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THROUGHOUT the universe, space-time is fizzing. Bubbles pop in and out of existence across the cosmos. Mostly, the froth is harmless. Yet at any moment, it could unleash a catastrophic reaction that rips through the fabric of space, destroying the universe and everything in it.

Clearly, a killer bubble hasn’t formed so far in the 13 billion years since the universe began. Yet Louis Clavelli believes that space-time bubbles are ripping stars apart on a daily basis. If he is right, the universe is a more dangerous place than you might think.

It’s a controversial idea. Most astronomers deny that anything as exotic is responsible for ripping stars apart. But Clavelli, who works as a physicist at the University of Alabama in Tuscaloosa, insists that his findings come from reputable physics.

The idea of a frothy universe springs from the view that space can exist in a number of different phases, similar to the way water can be a solid, liquid or gas depending on the amount of energy it contains. This view was bolstered in 1998 when astronomers studying distant supernovae discovered that the expansion of the universe is accelerating, and not slowing down as everyone had thought.

To explain the findings, cosmologists suggested that the vacuum of space is filled with a mysterious substance called dark energy.

No one knows what dark energy is, but its presence means that space is a far cry from the nothingness of a “true vacuum” which contains no energy. It is a distinction that has far-reaching consequences, because a false vacuum filled with dark energy can decay into a true vacuum. And because the decay is a quantum process, it can happen spontaneously.

Vacuum decay hit the headlines in 1999 when fears were raised over experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, New York. Could colliding nuclei trigger vacuum decay and promote a reaction that would destroy the universe? Reassuringly, most probably not. After all, cosmic rays smash into atoms in the upper atmosphere all the time without catastrophic results. Calculations show that in space the chances of a decay to the true vacuum are extraordinarily remote.

Even so, the process of vacuum decay continues to intrigue researchers because bubbles of true vacuum are forming spontaneously all the time throughout the universe. The bubbles come in a range of different sizes, although the laws of quantum mechanics overwhelmingly favour minuscule ones. Once formed, a bubble will either grow or collapse depending on its size and the density of its surroundings.

Physicists agree that in the emptiness of the cosmos, a bubble of true vacuum would have to be truly enormous to keep on growing. Yet it is a different story if it forms in the presence of matter. According to calculations by Clavelli, there is one place where a tiny bubble could swell with cataclysmic results: inside white dwarf stars.

White dwarfs are the super-dense cores of stars that have exhausted their nuclear fuel and collapsed under their own gravity to the size of the Earth. The only thing that prevents a white dwarf being crushed further is the pushing of atomic electrons against each other. The Pauli exclusion principle makes sure of that, because it forbids more than two electrons from sharing the same space.

Energy reserves

Electrons require a lot of energy to resist the crushing force of gravity. This means that white dwarfs are reservoirs of energy dotting the cosmos. “I don’t think enough people realise that in our universe the Pauli exclusion principle is a huge energy storage mechanism,” Clavelli says.

Together with his students Irina Perevalova and George Karatheodris, Clavelli looked at what would happen if the electrons suddenly stopped obeying the Pauli principle. “In that case they would collapse together,” he says. Then all the energy used by the electrons to resist the crushing force of gravity would be emitted almost instantaneously in the form of gamma rays.

According to their calculations, a white dwarf can spontaneously spring back to life. And in the 2 seconds it takes the revived stellar corpse to turn itself into a black hole, it releases enough energy to be seen across the universe as a gamma-ray burst. First spotted in the 1960s by military satellites, gamma-ray bursts are the most powerful blasts in the universe since the big bang. So far they have been too far away to threaten us, but exactly what powers them is one of the biggest mysteries in astronomy.

But isn’t brushing aside the Pauli exclusion principle scientific heresy? It would be if a respected theory of physics didn’t permit this. Called supersymmetry, the theory was devised in the 1970s, and is the leading contender in the hunt for a single, unifying force that will tidy up the mixed bag of fundamental forces we see today.

Supersymmetry’s answer is to introduce a heavyweight counterpart for each of the particles we see today. So far, no one has found any of these superpartners because none of today’s particle accelerators is powerful enough to churn out anything so hefty. But if supersymmetry is right, in the first split second after the big bang the universe would have been awash with the superpartners of quarks and leptons, dubbed squarks and sleptons, all acting under a single force.

For Clavelli, the crucial difference between regular particles and their superpartners lies with a quantum mechanical property called spin. It is spin that forbids more than two electrons from sharing the same space. Yet no such restrictions apply to their superpartners, selectrons, whose spin is different.

In the embryonic universe electrons and selectrons would have had the same mass and could continually turn into one another. But as the universe expanded and cooled, somehow this supersymmetry was broken. The superpartners gained mass and were frozen out, cut adrift from their lighter partners.

“We have yet to realise what it means to live in a universe governed by supersymmetry”

But Clavelli’s calculations show that supersymmetry lives on inside every bubble of true vacuum. A bubble that popped up inside a white dwarf would thrive in such superdense surroundings and rip through the dead star within a matter of seconds. As it engulfed the dead star, supersymmetry would take over and the electrons would once again be able to swap identities with their selectron superpartners. Free of the constraints of the Pauli exclusion principle, a white dwarf filled with selectrons would collapse in a burst of gamma rays to form a black hole (see Diagram).

