So, what is beyond the edge of the universe?

The fifth dimension is what is beyond the edge of the universe, say the creators of the idea. Though they argue that there is in fact no edge.

"There is only one universe," Ovrut said in a telephone interview. "It does not have a boundary. It's just one large extended brane that has been hit, heated up and is expanding."

The mind-bending concept does not involve multiple or parallel universes, as have been suggested by other researchers.

Instead, Ovrut explains, the fifth dimension is all there, is out there, and embedded in it are multiple branes. Each end of the fifth dimension is bounded by an infinite brane. Our visible universe is one of those, and before the collision it may or may not have contained normal matter. At the other end of the fifth dimension is a brane with physics unlike ours. The branes in between, while they may contain matter, are not universes, and they do not resemble the brane we inhabit.

There is no reason to assume, given this conceptual framework, that there are any other universes out there, Ovrut said.

Inflation attempts to account for the seeming uniformity of the universe. Look in any direction of the sky, and there are features in the universe -- galaxies and clusters of galaxies -- that very much resemble those in any other direction. The theory of inflation accounts for this by putting all matter in one spot at the beginning, then shooting it outward faster than the speed of light in a period of inflation whereby everything developed under similar rules regardless of where it was headed.

Ovrut said that in modeling a collision of branes, his group found that the result would be a universe that fits neatly with predictions of the Big Bang. It produces similar temperatures and causes the resulting universe to expand, for example, and creates matter with the same uniformity predicted by inflation.

"We are not attacking the theory of inflation," Ovrut said. "We're just presenting an alternative."

Turner, the University of Chicago cosmologist, said inflation theory has been so successful that it has killed all competing theories. But inflation doesn't address the idea that there might be other dimensions. Interest in this wild notion has grown among cosmologists in recent years.

In textbooks a century from now, Turner believes there will be one of the following two paragraphs:

"A hundred years ago, people were so desperate to try to understand how to put it all together, they invented additional spatial dimensions. What were they smoking?" Or: "A hundred years ago, people were so provincial that in spite of much evidence that there should be extra dimensions they refused to accept it."