20 May 00
They zoom through the Universe, ripping out stars' hearts and flouting the laws of physics. Hazel Muir investigates the wild world of Q-balls
LISTEN to Alexander Kusenko talking and you might think he's describing some quirky animal from a fantasy novel. "Q-balls can't eat very much," he says. And they'd certainly behave unconventionally on planet Earth. "They're so small that if you put one on your desk, it would plunge through the centre of the Earth like a needle."
The Q-balls Kusenko describes are tiny globs of exotic matter that might be roaming through outer space. Each one is like "a new universe in a nutshell", he says. Inside a Q-ball, the familiar forces that hold our world together don't exist. This has some startling consequences. It means that every Q-ball is on a mission to violate law and order in the Universe by assimilating normal matter and compelling it to live by Q-ball rules. And a single Q-ball can eat the heart out of a super-dense star, causing it to self-destruct in an almighty explosion.
No one has ever seen one of these oddballs. But a leading theory of particle physics predicts that they were created in the heat of the newborn Universe-and that they may still be common today. Over the next few years, scientists will be looking for their traces in all sorts of places, from the distant heavens to the depths of the Mediterranean Sea and the ice of Antarctica. Finding Q-ball footprints would resolve a host of cosmic mysteries, including the nature of much of the dark matter that astronomers are convinced pervades the Universe, and perhaps the origin of the brilliant but unpredictable gamma-ray bursters that flare up in the skies. Q-balls may even point the way to the ultimate theory of everything that will unify nature's fundamental forces.
The name Q-ball was coined some 20 years ago by Sidney Coleman, a physicist at Harvard University. It encompassed a whole mathematical zoo of exotic energy balls, of any size, which could have formed in the early Universe. Q was just a letter, chosen to represent some property of the Q-ball that makes it stable indefinitely.
Then in 1997, Kusenko, a theoretical physicist from the University of California at Los Angeles, and Mikhail Shaposhnikov of CERN, the European Laboratory for Particle Physics near Geneva, teased a more tangible Q-ball out of a theory called supersymmetry, or SUSY. For the past two decades, SUSY has been the leading theory attempting to take particle physics beyond the standard model, its current mainstay. The standard model successfully describes the particles that make up matter-quarks and leptons-and those that glue them together. But it has its flaws. For instance, it bungles calculations of the masses of certain particles, coming up with answers that are far higher than in nature. SUSY irons out this so-called "hierarchy problem". It also helps with efforts to unify nature's fundamental forces, tidying up the mixed bag of forces that we see today into a neat single force that prevailed just after the big bang.
It does all this by introducing a host of mirror particles-"superpartners" of the known particles-to square the mathematics. There is no trace of SUSY superpartners around us now, presumably because the current breed of particle accelerators can't muster enough energy to make them. But if supersymmetry is right, these superpartners would once have ruled the Universe. The theory predicts that just after a phase of rapid expansion of space-time called inflation, which took place 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.
This is where the Q-balls come in. What Kusenko and Shaposhnikov showed three years ago is that a sea of squarks and sleptons would inevitably contain tiny variations in density, and that the slightly denser parts could clump together (Physics Letters B, vol 418, p 46). "It's like a cloud, which can form clumps or drops that fall as rain," says Kusenko. If these clumps were large enough, they would form Q-balls that would survive today.
Supercomputer simulations by Shinta Kasuya and Masahiro Kawasaki of the University of Tokyo seem to back this up. Their simulations placed squarks at more than 2.6 million points in a cube-shaped lattice and watched how they evolved. Sure enough, the squark sea in the box clumped into more than 30 Q-balls, the largest containing around 2 × 1016 squarks (Physical Review D, vol 61, p 41 301).
These bags of myriad squarks and sleptons can exist because the particles are less picky than normal quarks about sharing their space. In a proton, for instance, there can be no more than three normal quarks, because they refuse to share their quantum state with another quark, and there are only three possibilities to choose from. But thousands of billions of squarks and sleptons would happily share the same state and live together in a tiny ball not much bigger than an atomic nucleus.
