JUST A MILLIMETRE AWAY from you, there could be another universe. You can’t
see it, smell it or reach out and touch it. You could only get to it by
travelling in a direction that is forbidden. Because we are all prisoners,
trapped in a nether dimension that physicists call the braneworld.
If they are to be believed, there’s far more to our Universe than meets the
eye, or even the most powerful telescopes. According to their theories, the
familiar three dimensions that contain all the stars and galaxies we see could
actually be a membrane—a brane for short—floating in a space of
five, six or more dimensions, like a soap bubble in the bathroom. There could be
other branes out there in the higher dimensions, other universes that we can
never see.
But that doesn’t mean we are unaffected by the exotic regions mapped out in
these extra dimensions. On the contrary, they may lie at the heart of some of
the fundamental features of the Universe. Dark matter, the antigravity of empty
space and even the beginnings of the Universe itself may all be bound up with
the fifth dimension and beyond.
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Unseen dimensions are nothing new for physicists. They crop up in string
theory, the leading contender for a “theory of everything” that will bring all
the forces of nature together under a single umbrella. String theory needs six
extra space dimensions for its tiny elemental strings to move through. These
extra dimensions are usually thought to be curled up into circles only
10-35 metres around, far smaller than the radius of a proton. No wonder
they are invisible.
But there is another possibility. According to a few theoretical physicists,
some of the extra dimensions could be a lot larger, perhaps a tenth of a
millimetre long—or even infinite, like our familiar space and time.
If that is so, then shouldn’t we be able to see these extra dimensions or
walk sideways into them? No, say the theorists, because matter and light are
stuck on our brane, leaving us blind to anything outside. We are stuck in
flatland. Only gravity is free to escape and roam in the higher
dimensions—what science fiction writers call hyperspace, and physicists
call “the bulk”.
It’s an attractive idea, because the existence of large gravity-only
dimensions would explain why gravity appears to be so weak, many orders of
magnitude feebler than electromagnetic and nuclear forces. This weakness may be
an illusion. On a fundamental level, gravity might be as strong as the other
forces, but appears to us to be much weaker because it leaks out into higher
dimensions.
The physicists scrambling to join in the brane game all have their own ideas
about what the braneworld is really like. How big are the extra dimensions? How
many of them are there? You can take your pick. “At the moment it’s like a
playground,” says Lev Kofman of the University of Toronto.
But if we’re stuck in our three dimensions, what is lurking beyond them,
filling the higher-dimensional hyperspace? One idea is that it’s the long-sought
dark matter that astronomers believe must be out there somewhere, keeping
galaxies and galaxy clusters in one piece. The matter we can see doesn’t exert
enough gravitational pull to keep these whirling bodies from flinging themselves
apart.
The conventional explanation for this mystery is that space must be filled
with a soup of peculiar particles that don’t radiate or reflect light, but only
exert gravity. Nobody has yet detected any such dark-matter particles, though,
so the case is still open.
Savas Dimopoulos of Stanford University thinks “dark matter” might really be
ordinary matter that is hidden from us. Last year, with Nemanja Kaloper of
Stanford, Nima Arkani-Hamed of the University of California at Berkeley and Gia
Dvali of New York University, Dimopoulos described a world where this matter
could be just millimetres away. This is the manyfold universe.
The basic idea is that our Universe is folded over many times
(see Diagram).
Light from distant objects has to travel along the brane and so would take
billions of years to reach us, but gravity could take the short cut through a
higher dimension and trick us into thinking that there’s unseen matter around
us. “Dark matter could be made out of the very same protons and electrons as we
are, and we may not even know it,” says Dimopoulos.
The “dark matter” seems to hang around galaxies and clusters on our fold, but
there’s no mystery in that. Gravity between matter on the separate folds would
tend to drag it together. That means a pattern of galaxies and clusters on the
next fold would more or less mimic ours— superficially, it would look like
our local Universe.
