• 28 July 2010 by Eugenie Samuel Reich

 

A shape-shifting fifth fundamental force could neatly explain the mystery of dark energy – and some other puzzling astronomical observations

ASK A cosmologist for a potted history of the universe, and it might go something like this: the cosmos began some 13.6 billion years ago with a big bang, exploding from a pinprick of searing heat and incredible density. Since then, it has been cooling and expanding: at first exponentially fast, but soon at a more measured, steady tempo.

At that point our friendly cosmologist might give voice to a little embarrassment. Because if measurements of the distance to faraway supernovae are to be believed, around 5 billion years ago the universe's expansion started to accelerate again. We don't know why. A mysterious "dark energy" permeating space is generally fingered as the culprit. But while this entity apparently flings galaxies apart with gusto, it has never been seen or produced in the lab and seemingly does not interact directly with light or matter on Earth or elsewhere. Such undetectability runs counter to the stuff of science.

Dark energy flings galaxies apart with gusto, but it has never been seen or produced on Earth

Or are we just overlooking evidence that is already there? Some inconsistencies in recent astrophysical observations, easy to dismiss as blips if taken on their own, might invite a startling conclusion when looked at together: that the cosmos is suffused by a fifth force in addition to the canonical four of gravity, electromagnetism and the strong and weak nuclear forces. What is unusual about this force is that its range changes according to its environment - a cosmic chameleon that might just explain the mysteries of dark energy.

The basic idea for this fifth force was hatched in 2004 by Justin Khoury and Amanda Weltman, then members of a team led by well-known string theorist Brian Greene at Columbia University in New York City. String theory is the favoured route to unifying gravity, the odd one out among the four forces, with the other three under the umbrella of quantum mechanics. It is a great playground for devising new fields and forces. The theory is formulated in 11 dimensions, seven of which are assumed to be curled up so small that we cannot see them. Disturbances in those curled-up dimensions might make themselves felt as "extra" forces in the four dimensions of space and time we do see.

For this picture to make sense, the effects in the visible dimensions must match our observations of the universe. Khoury and Weltman proposed one way of doing this: an extra force could be transmitted by particles whose mass depends on the density of the matter around them. That way, its effects could remained veiled on Earth.

How would that work? Well, in quantum mechanics, the range of influence of a force depends largely on the mass of the particles produced by the associated force field: the lighter the particle, the longer the force's range. Electromagnetic fields, for example, produce photons that have no mass whatsoever, so the range of the electromagnetic force is infinite. The particles that transmit the weak nuclear force, on the other hand, are extremely heavy and do not travel very far, confining the force to the tiny scales of the atomic nucleus. With the strong nuclear force, things are slightly more complex: the associated particles, called gluons, are massless but also have the ability to interact with themselves, preventing the force from operating over large distances.

Khoury and Weltman started from the observation that the average density of matter in Earth's vicinity is very high in cosmic terms, at about 0.5 grams per cubic centimetre. Under these circumstances, they proposed, the particle that transmits the chameleon force would be about a billion times lighter than the electron. The force itself would then have a range of not more than a millimetre - small enough for its effects to have remained undetected in the lab so far.

In the wide open spaces of the cosmos, however, where a cubic centimetre contains just 10^-29 grams of matter on average, the mass of the chameleon particle plummets by something like 22 orders of magnitude, producing a muscular force that could act over millions of light years. The lost mass is picked up as energy by the chameleon field.

Although the initial motivation was not to find a mechanism to explain dark energy, the idea that the chameleon might do so was always on their minds, says Weltman. With a few tweaks, it did. It could be made to create a kind of negative pressure that, on cosmic scales, would produce a repulsive effect in opposition to gravity. And with its dependence on density, the chameleon force could be made to appear 5 billion years ago, when the density of the expanding cosmos fell below a critical value. The force would propel galaxies away from one another at an ever-increasing rate, producing the kind of accelerated expansion we observe in the wider cosmos, all the while remaining hidden on Earth.

