide information that defines how dark energy behaves at microscopic scales, such as its energy density fluctuations, pressure, and interaction with other components of the universe.
“We currently have a huge zoo of dark energy models, and even though it’s a good thing to have multiple ways to explain a phenomena, it becomes quite hard for the field as a whole to discern what models could be more complete than others given that a lot of models can be made consistent with the current datasets,” Trivedi said. “If, however, dark energy could be successfully probed on the moon in the ways we are suggesting, then it would work to successfully trim down the set of possible models and allow us to better navigate this model zoo.”
Moving forward, the team suggests working on contributions to the design of the lunar laser interferometer, testing prototypes on Earth, integrating state-of-the-art analysis algorithms to experimentally probe potential dark energy signatures, and building the partnerships needed to deploy laser interferometers on the moon. This work would be part of a broader effort spearheaded by Vanderbilt Lunar Labs.
“By combining these measurements with cosmological surveys, we can move from simply detecting dark energy to directly probing its physical properties,” Gurrola said. “If successful, probing dark energy from the moon would establish a new pillar of observational cosmology, complementing telescopes and particle experiments and ushering the field into an era of true precision measurement.”