An important mystery facing cosmologists is that the rate of expansion of the universe appears to be increasing with time – physicists had expected this rate to decrease as the finite energy of expansion is depleted by the gravitational attraction that holds the universe together. Cosmologists have tried to explain this in terms of “dark energy”, which boosts the expansion of the universe by counteracting the effects of gravity. To be effective, dark energy must account for about 70% of all energy in the universe -- but it has yet to be observed directly.

Cosmologists have calculated that a new force associated with dark energy should become apparent at relatively short length scales – about 85 micrometres. It turns out that one of the best places to look for evidence of dark energy at this length is not in the deepest reaches of outer space, but rather in a simple laboratory experiment that measures the gravitational attraction between two plates and looks for any deviation from the classical inverse-square law (see figure "Attractive plates").

Dan Kapner and colleagues at the University of Washington have used a torsion balance to measure the force of gravity down to 55 micrometres and found that it still obeyed the inverse-square law well beyond 85 micrometres with 95% confidence. While this doesn’t rule out dark energy, it has allowed Kapner and colleagues to conclude that a new gravitational-strength force does not appear at this length scale.

Although other groups have measured the force of gravity on shorter length scales, Kapner told *Physics Web* that the Washington experiment offers the highest sensitivity at the length-scale associated with dark energy. This is because it employs more interacting mass at the required separations than other setups. The researchers are now building an improved balance that could boost the sensitivity of the measurements by a factor of 100.