LISTEN to Alexander Kusenko talking and you might think he鈥檚 describing some
quirky animal from a fantasy novel. 鈥淨-balls can鈥檛 eat very much,鈥� he says. And
they鈥檇 certainly behave unconventionally on planet Earth. 鈥淭hey鈥檙e so small that
if you put one on your desk, it would plunge through the centre of the Earth
like a needle.鈥�
The Q-balls Kusenko describes are tiny globs of exotic matter that might be
roaming through outer space. Each one is like 鈥渁 new universe in a nutshell鈥�, he
says. Inside a Q-ball, the familiar forces that hold our world together don鈥檛
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鈥攁nd 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鈥檚 fundamental
forces.
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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鈥攓uarks and leptons鈥攁nd 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 鈥渉ierarchy problem鈥�. It also helps with
efforts to unify nature鈥檚 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鈥斺�漵uperpartners鈥� of the known particles鈥攖o square the
mathematics. There is no trace of SUSY superpartners around us now, presumably
because the current breed of particle accelerators can鈥檛 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). 鈥淚t鈥檚 like a cloud, which can form
clumps or drops that fall as rain,鈥� 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 脳 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鈥攁bout 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鈥檇 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. 鈥淚t would be like
a bullet passing through a cloud of vapour,鈥� 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鈥檚 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鈥檚 core it would have slowed to a crawl.
Once there, it would eat any neutrons that came near it. 鈥淭he 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,鈥� 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. 鈥淭he neutron star is slowly
eaten from the inside by the Q-ball,鈥� says Kusenko. The energy escaping from the
star鈥檚 core would sap its mass, till eventually it weighed only a fifth of the
mass of the Sun.
鈥淎t that point, the force of gravity is no longer strong enough to keep
neutrons from decaying into electrons, protons and neutrinos,鈥� says Kusenko.
Suddenly, the mutinous neutrons would decay, releasing a huge amount of energy.
The whole neutron star would explode: 鈥淵ou would see a kind of mini supernova,鈥�
Kusenko says.
Quick snack
Kusenko and his colleagues calculate that it would take at least 10 million
years for a Q-ball to guzzle a neutron star鈥檚 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 (快猫短视频, 31 May 1997, p
28).
That鈥檚 one scenario, but it鈥檚 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鈥檛 conflict with Einstein鈥檚 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鈥檚 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鈥檙e
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鈥檙e 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鈥檒l 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. 鈥淭here 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,鈥� says Kusenko. 鈥淚 hear a rumour that Super-Kamiokande sees similar
events, but the experimentalists haven鈥檛 had time to analyse them yet.鈥� 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. 鈥淭here鈥檚 not a shred of
evidence that supersymmetry exists,鈥� 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
鈥渢heology鈥�, he says, has elbowed other promising ideas out of the limelight. 鈥淚
think that at least an equally good picture of nature emerges in complementary
sets of ideas that don鈥檛 involve supersymmetry. If we were really doing our jobs
as scientists, we鈥檇 be exploring all the possibilities.鈥�
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. 鈥淚鈥檓
not saying that these ideas aren鈥檛 worth pursuing, but I don鈥檛 think they ever
lead to clear answers.鈥� 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鈥� time, the most
powerful accelerator ever, the Large Hadron Collider (LHC), will start smashing
particles together at CERN. 鈥淭he LHC will be one of the most important
endeavours we ever undertake,鈥� says Hill. If there are no sightings of SUSY鈥檚
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鈥攁ncient or still
living鈥攚ill be much more compelling. And the nutshell universes in which
nature鈥檚 laws break down won鈥檛 seem such oddballs after all.