Published online 30 November 2007 | Nature | doi:10.1038/news.2007.292

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If quarks are made of preons, then stars made of the stuff should be detectable.

Bits within bits: is there something smaller than a quark, which in turn makes up protons and neutrons? ArSciMed / SCIENCE PHOTO LIBRARY

Are there pea-sized objects as heavy as the Moon out there in space? Perhaps so, if quarks, the constituent particles of atoms, are themselves made up of still smaller particles.

Fredrik Sandin and Johan Hansson of Luleå University of Technology in Sweden say that these hypothetical particles, called preons, might exist in super-dense chunks left over from the beginning of the Universe. Their work predicts that these heavy objects should be detectable with current astronomical techniques1. This helps to turn a highly speculative hypothesis into a testable idea.

If preon nuggets exist, they might account for a significant proportion of the mysterious dark matter known to make up a big chunk of the tangible mass of the Universe.

Smaller and smaller

It’s been long known that matter has a Russian-doll nature. Atoms are made of protons and neutrons (together called hadrons), along with lighter electrons. In turn, hadrons consist of particles called quarks, of which there are six varieties. In addition, there are six varieties of fundamental particles related to the electron, called leptons.

In 1974, physicists Jogesh Pati and Abdus Salam speculated that a small family of particles they called preons could explain the proliferation of quarks and leptons. In 1999, Hansson and his coworkers proposed that three types of preons would suffice to build all the known quarks and leptons23.

Then in 2005, Hansson and his student Sandin went on to explore whether some matter could have got stuck at the preon stage, rather than ‘condensing’ into quarks or hadrons4. They predict that it could. Such lumps of preons would be even denser than quark stars or neutron stars. Neutron stars, for comparison, are thought to compact the mass of our Sun into a ball the width of Long Island in New York.

Big bubbles

The lumps of preon matter they envisage wouldn't be made from collapsed stars, but would be relics from the Big Bang. As the newborn Universe expanded, the matter it contained gradually thinned out, switching from preon matter to quark matter and eventually to the atoms that now make up stars and interstellar gas. Sandin and Hansson say, however, that some of the preon matter might have got stuck as stable bubbles that never made the switch.

They calculate that these bubbles would be less massive than ordinary stars, at no more than 100 times the mass of Earth, and less than a metre across. There’s no lower limit, but Sandin and Hansson have considered preon nuggets down to the size of a pea, which would be a little less massive than the Moon.

Such objects, scattered sparsely through space, would be too small to see directly. But the researchers say that there are various ways they might reveal themselves.

Heavy bending

Such ultra-dense objects would bend light that passed nearby. This effect is called gravitational lensing: the objects acts as a kind of lens for the light of more distant stars that lie behind it when viewed from Earth. Gravitational lensing is well known for visible light bent by large, dark objects. But Sandin and Hansson say that, because preon nuggets are so small, they would exert their strongest influence on γ-rays, such as those emitted from extremely violent cosmic outbursts called γ-ray bursts. The preon lenses wouldn’t brighten the γ-ray signal, as a normal gravitational lens does, but would produce a characteristic wobble in the spectra.

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Another approach could spot two preon nuggets bound in mutual orbit by gravity. Such ‘binaries’ would emit gravity waves — ripples in space-time, which could be detected with gravity-wave detectors if the binary is near the Sun. Diminutive preon binaries would excite waves of high frequency, making them detectable by table-top devices rather than the immense gravity-wave detectors currently used to search for such waves from star-sized objects.

And tiny preon nuggets that collide with Earth would excite seismic waves that can be identified by seismic detectors. “They’re so small, they would just drill a hole through the planet”, says Hansson. But they would leave a trail of seismic waves along their path, which, being a straight line, would clearly differ from the rumbles created by the grinding of continents.

Shot in the dark

Making preon matter seems to be out of the question: it would involve recreating the conditions of the early Big Bang, before the putative switch to quark matter. That would require energies way beyond the reach even of the Large Hadron Collider, which is now nearing completion at Europe's particle-physics laboratory, CERN, near Geneva in Switzerland.

How do other physicists react to the notion of preon stars — quite literally a shot in the dark? “They’re either very enthusiastic, or they think it’s rubbish”, says Hansson. “There’s not much in between.”

But John Charap, a theoretical physicist at Queen Mary College in London, seems to sit in the middle ground. “It’s not a completely daft idea”, he says. “And after all, we need some pretty daft ideas to make any progress in understanding dark matter. We’re currently floundering around looking for ways to explain it. This might be as good a candidate as any.”