Take a Q-ball containing 1030 particles. It would only be roughly 10 times the size of an iron nucleus, but its mass would be in a different league-about a tenth of a milligram. And inside, thanks to the peculiar properties of squarks and sleptons, the strong, weak and electromagnetic forces that shape our world would not exist.
If these heavy Q-balls did form in the early Universe, they would still be around today. In that case, they could make up at least some of the unidentified dark matter that loiters around galaxies all over the Universe. We know this dark matter is there because its gravity distorts the paths of visible stars and galaxies.
So much for the idea. But if dark nutshell universes are really roving space, how would they make their mark? Because they are so tiny and would typically move at about 100 kilometres per second, they would zip straight through a planet or star without stopping for lunch. So they'd be pretty hard to spot. A Q-ball would speed through the Sun, for example, in less than four minutes and only lose around 0.001 per cent of its velocity as it did so. "It would be like a bullet passing through a cloud of vapour," says Kusenko. But for a different type of star, contamination by even a single Q-ball would mean certain death. Neutron stars form when a very massive star ends its life in a supernova explosion. The heavy stellar core that's left behind shrinks under gravity into a ball only about 20 or 30 kilometres across. The gravitational pull on matter inside is so great that electrons and protons are squeezed together into a dense soup of neutrons.
A single Q-ball visiting a neutron star would interact with so many neutrons that by the time it reached the star's core it would have slowed to a crawl. Once there, it would eat any neutrons that came near it. "The quarks inside the neutron suddenly discover that the forces that used to bind them have disappeared, so they split up and bounce around the Q-ball," says Kusenko. But not for long. Energetically speaking, it would be easier for the quark guests to blend in with the Q-ball by turning into squarks. The Q-ball would spit out the extra energy in the form of two or three quark-antiquark pairs called pions.
These would decay into various things, including tiny, fast-moving neutrinos that could escape from the infected neutron star. "The neutron star is slowly eaten from the inside by the Q-ball," says Kusenko. The energy escaping from the star's core would sap its mass, till eventually it weighed only a fifth of the mass of the Sun.
"At that point, the force of gravity is no longer strong enough to keep neutrons from decaying into electrons, protons and neutrinos," says Kusenko. Suddenly, the mutinous neutrons would decay, releasing a huge amount of energy. The whole neutron star would explode: "You would see a kind of mini supernova," Kusenko says.
Kusenko and his colleagues calculate that it would take at least 10 million years for a Q-ball to guzzle a neutron star's core (Physics Letters B, vol 423, p 104). Some neutron stars in the Universe are ten times that age, so Q-ball detonation could already be under way. If most of the energy from the neutron star explosion emerged as gamma rays, this might be the cause of gamma-ray bursters, says Kusenko. Till now, the source of these super-bright flashes of gamma rays has been a mystery (New Scientist, 31 May 1997, p 28).
That's one scenario, but it's equally possible that it could take up to 10 billion years for a Q-ball to munch through a neutron star core, in which case no neutron star could have existed long enough to succumb. All is not lost, though, because there is another way that Q-balls might reveal their influence: in the cosmic microwave background, the radiation left over from the big bang.
In 1998, Kari Enqvist at the University of Helsinki and John McDonald of Glasgow University showed that the clumping together of Q-balls would have subtly distorted this microwave background (Physics Letters B, vol 425, p 309). This might just show up in 2006, when the European Space Agency launches a satellite called Planck.
Another possibility is that the oddballs might show themselves closer to home. A Q-ball that arrived on Earth and started munching its way through protons would produce spectacular bursts of pions. These could register in several detectors around the world that pick up flashes of light from high-energy particles ploughing through water or ice. The calling card of a heavy Q-ball would be unmistakable, Kusenko says.
The bright flashes occur because the particles emitted by Q-balls would travel faster than light. This doesn't conflict with Einstein's famous cosmic speed limit: his theory said only that no particle can reach the speed that light travels in a vacuum. But in other media, light slows down dramatically. In water, for instance, it travels at only around 70 per cent of its speed in free space.