Seesawing strength

It all sounds very pretty, but where's the beef? Without evidence, the chameleon is just another theory to explain dark energy (see "Different routes to the dark side&quot. "The truth is, we know very little about the underlying physics of the dark sector," says Khoury. "My view is we'll let observations and experiments decide."

That is just where the chameleon force could outsmart its rivals. According to Khoury and Weltman's theory, the chameleon particle interacts with light and matter in specific ways, so unlike its reptilian namesake it should be eminently easy to spot. For a start, a photon in a strong enough magnetic field could occasionally decay into a chameleon particle, which could in turn change back into a photon. This seesawing between particles should modify the strength of the electromagnetic force, defined by a quantity known as the fine structure constant, or alpha.

Most astrophysical measurements of our cosmic neighbourhood suggest no such variation in alpha. Recently, Sergei Levshakov at the Ioffe Physical Technical Institute in St Petersburg, Russia, and colleagues showed that any change within our galaxy is less than 2 parts in 10 million. But there have been contrary indications from further afield. In 1999, an Australian team used the Keck telescopes in Hawaii to measure light emitted between 5 and 9.5 billion years ago by distant quasars. They concluded that alpha was once lower by about 11 parts per million, although that was revised to 6 parts per million in 2003 (Monthly Notices of the Royal Astronomical Society, vol 345, p 609). In June this year, Nissim Kanekar of the National Centre for Radio Astrophysics in Pune, India, and colleagues identified discrepancies in the spectrum of light from a molecular gas cloud 2.9 billion light years away that seem to indicate a value for alpha around 3 parts per million lower (The Astrophysical Journal Letters, vol 716, p L232010).

If photons travelling towards us over great distances pass through regions of space with strong magnetic fields and low densities of matter - ideal places for them to decay into chameleon particles - that is just the effect we would expect. "If the changes are real, it seems hard to explain them without a chameleon-like particle," says Douglas Shaw, an astrophysicist at Queen Mary University of London, UK.

Alpha might not be the only chameleon-affected quantity, either. In April this year, Levshakov and colleagues determined the ratio of the mass of the electron to that of the proton within ammonia atoms in gas clouds in our galaxy. They found the electron to be relatively heavier than it is on Earth by two parts in 100 million (arxiv.org/abs/1004.0783). As the chameleon particle changes its mass according to its environment, it is plausible that it would drag on the electron by a different amount to the proton. "It's very easy to fit the data to the chameleon model," says Shaw.

That, in a sense, is the problem: because the chameleon theory was conceived to fit observations and has yet to be derived from anything more fundamental, it is very easy to adjust its parameters to fit the data available. That same flexibility makes creating specific predictions, say of the amount of variation in a fundamental quantity under a particular set of circumstances, more of a problem.

Fortunately, there are other ways in which the chameleon might be pinned down. If a photon really can metamorphose into a chameleon particle and back, that should leave a signature in the polarisation of light that has travelled through regions where the chameleon force is strong. With Shaw, Clare Burrage of the German Electron Synchrotron DESY in Hamburg, Germany, caught a glimpse of something like that in 2009. They showed that some of the light reaching us from stars in other parts of our galaxy is polarised not only by the 2 per cent that can be explained by interstellar dust but also by an additional, if small, amount (Physical Review D, vol 79, p 044028). "We found a tentative indication of the chameleon," says Burrage.

Shaw cautions against reading too much into this: although the result was statistically significant, they studied only three stars. They are repeating the analysis for many more, including some "control" stars that are within 200,000 light years or so, nearby enough that there wouldn't have been time for any switching between photons and chameleon particles on the light's journey towards us.

Burrage thinks that chameleon-photon switching might also explain a discrepancy highlighted in 2003 by Martin Kunz of the University of Geneva, Switzerland, and Bruce Bassett of the South African Astronomical Observatory and the University of Cape Town. The most widely quoted value for the age of the universe, 13.6 billion years, comes from precise measurements of the cosmic microwave background, the relic radiation of the big bang. But two other methods come up with different answers. Calibrating the distances to supernovae of a standard brightness yields a figure of 13.1 billion years. Measuring radio galaxies of a standard size, meanwhile, comes up with 14.3 billion years.

