— Dark energy is so befuddling that it's causing some physicists to do their science backwards.
"Usually you propose your theory and then work out an experiment to test it," says Christian Beck of Queen Mary, University of London. A few years ago, however, he and his colleague Michael Mackey of McGill University in Montreal, Canada, proposed a table-top experiment to detect the elusive form of energy, without quite knowing why it might work. Now the pair have come up with the theory behind the experiment. "It is certainly an upside-down way of doing things," Beck admits.
Dark energy is the mysterious force that many physicists think is causing the expansion of the universe to accelerate. In 2004, Beck and Mackey claimed that the quantum fluctuations of empty space could be the source of dark energy and suggested a test for this idea. This involved measuring the varying current induced by quantum fluctuations in a device called a Josephson junction a very thin insulator sandwiched between two superconducting layers.
Beck reasoned that if quantum fluctuations and dark energy are related, the current in the Josephson junction would die off beyond a certain frequency (see A table-top test for dark energy?). But they hadn't worked out what exactly caused the cut-off.
Now the duo say they know, and last week Beck presented the theory at a conference on unsolved problems for the standard model of cosmology held at Imperial College London.
Quantum mechanics says that the vacuum of space is seething with virtual photons that are popping in and out of existence. Beck and Mackey suggest that when these virtual photons have a frequency below a certain threshold, they are able to interact gravitationally, contributing to dark energy.
Their theory is inspired by superconducting materials. "Below a critical temperature, electrons in the material act in a fundamentally different way, and it starts superconducting," says Beck. "So why shouldn't virtual photons also change character below a certain frequency?"
If so, virtual photons should behave differently below a frequency of around 2 terahertz, causing any currents in the Josephson junction to taper off above this frequency. Physicist Paul Warburton at University College London is building such a dark energy detector and could have results next year.
Some evidence that dark energy works like this may already have been found. In 2006, Martin Tajmar at the Austrian Research Centers facility in Seibersdorf and his colleagues noticed bizarre behaviour in a spinning niobium ring. At room temperature, niobium does not superconduct, and accelerometers around the ring measured that it was spinning at a constant rate. But once the temperature fell, the niobium started to superconduct, and the accelerometers suddenly picked up a signal (Gravity's secret).
"We measured an acceleration even though the ring's motion hadn't changed at all," says Clovis de Matos, who works at the European Space Agency in Paris and established the theory behind the experiment. He thinks the results could be explained if gravity got a boost inside the superconductor. "Beck and Mackey's gravitationally activated photon would have that effect," he says.
The controversial experiment seemed to fall foul of Einstein's equivalence principle, which states that all objects should accelerate under gravity at the same rate. It implied that "if you have two elevators, one made of normal matter and one made of superconducting matter, and accelerate them by the same amount, objects inside will feel different accelerations", de Matos says. Astronomers may have seen a similar violation of the principle (see "Two-speed gravity", below).
The odd acceleration detected in the niobium ring also suggests that energy isn't conserved in the superconductor another major violation of known physics. Dark energy could solve that problem, however. "We did the sums and found out that energy wasn't conserved, but perhaps that was just because we were missing dark energy," de Matos says.
Paul Frampton, a cosmologist at the University of North Carolina at Chapel Hill, thinks Beck and Mackey's reasoning is flawed. "I don't think for a second they'll measure dark energy, but they should certainly try."
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