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WHAT HAPPENED before the big bang? Try asking a cosmologist this and they鈥檒l
usually fob you off by saying it鈥檚 a meaningless question. Stephen Hawking
famously likened it to asking 鈥淲hat鈥檚 north of the North Pole?鈥� The big bang,
the idea goes, was the ultimate beginning. Time and space came into being then.
There was no 鈥渂efore鈥�.

But that may not be true after all. One daring physicist has come up with an
answer to the question no one is supposed to ask. If he is right, the Universe
began an unimaginably long time before the big bang. 鈥淔ar from being the
beginning of time, the big bang was merely an important turning point in the
Universe鈥檚 history,鈥� says Gabriele Veneziano of CERN, the European Laboratory
for Particle Physics near Geneva.

Veneziano鈥檚 quest for the era before the big bang started in the early 1990s,
when he and his colleague Maurizio Gasperini, then of the University of Turin,
set out to fix some of the major shortcomings of the standard big bang model. It
has plenty of them. For instance, if you imagine the expansion of the Universe
running backwards like a movie in reverse, the density and temperature increase
remorselessly until they skyrocket to infinity.

This infinite point, known as a singularity, is a big problem for physicists.
鈥淭he singularity tells us that our description of the Universe鈥擡instein鈥檚
theory of gravity, or general relativity鈥攊s not applicable in the earliest
moments of the Universe,鈥� says Veneziano. That鈥檚 because there is a point known
as the Planck time鈥攚ithin 10^-43 seconds of the big bang鈥攚hen
gravity is comparable in strength to the other forces of nature. Because of
this, to deal with the physics of the singularity you have to come up with a
quantum version of gravity鈥攁nd no such theory exists.

But physicists do have some ideas about how they might apply quantum theory
to gravity. Perhaps the most promising is string theory. According to this, the
fundamental particles of nature are impossibly tiny 鈥渟trings鈥� vibrating in a
space of nine dimensions, with all but three dimensions 鈥渞olled up鈥� smaller than
an atom. One of the fundamental vibration modes turns out to be a massless
particle that looks just like the hypothetical carrier of the gravitational
force, the 鈥済raviton鈥�. That鈥檚 why Veneziano decided to see whether string theory
could help with the singularity problem. 鈥淚 was convinced that cosmology of the
very early Universe was the right arena for applying and testing string theory,鈥�
he says.

Beyond the wall

What Veneziano found was that string theory can remove the troublesome
singularity at time zero. Because strings have a finite size, the Universe still
shrinks as time runs backwards but it never reaches zero volume so the
singularity never arises. In the standard model of the big bang, the singularity
acts like a brick wall. In string cosmology, there is no such wall and it is
possible to venture back into the era before the big bang.

Having an epoch before the big bang solves another problem with the standard
big bang model. If you imagine the expansion of space running all the way back
to the earliest time reliably described by general relativity, you discover
something extremely odd about the Universe. Although the content of today鈥檚
observable Universe was squeezed into a volume only about 1 millimetre across,
the distance light could have travelled since t = 0 was 10³¹ times
smaller.

This has profound implications for the early Universe. The only way one
region of space can 鈥渒now鈥� about the conditions in another is if there has been
time for some kind of influence to go back and forth between them鈥攁nd the
maximum speed of any influence is that of light. Hence, the early Universe
consisted of 10⁹³ regions which were entirely separate, or 鈥渃ausally
诲颈蝉肠辞苍苍别肠迟别诲鈥�.

The problem is that today鈥檚 Universe is homogeneous. Not only is the density
of matter essentially the same everywhere but so too is the temperature of the
leftover radiation from the big bang. Contrary to all expectations, the 10⁹³
disconnected regions must have found a way to learn about each other.

In the standard model of the big bang, there is not enough time for the
regions to have learned about each other and equalised their density and
temperature in the ultra-short interval between the Universe鈥檚 birth and the
Planck time. Cosmologists have no choice but to patch things up later by
postulating a brief phase of superfast expansion that 鈥渋nflates鈥� the entire
observable Universe from a single causally connected region. Inflation doesn鈥檛
just explain why the Universe is so uniform. It also provides a mechanism for
blowing up tiny quantum fluctuations into the giant conglomerations of galaxies
we see in today鈥檚 Universe. However, the theory has its own problems. For
instance, the mechanism responsible for blowing up space鈥攖he 鈥渋nflaton鈥�
field鈥攊s plucked from thin air by theorists, and for it to work you have
to pick the field鈥檚 initial state with extraordinary care.