Dvali, along with Gregory Gabadadze of the University of Minnesota at
Minneapolis, thinks the braneworld might also explain another cosmological
conundrum—why the world is made mostly of matter, and not antimatter. In
the big bang, matter and antimatter should have been created in equal
quantities. When a particle of matter meets its antimatter
counterpart—which has the same mass but opposite charge and other
properties—they annihilate each other in a burst of energy. So if nature
treated them equally, there would be nothing left in the Universe except
radiation.
Particle physicists think that nature must be somehow lopsided: a phenomenon
called CP violation has the potential to discriminate between matter and
antimatter. But CP violation can’t change the overall balance between matter and
antimatter on its own. There has to be some way to hide the antimatter in the
Universe.
Dvali and Gabadadze have a braneworld explanation. They say
quantum-mechanical fluctuations in the early stages of our Universe cause the
brane to wriggle so much that parts of it come loose. These “baby branes” pull
away from our Universe and head off into the bulk, capturing some particles from
the main brane and carrying them away. If CP violation somehow arranges it so
that more antimatter than matter is carried off, our world is left with a net
balance of matter.
Brane cosmology might also explain one of the most shocking properties of the
Universe. A few years ago, astronomers discovered that the expansion of the
Universe seems to be accelerating. Distant supernovae—huge explosions that
mark the death of a giant star—look dimmer than they should. That can be
explained if the Universe’s expansion has speeded up, putting the explosions
further away than we thought. What force could be responsible for this
accelerated expansion?
Ripped apart
Einstein worked out that that if space has some energy and pressure of its
own, it would exert a kind of repulsive force or negative gravity. This comes
into his theory of general relativity as an optional quantity called the
cosmological constant—a property of space he eventually rejected, but one
we may now have to take seriously. But that raises another question: why should
space have its own energy?
Physicists seemed at first to have a ready explanation. Quantum theory says
that space is filled with short-lived particles and antiparticles, continually
appearing and disappearing. This would give space its own inherent energy, and
therefore a repulsive force.
But there’s a problem. When you work out the numbers, the theory predicts an
energy density that is10120 times too high. It would lead to a repulsive force
so powerful it would rip apart the bonds that hold atoms and molecules
together.
Branes might come to the rescue again. Arkani-Hamed, Dimopoulos, Kaloper and
Raman Sundrum of Johns Hopkins University in Baltimore suggested in 1999 that
the braneworld would allow most of this antigravity to leak away into the higher
dimensions of hyperspace, just as ordinary gravity does.
It’s a neat idea, but actually calculating whether it pans out to produce
exactly the right dilution of antigravity is another matter. Braneworld physics
isn’t yet a precise science. Sundrum likens it to banging on a faulty TV set:
sometimes you get lucky and the problem is fixed. “Braneworlds are certainly
banging on gravity, but we still have to see if they help with the cosmological
constant problem,” he says.
Meanwhile, could braneworlds bang on the big bang? “They provide a natural
candidate for the beginning of the Universe,” claims Neil Turok of Cambridge
University. This year Turok, along with Paul Steinhardt and Justin Khoury of
Princeton University and Burt Ovrut of the University of Pennsylvania, came out
with the idea that the big bang might have been more of a grand slam.
In their picture, our 3-brane universe was once cold and empty. Then one day
another brane came out of the fifth dimension and smacked into ours, releasing
enough energy to create all the matter in out Universe
(see Diagram).
The team christened this model the “ekpyrotic universe”, referring to an
ancient Greek cosmology where the Universe is created in a burst of fire
(New Scientist, 14 April, p 7).
The ekpyrotic universe does away with the mystery at the beginning of time.
In conventional cosmology, if you play the film of the Universe backwards, you
see all the galaxies which are now moving away from each other move back
together. Do this long enough, and you will see everything coalesce at a point
of infinite density and temperature called a singularity. But then the film
projector breaks and we are left in the dark. At a singularity, physical
equations no longer make sense because everything is infinite, which is why
science can’t tell us what, if anything, happened before the big bang.
In the ekpyrotic universe, there is no singularity. Before the braneworld
collision, our Universe was still there, but it was empty. The question of what
happened before the big bang becomes a sensible one, even if the answer may be
“not much”. “This takes you from a universe of finite age to one of potentially
infinite age,” says Steinhardt.