The discrepancy could just be an unlucky fluke: the two measurements have an associated uncertainty and there is roughly a 1 in 20 chance of the figures disagreeing by this amount, says Bassett. Burrage suggests an alternative: supernovae could appear brighter than expected if chameleon particles created in highly magnetic environments oscillate into photons on their way towards us. That would mean the supernovae are further away than they seem, which in turn would increase the age of universe extracted using this method. "If the effect is real, this would be one natural explanation," agrees Bassett, while adding that further data from the Sloan Digital Sky Survey, a comprehensive project to catalogue the heavens, might resolve the discrepancy without the chameleon.

By Khoury and Weltman's calculations, direct evidence for the chameleon could also be lurking closer to home: in variations in the gravity experienced by small test masses in areas of differing ambient densities. That idea could be easily tested in space, or even by balloon experiments around 30 kilometres above Earth's surface. The French space probe Microscope, planned for launch in 2012, will measure the acceleration of test objects in free fall. If the chameleon theory holds true, the theory predicts that the probe should see objects some distance above Earth accelerate slightly faster as they fall than we would expect from their position in the Earth's gravitational field.

Direct evidence for the chameleon could be lurking in subtle variations in gravity closer to home

Ultimately, the most persuasive evidence for the chameleon force would be to see its effects here on Earth. The chameleon was designed to explain why we haven't seen evidence for a fifth force in the lab, but it might be tested if experiments are designed with chameleon properties in mind. In a paper published this June, Shaw and colleagues suggest just such a test: measuring the force between two parallel plates as a gas is pumped in between them. Normally, this force should depend simply on the density, but an extra attractive force might be expected at low densities if the chameleon force kicks in (Physical Review Letters, vol 104, p 241101).

Another experiment that might detect the chameleon is the GammeV experiment at Fermilab in Batavia, Illinois. It involves bouncing a laser beam around a cavity with glass windows for about 5 hours, and then switching it off. If the chameleon theory holds true, some photons will oscillate into chameleon particles that will bounce off the windows rather than pass through. Some of these will oscillate back into photons and escape, creating a visible afterglow.

So far the GammeV experiment has seen nothing, while ruling out several possible masses for chameleon particles. The same is true of the Axion Dark Matter Experiment at Lawrence Livermore National Laboratory in California, which was built to detect dark matter by looking for surges in microwaves bouncing around in a cavity. But the fact that the theory is being actively tested by experiments shows how seriously it is being taken, says Khoury. "It's flattering," he says.

Shaw says that with lab and space-based tests likely to improve, the chameleon theory stands to be confirmed - or ruled out - within the next 10 years. In contrast, there is little hope of submitting the other potential answers to the dark energy puzzle to a direct experimental test any time soon. Weltman says that she sees hints rather than hard evidence for chameleons so far, but thinks the theory is well worth pursuing. "I love that it is so testable," she says. "It appeals to all of my sense of how science should be."

Different routes to the dark side

The chameleon is by no means the first attempt to explain dark energy.

The COSMOLOGICAL CONSTANT is the standard explanation. This additional term in the equations of general relativity, a repulsive force to counter gravity's attraction, was introduced by Einstein to ensure the evolving universe neither expanded nor contracted. He is said to have later called this fudge "the greatest blunder of my life". Since 1999, when observations of the dimming of distant supernova suggested that the cosmic expansion has been accelerating, the idea has been back in vogue.

An alternative explanation is provided by QUINTESSENCE, a field that permeates the universe. Five billion years ago, the pressure associated with the field turned negative, causing cosmic expansion to speed up. The chameleon force is similar to quintessence, but crucially it also interacts with matter, making it potentially easier to observe.

In INHOMOGENEOUS UNIVERSE MODELS, also known as "Swiss cheese" universes, the accelerated cosmic expansion is an optical illusion caused by an uneven distribution of matter in the universe (New Scientist, 8 March 2008, p 32)

 

http://www.newscientist.com/article/mg20727711.100-is-a-cosmic-chameleon-driving-galaxies-apart.html?full=true&print=true