All this means that life would be far easier for cosmologists if the big bang
was not the beginning of the Universe, and a long prehistory preceded it. There
would be plenty of time before the Planck time for the densities and
temperatures in 10⁹³ regions to be equalised, and it would not be necessary to
patch things up with standard inflation. 鈥淎 prehistory is the obvious way to
homogenise the Universe,鈥� says Veneziano. And that鈥檚 just what string theory can
provide.

So what would the pre-big-bang period be like? You can see clues to the
answer, Veneziano says, by looking at the Universe鈥檚 symmetry. The standard
cosmological solutions of Einstein鈥檚 general relativity equations鈥攌nown as
Friedmann big bang models鈥攑ossess a very simple symmetry. They are
invariant under time-reversal: in other words, if you take a solution of the
equation and replace time by minus time you get another solution. Rather than
expanding from a big bang, this time-reversed Universe contracts down to a big
crunch.

The cosmological solutions of string theory possess this same symmetry, and
another one too: they are also symmetrical if you replace the scale factor of
the Universe, which sets its size, by its reciprocal. Apply both of these
changes鈥攖ime reversal and the scale-factor change鈥攁nd you get an
unusual type of expanding universe in which the expansion is accelerating rather
than decelerating. In other words, it is an inflationary universe. 鈥淔or every
big bang solution, there turns out to be a solution in which space inflates from
t = 鈭掆垶 to t = 0鈥攖hat is, towards the big bang,鈥�
says Veneziano.

String theory therefore suggests that the Universe underwent a phase of
accelerated expansion from t= 鈭掆垶 to until just before t
= 0. Just after t = 0, this changed into the more leisurely
decelerating expansion of the old Friedmann big bang models.

Now we see the big bang in a new light. It was the time of transition from
inflation to the expansion we see around us today. 鈥淎t the big bang, the
Universe had maximum curvature, maximum expansion rate and maximum temperature,鈥�
says Veneziano. 鈥淭he big bang emerges not as the beginning but an important
turning point in the history of the Universe.鈥�

Some others are in broad agreement. 鈥淚 think it鈥檚 very likely that the big
bang is indeed a later stage of the Universe,鈥� says Gordon Kane, a particle
physicist at the University of Michigan.

One of the huge advantages of the string picture is that the pre-big-bang era
is automatically inflationary and there is more than enough time for inflation
to equalise the density and temperature of the Universe. But a description of
how space evolved in the pre-big-bang era is only half the picture. To get the
rest of it we need to know what the Universe evolved from鈥攊ts initial
conditions. What was the Universe like at t= 鈭掆垶?

In the standard model of the big bang, the Universe started out in an
extraordinarily special state in which the temperature and density are
fine-tuned to be precisely the same in 10⁹³ separate regions. Faced with a
special state, a physicist鈥檚 instinct is always to look for a not-so-special
state from which it evolved. In the case of a crystal, for instance, everyone
recognises that the fantastically ordered arrangement of atoms evolved via a
phase transition from a more amorphous, and far less fantastic, liquid state.
鈥淚n the same way, the Universe must have evolved from the simplest, most
ordinary state that can be imagined,鈥� says Veneziano.

The simplest possible universe, according to Veneziano is infinite in extent,
empty, cold and 鈥渇lat鈥濃�攚hich means space has a very low curvature.
Veneziano and his colleagues Alessandra Buonanno at Caltech and Thibault Damour
at the Institute of Advanced Scientific Studies (IHES) near Paris, call it a
principle of 鈥渁symptotic past triviality鈥�.

Not everyone is happy with this. 鈥淚t鈥檚 equivalent to assuming homogeneity in
the first place,鈥� says cosmologist Paul Steinhardt of Princeton University.
鈥淕abriele has not solved any initial conditions problems鈥攈e has
`trivially鈥� set them by his initial conditions.鈥�

Actually, complete flatness does not fit the bill. Veneziano points out that
the most general solution of the string equations is filled with a chaotic sea
of gravitational waves鈥攔andom fluctuations in the curvature of space. 鈥淚f
space were completely flat it would be trivial in a special way,鈥� he says.

