30 MAR 02: OUR UNIVERSE,
BORN OUT OF BOUNDARY BRANE FIRE
Scientist magazine, vol 173 issue 2334, 16/03/2002, page 26:
Cycles of creation
Our Universe may be stuck
in an endless loop of death and rebirth. It’s an old idea, says Marcus
Chown, but the strange power of nothingness has given it a new lease
WHAT happened before the
big bang? If some physicists are to be believed, the question is about
as meaningless as asking what is north of the North Pole. But others don’t
give up so easily.
to two cosmologists, before the big bang there was another big bang. And,
before that, another. “If we’re right,” says Neil Turok of the University
of Cambridge, “the big bang is but one in an infinite series of big bangs
stretching back into the eternal past.” And into the eternal future.
Turok and his colleague Paul Steinhardt of Princeton University are advocating
is a new version of an idea that dates back to the 1920s. Back then the
Russian physicist Aleksandr Friedmann, the father of the big bang idea,
realised that if the gravity of all the matter in the Universe is powerful
enough, it could stop the expansion of the cosmos and turn it around. The
Universe would then carry on contracting down to a “big crunch”. If both
expanding and re-collapsing universes are permitted, it’s a simple step
to imagine the one changing seamlessly into the other. From the big crunch
the Universe would bounce or rebound in a new big bang and the whole cycle
would begin again.
a popular idea until the 1960s, when Roger Penrose and Stephen Hawking
scuppered it. Using Einstein’s general theory of relativity, which explains
gravity as a warp in space-time, they showed that the big bang must have
started in a singularity. A singularity is a point of infinite density
and temperature, and it’s a big problem for anyone taking a hard look at
the physics of the big bang. That’s because when everything in your equations
goes to infinity, the equations are meaningless. Physics breaks down.
doesn’t rule out a cyclic Universe. But the singularity is like an opaque
curtain, preventing a view through the big bang to earlier times. With
the singularity in the way, it makes no sense to talk about any continuous
existence. If the Universe passes through a singularity, everything gets
scrambled in the breakdown of physics. Nothing in the new universe can
be affected by what happened before, so the previous cycle might as well
not have existed. This was very discouraging, and people abandoned the
idea of a cyclic Universe.
has come about because physicists are now convinced that Einstein’s theory
of gravity breaks down at the big bang. It’s all because of quantum mechanics,
which seems to impose a fundamental fuzziness on things. Quantum theory
is usually applied to particles of matter, but many physicists think it
must affect space-time too. The implication, they say, is that nothing
can collapse to a point. Instead there is a minimum size for anything.
The Universe may once have been pretty amazingly small, but it wasn’t infinitesimal,
so its temperature and density weren’t infinite. “The Universe may not,
after all, have begun in a singularity,” says Turok.
the past decade or so, this idea of space-time fuzziness has encouraged
some physicists to think about what happened before the big bang. But on
its own, it doesn’t prove there was anything, or give any hints about what
last year, Turok and Steinhardt came up with the first part of their new
theory. It builds on what are called brane-world scenarios, an outgrowth
of the idea that extra dimensions in space are needed to explain the fundamental
forces of nature. To explain why we experience only four of these dimensions,
physicists have come up with the peculiar idea that the matter and non-gravitational
forces of our Universe are stuck firmly to a four-dimensional island, or
“brane”, floating within a higher-dimensional space. Whereas most of the
extra space dimensions are supposed to be rolled up much smaller than an
atom, it may be that one of them is relatively large, and we simply don’t
see it because it is the exclusive realm of gravity ( New Scientist, 29
September 2001, p 26) .
brane-world scenario suggests a possible explanation for the big bang,”
says Turok. Branes have their own mass, so a moving brane has an enormous
amount of kinetic energy. And if our brane collided with another brane,
this kinetic energy would be liberated, he thinks. “This could have created
the fireball of the big bang and ultimately all the matter we see in today’s
galaxies and stars”.
and Steinhardt, who developed this idea with Justin Khoury of Princeton
University and Burt Ovrut of the University of Pennsylvania, call it the
“ekpyrotic” universe, from the Greek for “born out of fire”. They have
thought through several colliding-brane scenarios, some involving three
branes. But what they’ve ended up with is a relatively simple scenario,
in which two four-dimensional branes approach each other along a fifth
dimension. Turok and his colleagues call them “boundary branes” because
they form the ultimate boundaries of the Universe.
we have done is explore what would happen if one brane passes through the
other,” he says. They found that the kinetic energy of the colliding branes
is converted into heat energy within the branes when they collide with
each other, effectively conjuring real particles out of the vacuum. What’s
more, it naturally produces a Universe that is smooth on the largest scales,
but has small lumps and bumps in it to turn into galaxies and galaxy clusters.
basic model, there’s still no cycle. Just a phase of approaching, empty
branes before the big bang. Then Steinhardt and Turok asked themselves,
what could pull the branes together before their collision? That something
can only be the vacuum in between them, says Turok-because there’s nothing
as it turns out, changes everything. “The vacuum is like a spring between
the plates, or branes,” says Turok. Within our Universe it appears to be
generating a repulsive force-the so-called cosmological constant-which
is driving apart the galaxies. An attractive force would seem to be incompatible
with that. But it turns out that even while there is a repulsion along
the space dimensions inside each brane, there can also be an attraction
between the branes along the fifth dimension.
team is considering a number of possible mechanisms that might be behind
this force. One suggestion is that there is a charge imbalance between
the two branes that creates an attractive force between them. “We don’t
have a complete theory in which this could be calculated,” Turok says.
