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. "Our new model solves the same problems," 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 "3-brane" to cosmologists, is just one of the two boundary surfaces of a thin four-dimensional "bulk space", 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 "brane storm" led to a collision, the energy released resulted in the big bang.

Unlike inflation, the brane-storm model -- dubbed the "ekpyrotic universe" by the authors of the paper, related to the Greek idea of "cosmic fire" -- fits in neatly with the popular string theory of particle physics. "That's one of the exciting things about it," says Neil Turok of Cambridge University, another co-author.

"I'm delighted to see an alternative picture for the early Universe," says cosmologist Jim Peebles, also of Princeton. "It has been frustrating to me not to have any alternatives to inflation, which I feel has been accepted by many cosmologists too easily."

As for the credibility of the new model, he says: "I haven't heard my string theory colleagues complain. That's a positive sign." 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't be ruled out.

Luckily, the fact that Newton's gravitational constant doesn't appear to be changing implies we're safe from a collision for many billions of years.

This article appeared in April 14, 2001 issue of New Scientist

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