Inflation theory has been cock of the walk since the early 1980s, when physicist Alan Guth concocted it to solve several problems that had been plaguing big bang theorists. For instance, the universe appears to be "flat" (a technical term describing the large-scale curvature of space) and isotropic (it has roughly the same properties everywhere)--features that a simple big bang model can't easily explain. Inflationary theory proposes that the very early universe went through an amazingly violent and rapid expansion for less than 10^-32 second. For 20 years no one has come up with a new scenario that so well matches scientists' observations.

Now, physicist Paul Steinhardt at Princeton University and his colleagues have created what they call the "ekpyrotic model": a version of the early universe that explains flatness and isotropy without invoking inflation. At first glance, the new model--based on an extension of string theory known as M-theory--seems surreal. It takes place in 11 dimensions, six of which are rolled up and can safely be ignored. In that effectively 5-dimensional space float two perfectly flat four-dimensional membranes, like sheets drying on parallel clotheslines. One of the sheets is our universe; the other, a "hidden" parallel universe.

Here's how the universe came to be, according to the new theory: Provoked by random fluctuations, our unseen companion spontaneously shed a membrane that slowly floated toward our universe. As it moved, it flattened out--although quantum fluctuations wrinkled its surface somewhat--and gently accelerated toward our membrane. The floater sped up and splatted into our universe, whereupon some of the energy of the collision became the energy and matter that make up our cosmos. Because both the moving membrane and our own membrane started out roughly flat, our postcollision universe remains flat as well. "Flat plus flat equals flat," Steinhardt says.

"It's the first really intriguing connection between M-theory and cosmology," says David Spergel, an astrophysicist at Princeton University. "This is sort of an Ur-big bang." And though ekpyrotic theory might seem totally detached from reality, future experiments should be able to tell whether it or inflation is correct. The two models predict that different sorts of gravitational waves are rattling around the universe--waves that might one day be detectable by successors to current gravitational-wave experiments.

--CHARLES SEIFE

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