In this week's Physical Review Letters2, a team led by physicist Hartmut Abele at the Technical University of Vienna shows that the ordinary laws of gravity are still valid even when measured over the scale of a few micrometres. The researchers measured quantized gravitational energy levels with a precision that is 100,000 times better than in previous experiments3.
That precision is sufficient to test some proposed explanations for dark energy — the unknown force that seems to be accelerating the expansion of the Universe. Some models of dark energy put constraints on particular gravity-like forces that would subtly distort the quantum levels at these micrometre scales. “It’s really a beautiful experimental tour de force,” says Geoffrey Greene, a physicist at the University of Tennessee in Knoxville who was not involved in the study.
'Chameleon' dark energy is one such hypothesized force. It derives its name from the way the range over which it acts is reduced drastically for dense objects, which would account for why we fail to see it in Solar System measurements. Such a 'fifth force', existing alongside the known electromagnetic, strong, weak and gravitational forces, would tweak the neutrons' energy levels from those predicted by gravity alone, says Amol Upadhye, a theoretical physicist at Ewha Womans University in Seoul, who was not part of the research team.
The team’s results put a limit on how strong that force could be. “This limit is one hundred times better than the previous such limit,” says Upadhye. This does not eliminate chameleon theories as possible explanations for the dark energy, he adds. “There are still some seven orders of magnitude to cover … but this goes a long way towards closing that gap.”
The results also constrain the properties of a potential candidate for dark matter, the substance thought to make up 85% of matter in the Universe but which seems to be undetectable except for its gravitational pull at cosmic scales. Very light hypothetical particles called axions would cause a deviation from the ordinary law of gravity at short distances. The absence of such an effect in this latest study limits how strong these interactions could be.
"It's truly remarkable that experiments such as this are possible at all," says Upadhye. The researchers call the technique gravity resonance spectroscopy, because it mirrors other kinds of spectroscopy, which measure the energy states of electrons in the electromagnetic field of an atom. These have found a wide range of uses — from determining the composition of faraway galactic objects to atomic clocks. “This first application of the new technology is a big step," says Greene.