Justin Khoury

A dark energy project developed by Penn’s Justin Khoury and continued in collaboration with UC, Berkeley, was awarded a $1 million W. M. Keck Foundation Science and Engineering Grant to support both the experimental work at Berkeley and the theoretical work at Penn.

Dr. Khoury is a professor in the department of physics & astronomy in the School of Arts and Sciences. He initially developed the project in 2004, during his postdoctoral studies at Columbia University. It is based on a class of dark energy theories called chameleon theories, which views dark energy as dynamic and make predictions for experiments on Earth. 

“Chameleons are basically a form of dark energy whereby the agent that gives rise to dark energy can change its properties depending on the environment,” Dr. Khoury said. “In particular it uses the large density of the local environment to hide itself from experiments. If you have large objects such as Earth or the sun, the effects of the chameleon will be efficiently hidden. But if you take a small particle, such as an atom, it can have a relatively substantial effect.”

Berkeley researchers, led by Holger Müller, an associate professor of physics and the principle investigator of the project, are investigating these ideas using a technique called atom interferometry, which is based on the principle of superposition, which states that particles can be in two places at the same time.

The researchers release a cloud of atoms into a source sphere in a vacuum chamber. Using lasers, they split the atoms’ wave functions so that they go through two different paths at once. Then they recombine the atoms, measuring the force between the source mass and the atoms. The purpose of the project is to determine whether or not a chameleon force is influencing the motion of the atoms.

“The motion of these atoms is governed by gravity,” Dr. Khoury said. “But if there were a chameleon force, then that would also influence their motion. By using atom interferometry, they can actually tell whether or not there’s a chameleon force on top of gravity. It’s a really powerful, extremely precise experiment. It’s also cutting-edge. This field of atom interferometry is relatively new, and it’s a very hot field, and so this collaboration is bringing Berkeley’s expertise to try to constrain or potentially discover this theory of dark energy.”