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IT was mighty quiet in our Universe: devoid of all matter and energy. Then
another universe collided with it. Suddenly space became a searing soup of
particles and radiation, far hotter and denser than the centre of the Sun.

This, says a team of physicists, is how the big bang happened. The approach
of the parallel universe caused the expansion of space, and the collision itself
produced all energy and matter. A comprehensive description of this new theory
has been submitted to the journal Physical Review D.

The model is a viable alternative to the theory of cosmic inflation, which
was cooked up to solve some of the problems of classical big bang theory.
According to inflation theory, the Universe underwent a brief period of
exponential expansion in the first split second of its existence. 鈥淥ur new model
solves the same problems,鈥� says co-author Paul Steinhardt of Princeton
University, who was one of the founders of inflation some 20 years ago.

So how might this primordial collision have come about? According to the
theory, our three-dimensional Universe, known as a 鈥�3-brane鈥� to cosmologists, is
just one of the two boundary surfaces of a thin four-dimensional 鈥渂ulk
space鈥�, rather like one of the two surfaces of a CD. But there was another
universe, or bulk brane, inside the bulk space which encroached on our boundary
brane. When this 鈥渂rane storm鈥� led to a collision, the energy released resulted
in the big bang.

Unlike inflation, the brane-storm model鈥攄ubbed the 鈥渆kpyrotic universe鈥�
by the authors of the paper, related to the Greek idea of 鈥渃osmic
fire鈥濃�攆its in neatly with the popular string theory of particle physics.
鈥淭hat鈥檚 one of the exciting things about it,鈥� says Neil Turok of Cambridge
University, another co-author.

鈥淚鈥檓 delighted to see an alternative picture for the early Universe,鈥� says
cosmologist Jim Peebles, also of Princeton. 鈥淚t has been frustrating to me not
to have any alternatives to inflation, which I feel has been accepted by many
cosmologists too easily.鈥� As for the credibility of the new model, he says: 鈥淚
haven鈥檛 heard my string theory colleagues complain. That鈥檚 a positive sign.鈥�

But if there are other branes floating around in 4-D bulk space, might we be
in for another big bang? Turok says it can鈥檛 be ruled out. Luckily, the fact
that Newton鈥檚 gravitational constant doesn鈥檛 appear to be changing implies we鈥檙e
safe from a collision for many billions of years.