“Our scenario is more of a guide as to how things could work.”
that today, the spring is still being stretched, but in the far future
it will reach its maximum extension. Once that happens, the branes will
begin to accelerate towards each other until they collide again.
the new picture, the oscillation occurs only along the fifth dimension.
It happens like this: two branes are pulled together by the vacuum, and
collide. Inside both branes a huge amount of energy is released, and the
branes expand (if you can imagine an infinite rubber sheet being stretched
out, it’s a little like that). We brane-bound creatures call this event
the big bang.
branes expand and cool, matter and galaxies form. The galaxies drift apart
and age. After a while, the gently repulsive vacuum inside the branes makes
this expansion accelerate, so the galaxies fly apart faster still. The
end looks bleak.
the two branes have moved apart and then been pulled back together by the
attractive vacuum in between them. They rush towards a collision once more,
and a new big bang overwhelms both universes.
the perspective of someone stuck on the brane, space-time just keeps on
expanding, though the expansion is given repeated pushes by successive
bangs-that is, brane collisions. In other words, from the off-brane perspective,
we have something more like the traditional cyclic universe, yo-yoing back
and forth. Meanwhile, from the brane perspective, we have an altogether
different kind of cycle in an eternally expanding Universe.
overcomes another big problem with the old-style cyclic universe. In each
cycle, stars radiate heat into space, but these cyclic models involve closed
universes, so each bang is hotter than its predecessor. Looking backwards
in time, then, the cycles get progressively cooler. The inescapable conclusion
is that the cycles must have begun at some time in the past. “But simply
pushing the origin of the Universe back before the big bang is not very
aesthetically pleasing,” says Turok. “This is another reason why the cyclic
universe was seen as unsatisfactory.”
cyclic universe avoids this problem. After the branes have passed through
each other, the spring of the vacuum is in compression and causes the space
of the branes to expand for a long time. That dilutes the heat from stars
so that the patch of space that experiences each new bang has essentially
the same temperature as the previous cycle. Consequently, all cycles are
the same and the universe can have oscillated for ever. “Such a universe
is more aesthetically pleasing than a big bang universe since the question
of what happened before is no longer a nagging problem,” says Turok. “The
Universe has been around for ever. There was no beginning.”
galaxies and life may therefore have existed in previous cycles of the
Universe. But, if the cycles are all identical, wouldn’t such endless repetition
be mind-numbingly dull? Turok and Steinhardt think not, because random
events will change the details each time. You won’t get the same galaxies,
planets and people each cycle. “Just because the cycles repeat does not
mean the events in each cycle are identical,” says Turok.
speculatively, he points out that the extra rolled-up dimensions might
vary their sizes between cycles. The significance of this is that the fundamental
forces are suspected to be manifestations of the sizes of these extra dimensions.
“The laws of physics could change from cycle to cycle,” says Turok.
physical laws can change, they might be driven ever closer to some particular
set, what physicists call an attractor. “If we are lucky, we might find
that the sizes of the extra dimensions home in on particular values,” he
says. “We might then finally have an explanation for, say, the mass of
both Turok and Steinhardt are excited by all these possibilities. Reactions
from their colleagues are more mixed. “At the moment I have an open mind
on the ekpyrotic universe and its latest oscillating version,” says Tom
Kibble of Imperial College in London. “There is no doubt an element of
hype here, but I think they are right to be excited.”
most outspoken opponent is Andrei Linde of Stanford University. “This is
mostly hype,” he says. He thinks the whole model is unnecessarily complicated,
like the epicycles that medieval astronomers used to describe the orbits
of the planets in our Solar System.
Steinhardt and Turok are right after all, the future is less bleak and
more dangerous than we have been told. Some cosmologists suggest that,
because the galaxies are now accelerating apart, the future holds nothing
but an ever emptier, cooler Universe. Now we have an alternative to look
forward to: an almighty surprise, one day, when we and our fellow universe
come together and collide once more in a spectacular finale. And who knows
what will emerge from the fire?
”The ekpyrotic universe:
colliding branes and the origin of the hot big bang” by Justin Khoury,
Burt Ovrut, Paul Steinhardt and Neil Turok, ( http://www.arxiv.org/abs/hep-th/0103239)
“From big crunch to big
bang” by Justin Khoury, Burt Ovrut, Nathan Seiberg, Paul Steinhardt and
Neil Turok ( http://www.arxiv.org/abs/hep-th/0108187)
Paul Steinhardt’s website
is at http://feynman.princeton.edu/~steinh Neil Turok’s website is at http://www.damtp.cam.ac.uk/user/ngt1000