Clearly, the Universe did not stay trivial or we wouldn鈥檛 be here today. As
time passed鈥攁nd there鈥檚 been an awful lot of that鈥攖here was an
opportunity for classical fluctuations to create regions that had a greater
energy density than average. As in today鈥檚 Universe, these fluctuations could
become pronounced enough to give birth to black holes.

It is here that we see the real birth of the Universe we recognise. According
to Veneziano, the solution of string theory that applies to the inside of a
black hole is exactly the same as the accelerated expansion that string theory
predicts for the pre-big-bang era. 鈥淥ur Universe is a patch of the inside of a
black hole,鈥� he says.

Veneziano and Gasperini have shown over the past few years that as the
Universe expands towards t = 0, space-time becomes increasingly curved,
resulting in a dramatic increase in temperature and energy density. A split
second after time zero, a millimetre-sized, three-dimensional region within this
vast expanse could look just like the superdense, superhot patch in standard
inflation theory.

Production line

Of course, to be valid any scenario must supply the particles of matter we
see around us today. According to Veneziano, particles such as electrons,
positrons and photons are conjured into existence by fluctuations in the
geometry of space. This is where quantum mechanics comes into play. Just as
strong electric fields can create electron-positron pairs, strongly varying
gravitational fields lead to quantum production of all sorts of particles.
Furthermore, the particles are born with a large amount of kinetic energy, and
as a result the Universe gradually warms up. 鈥淭his can be contrasted with the
standard picture in which the particles are produced and heated up after the end
of inflation,鈥� says Veneziano.

For the pre-big-bang scenario to be anything more than science fiction it has
to make predictions about the Universe. Fortunately it does make several
testable predictions which are at odds with those of the standard model of
inflation. If Veneziano is right, the Universe should be filled with a chaotic
sea of gravitational waves left over from its trivial past. The waves will be
weak, and the prospects of seeing them with the current generation of
detectors鈥攐r even their immediate successors鈥攁re not good. However,
Veneziano maintains that third-generation detectors should be able to pick up
this gravitational-wave background.

Effects from before the big bang should also be visible in the cosmic
microwave background, the 鈥渁fterglow鈥� of the big bang. Veneziano predicts
different angular positions of the peaks in the power spectrum of the cosmic
background, compared with standard theories. This could be tested by the
American MAP satellite, which is due for launch in November, or the European
Planck probe scheduled for 2007.

The pre-big-bang scenario still has problems. Many might accuse Veneziano of
simply moving the beginning of the Universe to an earlier time, without doing
any more to explain how it came into being. Veneziano pleads that this is too
harsh. 鈥淚n the pre-big-bang scenario, the beginning is still not nothing but
it鈥檚 far more trivial and natural than the big bang,鈥� he says. 鈥淚 believe this
is a big improvement on the standard big bang.鈥�

Another problem is what happens at time zero. Somehow, the accelerating
Universe of the pre-big-bang era must change seamlessly into the more leisurely
expanding Universe we see around us today. This 鈥減atching together鈥� problem is
exactly what string theory, with its abhorrence of singularities, should solve.
But it is not yet clear exactly how the patching would work.

While Veneziano doesn鈥檛 yet have all the answers, many cosmologists think he
has done them a huge service. 鈥淭here is a tendency to be very sceptical on the
part of most people who think about this stuff,鈥� says Andreas Albrecht of the
University of California at Davis. 鈥淏ut others are happy to hear that someone is
trying to do some cosmology with stringy connections.鈥�

鈥淕abriele鈥檚 ideas are extremely good,鈥� agrees Kane. 鈥淚t鈥檚 less clear they are
right, but if not they will surely stimulate thinking about these issues. This
is probably the most important point鈥攕tudying serious, testable (at least
in principle) models about before-the-big-bang physics is now good physics.鈥�

Others are not so positive. Steinhardt says: 鈥淲hile everyone really
appreciates Gabriele for his creativity and charm, I think most of us think that
he has been on the wrong track, or a very unlikely track, for a number of
测别补谤蝉.鈥�

The risk that he鈥檚 wrong doesn鈥檛 bother Veneziano. He is perfectly willing to
admit this is a possibility. But, he says, 鈥渨e have shattered a wall鈥�. Want to
know what came before the big bang? Then ask away. It鈥檚 no longer taboo.