Thankfully, the rest of the universe would be spared from destruction: Clavelli’s theory shows that the swelling bubble stalls when it reaches the surface of the white dwarf, thwarted by the low density of the surrounding space.

The governor

Clavelli’s version of what lies behind gamma-ray bursts is highly controversial. Most astrophysicists deny that gamma-ray bursts are anything more exotic than explosions created by the collapse of titanic stars. But Clavelli says that is because they have yet to realise what it truly means to live in a universe governed by supersymmetry.

While supersymmetry is a respected theory, there is still no experimental evidence for it. So Clavelli’s team knew just how heretical a supersymmetric origin for gamma-ray bursts would seem – not least because many astronomers who study the bursts believe they already have a good working model for the blasts. After a long tussle with one of the journal’s referees they have finally had a paper accepted by Physical Review D.

What is making all the difference is that as astronomers pick up more gamma-ray bursts, they realise it is harder to fit them all into the accepted theory. First put forward in 1993 by Stanford Woosley at the University of California in Santa Cruz, the collapsar model is based on a star more than 30 times as massive as the sun, spewing gamma rays as it collapses under its own gravity to form a black hole (see Collapsar star).

“So far, the models have been based upon one or two well-observed bursts. So, I think the jury is still out on whether the collapsar model is right for all bursts,” says Keith Mason of the Mullard Space Science Laboratory at University College London.

And the collapsar model it not watertight: supporters of Woosley’s theory worry about the way in which jets of matter are launched from the vicinity of the black hole and accelerated close to the speed of light. “I wish I knew how to accelerate the jets,” says Mason. “You can wave your hands about and say that magnetic fields must somehow be involved but the truth is it’s still a real puzzle.”

Mason hopes astronomers will glean some clues from observations of active galaxies and the sun, which also emit high-speed jets. “I think that active galaxies and solar flares are opposite ends of a continuum and that magnetically driven jets can occur on many scales,” says Mason. If he is right, all that is needed is a eureka moment for the mathematics to fall into place.

Nevertheless, Clavelli believes he can exploit such chinks in the collapsar model’s armour. “I am so convinced by the supersymmetry mechanism that I am spending all my time on it,” he says. One of the advantages of Clavelli’s idea is that it does away with the troublesome jets. According to his model, gamma-ray bursts are a natural consequence of the supersymmetric reaction.

“Space-time bubbles are ripping stars apart on a daily basis”

Observations appeared to back him up too. According to the collapsar model, every gamma-ray burst should produce an explosion of visible light as jets of matter rip through the rest of the star. In contrast, Clavelli’s supersymmetric model rules out optical afterglows.

Swift conclusion

And until recently, no detectable optical afterglow was visible for up to two-thirds of gamma-ray bursts. Supporters of the collapsar model argued that this might be due to gamma-ray bursts exploding in dusty regions of space that hid visible light. But most of them believed that their telescopes were not fast enough to detect the rapidly fading afterglow.

As astronomers have become better at turning their telescopes towards gamma-ray bursts within moments of the blast, so the number “dark” bursts has fallen. Nevertheless, many afterglows remain dimmer than astronomers expect and about 1 in every 5 gamma-ray bursts remains totally dark.

Clavelli concedes there are plenty of problems to overcome before his theory is accepted. Telescopes reveal the bursts as pencil-like beams of gamma rays, whereas in Clavelli’s model gamma rays spray out in all directions. To explain the discrepancy, his group has developed a computer simulation of a white dwarf being engulfed by vacuum decay.

What they found is encouraging. Any fluctuation in the density of selectrons inside the white dwarf is amplified as it gives out gamma rays. As these gamma rays pass through the surrounding selectron cloud, they stimulate further emission in the same direction, reminiscent of the way light emerges as a narrow beam from a laser. This suggests that the gamma-ray bursts would tend to gain strength in a particular direction.

So far the group has yet to get to grips with the detail of the process. Clavelli points out that his model is still being refined. But that has failed to sway Woosley.

In fact, Woosley is scathing about the whole thing. “The particle physics may be interesting, but as a gamma-ray burst model it is not very good,” he says. “The time has passed when one can simply point at some phenomenon that releases a lot of energy and say that makes it a gamma-ray burst model.”

Clavelli remains defiant: “Astronomers talk about things like central engines without explaining the details of how those things work. To me it seems more speculative than the supersymmetry model.”

Astronomers will know if Clavelli’s model is worth pursuing when NASA’s Swift gamma-ray observatory starts taking data next month. Swift is designed to scour a sixth of the sky at a time for gamma-ray bursts. Within seconds of spotting one, it will point its X-ray and optical telescopes at the afterglow of the blast.

Since Swift was launched in November, scientists have been putting the observatory through its paces, calibrating its three telescopes in readiness for the science phase starting in April. Within two years, the Swift team expects the observatory to have captured gamma rays, X-rays, ultraviolet and optical light from hundreds of gamma-ray bursts. Swift promises to end a lot of speculation.

If optical afterglows are there, its ultraviolet and optical telescope will see them. And if the number of optical afterglows falls short of expectations, at the very least the collapsar model will need serious revision. That might be just the break Clavelli needs for astronomers to take his idea seriously.