It's a study in contrasts. For more than a decade, physicists have been puzzling over dark energy, the mysterious stuff that’s blowing space apart and has been detected only by studying the universe on the largest scales. Now, researchers have probed its properties using about the smallest tools available—atoms falling freely in a vacuum chamber. The experiment, reported today in Science, doesn't reveal what dark energy is, but it helps nail down what it isn't. In particular, it narrows the prospects for one popular idea: that dark energy resides in hypothesized "chameleon particles" hiding in plain sight.

"I find it exciting to be able to use laboratory-scale experiments to test such ideas," says Amol Upadhye, a theoretical physicist at the University of Wisconsin, Madison, who was not involved in the work. The test doesn’t entirely rule out chameleons, he says, but future improvements might put the idea to the ultimate test.

The discovery of dark energy rocked physics and cosmology. Scientists thought the expansion of the universe was slowing, as the galaxies tugged on one another with their gravity and counteracted the expansion that began with the big bang. However, in 1998, two teams of cosmologists showed that in fact the expansion is accelerating by studying stellar explosions called supernovae. The result has been bolstered by analyses of galaxy clusters, the afterglow of the big bang (the cosmic microwave background), and other cosmological phenomena. Physicists attribute the acceleration to some sort of space-stretching dark energy.

But what is dark energy? There are two possibilities. It could be energy hidden in the vacuum of empty space itself—a cosmological constant, as Albert Einstein hypothesized in 1917. Or it could be a quantum field that fills space and blows it up like a balloon. Both alternatives have problems. Given the standard model of particle physics, theorists can calculate what the cosmological constant should be, and they get a value vastly too big to explain the relatively modest acceleration—suggesting some unknown physics just zeroes it out. On the other hand, the presence of a quantum field would affect things like the orbits of the planets in the solar system—but dark energy doesn’t seem to.

That's where chameleon particles come in. The hypothetical particles would make up just such a quantum field, but they would interact with matter in a way that would make the field vanish wherever the density of matter was high. Thus the field would exert no noticeable effect on things like planets. "The chameleon, like many other theoretical ideas, has a small probability of being there," says Justin Khoury, a theoretical cosmologist at the University of Pennsylvania and co-inventor of the concept. "Nonetheless we should test it if we can."

That's just what Khoury, Holger Müller, an atomic physicist at the University of California, Berkeley, and colleagues have done. To search for a chameleon field, they studied the interactions between an aluminum sphere 9.5 millimeters in diameter and a puff of 10 million ultracold cesium atoms within a vacuum chamber. If there were a chameleon field within the vacuum, then the sphere would squash it. And like a bowling ball on a trampoline, the sphere would bend the field just outside its surface, causing the field’s strength to taper to zero. The cloud of atoms would slide down the sloping field, experiencing a short-range force toward the sphere. Crucially, the cloud itself was not dense enough to suppress the field and spoil the effect. "In the simplest terms, we're looking for a funny force between the sphere and the atoms," Müller says.

That force would come in addition to the pull of Earth's gravity. So, the researchers repeated the experiment in two different configurations. In one, they dropped the atoms from 8.8 millimeters above the sphere, close enough for the sloping chameleon field to exert a force. They used an exquisitely sensitive technique called atom interferometry to measure the cesium atoms’ acceleration as they fell for about 20 milliseconds (see figure). In the other configuration, they dropped the atoms well to the side of the sphere, where the chameleon field should have been uniform and produced no force. So, if there were a chameleon field, the atoms would accelerate downward faster when dropped above the sphere. In fact, in both configurations, the atoms accelerated at the same rate to within a precision of 1 part in 1 million.

Curiously, neither Müller nor Khoury thought up the experiment. Instead, it was proposed by Clare Burrage and Edmund Copeland of the University of Nottingham in the United Kingdom and Edward Hinds of Imperial College London, in a paper they posted to the arXiv preprint server a year ago. "Of course I was disappointed that they did it before us," Hinds says, "but they already had a suitable apparatus, while we have had to build an experiment specifically for the purpose."

At its current precision, the experiment rules out only chameleons whose interaction with matter—the thing that makes the field go away where the matter density is high—is much stronger than gravity, Khoury says. Those that interact with matter more weakly are still viable, he says. Müller says that his team aims to improve the precision of their experiment to 1 part in 1 billion, which should put the chameleon to the ultimate test. Hinds is trying to beat Müller to that goal. And even if the chameleon concept dies, there are other ways to hide a quantum field that would produce dark energy.