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THE nature of dark energy, the mysterious stuff that is relentlessly pushing the universe apart, could be revealed by a simple table-top experiment.

Physicists dreamed up dark energy in 1998, when they found that distant supernovae appeared fainter than expected, showing that they were farther away than previously thought. To explain this, they concluded that the expansion of the universe must be accelerating and that dark energy was responsible.

One possible origin for dark energy arises from a prediction made by quantum physics: that the vacuum of space is a choppy sea of “quantum fluctuations”. This quantum vacuum could be the source of dark energy.

The effects of the quantum vacuum have already been observed in a device known as the Josephson junction, an extremely thin layer of insulator sandwiched between two superconducting layers. Josephson junctions have the curious property of being able to contain a varying current even in the absence of an external voltage and in 1982, Roger Koch and colleagues, then at the University of California, Berkeley, and the Lawrence Berkeley Laboratory, showed that the jitter of the quantum vacuum could cause high-frequency fluctuations in the varying current within the Josephson junction.

Now Christian Beck of Queen Mary University of London and Michael Mackey of McGill University in Montreal, Canada, have pointed out that if the quantum vacuum and dark energy are the same, then the fluctuations in a Josephson junction should peter out at frequencies above about 1700 gigahertz. Koch’s team originally measured these fluctuations up to a frequency of 600 gigahertz. “So it is necessary only for someone to improve on his experiment by a factor of 3,” says Beck.

If such an experiment showed no cut-off in the frequency of fluctuations in the Josephson junction, the energy density in the fluctuations will exceed the density of dark energy observed in the universe. And since the fluctuations in the device are caused by quantum vacuum, this will mean that quantum vacuum is not the source of dark energy. “We will have made some progress, but the mystery of what the dark energy is will have deepened,” says Beck. But if the fluctuations do cut off at the predicted frequency, this would strongly support the idea that the quantum vacuum is the source of dark energy.

“It would be wonderful if a table-top experiment would help in the search for dark energy,” says Michael Doran of Dartmouth College in New Hampshire. But he points out that such cut-offs have not been seen in measurements of a related phenomenon known as the Casimir effect, which is a small attractive force that arises due to the quantum vacuum between two parallel and uncharged conducting plates just hundreds of nanometres apart. “Then again, experiments have always been good for surprises,” he says.