That's what gives physicists a chance to see the speedy particles from Q-balls. As they pass through water in a lake, for instance, they emit amounts of radiation that would usually be far too tiny to measure. But because they're now travelling faster than light, this radiation bunches up into blue flashes known as Cerenkov radiation, in the same way that sound waves from aircraft moving faster than sound bunch together into a deafening sonic boom. Using sensitive photomultipliers to amplify the Cerenkov flashes, physicists can spot the particles as they pass through.
If a heavy Q-ball carrying no electric charge hit water, it would start to guzzle protons and neutrons, and spit out a high-energy spray of pions, which in turn would decay into a mixed bag of other particles, including muons, electrons and positrons. This assorted debris would produce a dazzling stream of Cerenkov flashes.
From 1984 to 1990, a string of Cerenkov detectors called Gyrlyanda was operating in Lake Baikal in southern Siberia. Physicists looking back at the data Gyrlyanda collected have found no sign of any Q-balls. The same goes for an underground detector in Italy called MACRO, at the Gran Sasso national laboratory north-east of Rome.
This means that if Q-balls exist, they must be rare. And if they're rare, yet account for much of the unidentified dark matter in the Universe, they must be heavy. This kind of reasoning allows scientists to calculate that if dark-matter Q-balls exist they must contain more than 1022 particles.
Q-balls like this might well turn up at other, larger particle detectors that have more chance of bagging rare visitors. For example, the ANTARES detector being built 2 kilometres down on the Mediterranean seabed off Toulon in France is made up of 13 strings of Cerenkov detectors. An international team of scientists aims to complete it by 2003, when it will cover an area of 0.1 square kilometres. They hope eventually to extend ANTARES to a square kilometre.
Another team is hoping to get the go-ahead for a similar detector in Antarctica. They want to build a 1-cubic-kilometre detector, called IceCube, by drilling 81 holes 2.4 kilometres down into the clear ice at the South Pole. Then they'll lower strings of Cerenkov detectors. The plan is to have IceCube up and running around 2008.
Tantalisingly, a couple of Q-balls just might have left their mark in a detector called Super-Kamiokande, a tank containing 50 000 tonnes of water in a mine beneath Mount Ikenoyama in Japan. "There were a few events observed by the Kamiokande experiment, the predecessor of Super-Kamiokande, that no one could identify. Something came in and blinded the detectors because they saw so much light," says Kusenko. "I hear a rumour that Super-Kamiokande sees similar events, but the experimentalists haven't had time to analyse them yet." The next few years promise to be an exciting time for researchers eager to spot these cosmic anarchists.
But as long as SUSY remains unproved, there are those who continue to regard Q-balls as mere figments of fertile imaginations. "There's not a shred of evidence that supersymmetry exists," says Chris Hill, a theorist at Fermilab near Chicago. SUSY is elegant in many ways, Hill admits, but he accuses physicists of treating it more like religion than a scientific theory. This "theology", he says, has elbowed other promising ideas out of the limelight. "I think that at least an equally good picture of nature emerges in complementary sets of ideas that don't involve supersymmetry. If we were really doing our jobs as scientists, we'd be exploring all the possibilities."
Hill even questions whether predictions about neutron star explosions or the microwave background can ever say anything useful about SUSY or Q-balls, because the large-scale Universe is such a minefield of complexity and unknowns. "I'm not saying that these ideas aren't worth pursuing, but I don't think they ever lead to clear answers." Far better, he says, to test basic ideas directly, using powerful particle accelerators that can look at the laws of physics on the tiniest scales.
The moment of truth may not be far away. In around five years' time, the most powerful accelerator ever, the Large Hadron Collider (LHC), will start smashing particles together at CERN. "The LHC will be one of the most important endeavours we ever undertake," says Hill. If there are no sightings of SUSY's mirror particles in the debris of collisions, the theory will be in deep trouble.
But if the mirror particles do show up, SUSY will join the ranks of the tried-and-tested laws of nature. The case for Q-balls-ancient or still living-will be much more compelling. And the nutshell universes in which nature's laws break down won't seem such oddballs after all.