Is a cosmic chameleon driving galaxies apart?

Where do we look to observe this accelerated expansion? (Image: NASA)


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.