Maybe there’s a good reason why it has been so difficult to pin down the nature of dark energy – the mysterious force that is making the universe expand faster. Dark energy could be mediated by a chameleon-like particle that changes character to suit its surroundings.
Ever since dark energy was suggested in the late 1990s, cosmologists have struggled to explain just how it can counter gravity on the cosmological scale, but apparently leave it untouched here on Earth.
In 2003, Justin Khoury of the Perimeter Institute in Waterloo, Canada, and Amanda Weltman of the University of Cape Town in South Africa suggested the existence of a curious particle that could provide the answer. Dubbed “chameleons”, these are theoretical particles that mimic their immediate surroundings: they are heavy in regions that are packed with matter, and light where matter is in short supply. Khoury and Weltman’s colleagues calculated that on cosmological scales, the chameleon would be nearly massless, and would mediate a force that reproduced the effects of dark energy. But here on Earth, chameleons would be too heavy to produce any measurable force – making them undetectable in experiments designed to test deviations from Newtonian gravity on Earth.
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“On cosmological scales the chameleon would mediate a force that reproduced the effects of dark energy”
It has been tough to find evidence for particles that are, by definition, adept at hiding. However, it seems the elusive chameleon may have been spotted by the at the National Laboratories of Legnaro in Italy. The experiment sends a linearly polarised laser beam through a vacuum chamber bathed in an intense magnetic field and measures changes in the polarisation of the photons as they leave the vacuum chamber.
PVLAS hit the headlines in July 2006, when researchers announced that they had measured a rotation in the polarisation of a laser beam that was 10,000 times larger than could be explained by standard physics, along with an unexpected change from linear to elliptical polarisation.
Almost immediately, the results were ascribed to the disappearance or conversion of a tiny fraction of the photons to axions, the much sought-after hypothetical particles that are candidates for dark matter (żěè¶ĚĘÓƵ, 18 July 2006, p 35).
At the same time, Anne-Christine Davis at the University of Cambridge and her colleagues began to wonder if a dark energy particle, the chameleon, was responsible for the shift in polarisation. Within the near-perfect vacuum of the PVLAS chamber the chameleon, in keeping with its shifting nature, would have a mass very close to zero. The team’s calculations showed that it would be very easy for photons in the laser beam to transform into chameleons in such a vacuum.
But more detailed calculations revealed problems. According to the team’s model, chameleon particles would remain trapped in the vacuum chamber, bouncing back and forth, and cancelling out any rotation in polarisation in the laser beam rather than creating the observed huge rotation. It seemed the chameleon could not, after all, explain the PVLAS results.
Then there came a surprising twist. Last month, researchers ruled out the axion explanation (see “Particles that pass through plates”). A few days later, the PVLAS collaboration retracted their original findings. They had upgraded their equipment and could not reproduce their earlier results. The new experiment no longer showed that the photon polarisation had rotated, but it was still elliptical at low settings of the magnetic field ().
This was good news for Davis and her team. They redid their calculations and found that the chameleon – whose presence predicted a change in ellipticity but no rotation – now fitted the bill. Also, the strength of the chameleon field needed to explain the PVLAS result was just the right size to account for the density of dark energy observed by astronomers on the cosmological scale. “We got the accelerating universe thrown in for free,” says Davis, who presented her work at a conference on particles, strings and cosmology at Imperial College London on 5 July.Astrophysicist Andy Albrecht at the University of California, Davis, is impressed that the group has proposed a solution for dark energy that can be tested. “There is so much unconstrained speculation about dark energy, I cannot help but feel that this is a breath of fresh air,” he says.
And while Glennys Farrar, an astrophysicist at New York University, is intrigued by the link between the PVLAS findings and dark energy, she is cautious about the reliability of the latest PVLAS results, given that the researchers retracted their original claims.
PVLAS team member Giovanni Cantatore is also keen to play down claims that the dark energy particle has been discovered. “We won’t say that we have definitely found it,” he says. “But the impact of such a discovery would be enormous, so I encourage independent groups to look into this further.”
Carlo Rizzo at the French national research agency (CNRS) in Toulouse agrees that the PVLAS retraction is “troubling”. He and his colleagues are setting up their own experiment to double-check the latest PVLAS results and will also test for the chameleon. “We could flood the vacuum chamber with a dense gas during the experiment,” says Rizzo. “Perhaps then we will see the chameleon changing character before our eyes.”
Space tests for the chameleon are also being lined up. Precise tests of the force of gravity carried out on the SEE, MICROSCOPE, Galileo Galilei and STEP satellites should reveal if a significant chameleon force does indeed come into play in regions of sparse matter. “It will be very tough for the chameleon to keep on hiding out there,” says Weltman.
Particles that pass through plates
Carlo Rizzo at the French national research agency (CNRS) in Toulouse and his team passed a laser beam through a vacuum chamber in the presence of a magnetic field. Unlike the PVLAS experiment in Italy, the team blocked the far end of the vacuum chamber with an aluminium plate.
The plate would block the photons from the laser beam, but if photons were being converted to axions – candidate particles for dark matter – then they would be able to pass through the plate and change back into photons. No such photons were detected by the team, ruling out axions ().
This doesn’t mean chameleons are the only answer. So-called mini-charged particles (MCPs) – predicted to have between 10-15 and 10-7 times the charge of an electron – can also explain the latest PVLAS results, without falling foul of Rizzo’s experiment. If they were being produced in the vacuum chamber, then after passing through the aluminium plate, pairs of MCPs would spiral apart and not recombine to create new photons, says Andreas Ringwald, with the DESY collaboration in Hamburg, Germany.
In theory, MCPs can travel through solid objects – potentially allowing for cable-free telecommunication directly through Earth. “MCPs are more exciting [than chameleons] because their discovery could actually change people’s day-to-day lives,” says Ringwald.