Inflation punctured
The theory also provides an alternative to cosmic inflation, which for 20
years has been the favoured explanation for some of the grandest properties of
the Universe.
For example, the cosmic microwave background radiation, a remnant of the big
bang, is almost exactly the same temperature and brightness in all directions.
On the face of it, there’s no reason why the Universe should have this
large-scale smoothness. Right from the moment of the big bang, opposite ends of
the expanding Universe would have been too far apart for light to have reached
from one to other. This would have left them with no way to equalise their
temperatures.
Inflation gets round this by proposing that early in its life, the Universe
went through a brief but incredibly rapid period of expansion. The whole
Universe we see now (and in the microwave background era) was amplified from
just one tiny patch of the original big bang. That explains why one side of the
visible Universe is much the same density and temperature as the other side. It
also explains the small irregularities we see in the background radiation: they
started out as minuscule quantum fluctuations, and were expanded by
inflation.
Inflationary cosmology has also been successful in explaining other big-bang
problems. The theory comes in many different guises and no one of these is
wholly satisfactory, but there’s no workable alternative—apart, perhaps,
from the ekpyrotic theory.
In this view of things, when the branes collide, the conflagration occurs all
along the brane with the same violence. That would account for the large-scale
smoothness of the Universe. Meanwhile quantum ripples along the incoming brane’s
surface cause small fluctuations in temperature, and seed galaxy formation.
“A theory which competes with inflation is so important that it must be
checked out,” says Andrei Linde of Stanford University, one of the fathers of
inflationary cosmology. But Linde reckons it doesn’t make the grade. He points
out that it requires very finely balanced initial conditions. For example, the
two colliding branes must be almost perfectly parallel to begin with.
Theory alone won’t tell us who’s right. We need some solid experimental
evidence— and fortunately there may be a way to investigate the higher
dimensions. Prisoners in the braneworld we may be, but we might yet be able to
hear echoes from outside its walls. These echoes would come to us in the form of
gravity waves.
According to general relativity, when matter is accelerated it creates
ripples in space-time. So when stars collapse into black holes, or when two
black holes collide, they should emit bursts of gravitational waves. Astronomers
who hope to detect these waves are building several new detectors.
The biggest so far is LIGO—actually a pair of detectors, one in
Louisiana and another thousands of kilometres away in Washington state. In each
of them, lasers measure the lengths of two 4-kilometre tunnels, looking for the
tiny distortions that a passing gravitational wave would create.
If there is a fifth dimension, these detectors could find more than they
bargained for. In the manyfold universe, for example, gravitational waves from a
collapsing star that is billions of light years away along the 3-brane might be
able to reach us by travelling just a few millimetres across a higher dimension.
If these events happen as frequently on other folds as they do on ours, LIGO
should detect far more of them than we’d otherwise expect—up to 30 times
as many, according to Dimopoulos. “If we find an excess, it could signal the
presence of other universes,” he says.
The search for gravitational waves could also arbitrate between the competing
descriptions of what went on in the early Universe. If inflationary cosmology is
correct, we could see a background of primordial gravitational waves which were
generated during the rapid expansion period. The signals would be too weak for
LIGO and other new detectors to spot, but they might leave behind subtle
patterns in the microwave background
(New Scientist, 31 March, p 26).
Because the ekpyrotic model has no rapid expansion period, it should produce no
gravitational wave background. “If we find gravitational waves using the
background radiation, then the ekpyrotic scenario is ruled out,” says Linde.
Meanwhile, the fifth dimension is giving physicists plenty of room for
speculation. If there are hidden large dimensions out there, all kinds of
possibilities open up.
Could we use gravitational waves to communicate with other branes? Or voyage
across hyperspace in baby branes of our own making? Dimopoulos wonders whether
the higher dimensions might be home to new types of force that we could one day
harness. “You can get pretty wild with this,” he says.
If the braneworld is a prison, maybe the inmates are getting ready to revolt.