Published online 29 May 2009 | Nature | doi:10.1038/news.2009.531

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Reported 'chameleon' particle would change its mass to match its environs.

Cosmologists don't usually take their lead from the animal kingdom. But a model that postulates the existence of a 'chameleon' particle — which would change its mass depending on its surroundings — is gaining attention.

Photons traveling from active galactic nuclei, such as at the heart of the galaxy M87, could reveal the proposed chameleon particle. NASA/CXC/CfA/W. Forman et al.; Radio: NRAO/AUI/NSF/W. Cotton; Optical: NASA/ESA/Hubble Heritage Team (STScI/AURA), and R. Gendler

A new paper1 claims to have spotted signs of this elusive particle, whose existence was first postulated in 20032 to explain the accelerating expansion of the Universe, which has been attributed to some unknown 'dark energy'. The changing mass of a chameleon particle would modify the range at which its force can act, thus possibly explaining why whatever causes the Universe's acceleration hasn't been detected on Earth. On Earth, the chameleon would be too heavy to create any noticeable force, but in the tracts of empty space, its effect would be huge.

In theory, photons that travel through magnetic fields can turn into chameleons, reducing the amount of light that reaches Earth from distant sources. The amount of dimming depends on the light's frequency. By comparing light emitted across a range of frequencies from the luminous centres of 77 active galaxies, Douglas Shaw at Queen Mary University of London and his colleagues have found what they call "good evidence" that some photons have gone missing in transit.

"It's absolutely an interesting way of looking for [exotic] particles, and the results are certainly intriguing," says Frank Wilczek, a particle physicist at the Massachusetts Institute of Technology in Cambridge who was not involved with the work. But it's too early to interpret the findings as an unambiguous detection of chameleon particles, he says.

Missing in action

By themselves, the observations of dimmed light by Shaw and his colleagues can't distinguish between models that rely on chameleons and models in which photons turn into other 'axion-like' particles. Either "would be an interesting discovery," says Shaw.

However, only the chameleon model predicts that the photons' polarizations should be aligned with the magnetic fields they traversed. So far, the team has studied data on light from three stars in the Milky Way galaxy and in each case found the required polarization3.

"It's an ingenious and original technique," says astronomer Malcolm Fairbairn of King's College London. But he adds that astronomers don't yet fully understand how light is produced in these distant sources, so it is tough to predict the signals one might expect in the absence of chameleons.

Still, the group's analysis appears to get a boost from an independent study into an unusually high flux of high-energy photons spotted by the MAGIC telescope on La Palma and the VERITAS telescope in Arizona4,5. The results have perplexed astronomers because very high-energy photons should be kept from reaching Earth by interactions with the cosmic microwave background radiation. However, Miguel Sánchez-Conde at the Institute of Astrophysics of Andalucia, Spain, and his colleagues have shown that if the photons convert into axion-like particles for part of their journey, they could reach Earth undisturbed6.

Bernard Sadoulet, an astronomer involved in dark-matter searches at the University of California, Berkeley, says that the wider community is likely to need more convincing. "The correct next step," he says, "should be to get back-up data in the laboratory."

Time for a change

Amanda Weltman at the University of Cambridge, UK, and one of the originators of the chameleon idea, is attempting to do just that with the GammeV group at the Fermi National Accelerator Laboratory in Batavia, Illinois. They are shining laser light through a tube with windows at either end that is immersed in a magnetic field. The chameleon model predicts that some photons should convert to chameleon particles. When the laser is switched off, the chameleons should slowly turn back to photons and create a faint afterglow. The experiment has completed its first phase without seeing any signs of a chameleon; however, the team has not yet searched for the chameleons with parameters that match those possibly sighted by Shaw's team7. The GammeV group is now preparing to test for chameleons in that "interesting range", says Weltman.

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With other tests planned to sniff out the chameleon, Weltman believes the particle will be confirmed or ruled out within the next decade. The European Space Agency's MICROSCOPE satellite, due to launch in 2012, will look for chameleon-induced deviations by studying the relative motion of two masses in orbit. The chameleon particle could also affect how stars and hydrogen gas clouds move in relation to each other under the influence of gravity in galaxies, says Lam Hui of Columbia University in New York.8 "Those data have already been collected by astronomers, so it's a question of analysing them," he says.

Whatever the outcome, the accumulation of strange astrophysical observations signal "exciting times", says Sánchez-Conde. "Everything seems to point to something new happening in physics."

Editor's note: The chameleon theory was first proposed by Justin Khoury along with Amanda Weltman. Both were at Columbia University at the time.