The Big Bang (one more time)

The inflationary model has fended off all comers; it matches many observations and has described features of the early universe that astronomers searched for and found, Steinhardt says.

For Dr. Guth, a physics professor at the Massachusetts Institute of Technology, inflation was a solution to a particle-physics conundrum. The problem emerged from efforts in the late 1970s to develop grand unified theories to describe the emergence of three of the four forces of nature and the associated subatomic particles from the big bang.

At the time, Guth and collaborator Henry Tye determined that grand unified theories predicted – and sometimes required – formation of magnetic monopoles. These "outrageously heavy" particles "had always been consistent with the laws of physics, but no one had ever seen one and there was no real reason to believe they existed," Guth says.

Prodded by Dr. Tye, Guth says he reluctantly calculated the number of these particles that would have been created during the big bang. They discovered that monopoles would have been as ubiquitous as protons – leading to a universe that would look much different from the one we inhabit.

Guth's calculations led him to conclude that a brief period of inflation stifled monopole formation. As a bonus, inflation also appeared to solve problems cosmologists had in squaring conventional big-bang thinking with astronomers' observations of the universe.

Particle-physics theorists were the most receptive to this view, Guth says. By contrast, he says, astronomers and cosmologists "took a wait-and-see attitude."

This same interplay between the world of the very tiny and the world of the very large weaves its way through Steinhardt's and Dr. Turok's cyclical universe. They hold that much of their model works well in a four-dimensional universe of height, depth, width, and space-time. They add that it finds its true home in the nine to 10 dimensions of string theory, which tries to explain how the four forces of nature emerged from one unified force early on.

One variation, known as M theory, holds that the universe consists of two parallel sheets, or membranes. The two membranes are separated by a "fifth" dimension a tiny fraction of a centimeter wide.

Steinhardt and Turok's calculations describe the membranes meeting in a slap, triggering the big bang. On the membrane humans inhabit, the bang yields the particles, energy, and forces familiar to scientists. The other contains "we know not what," Steinhardt says. The duo posits the second one may be home to "dark matter."

Over trillions of years, the membranes expand, growing darker, colder, and less dense, until the logic of Steinhardt's equations brings them back together in another cosmic slap. The membranes resume expanding, even as they drift apart, only to repeat the cycle.

Steinhardt says this model yields all the features of the inflationary model, without inflation. What his calculations don't show is what happens when membranes "bounce."

As always, nature will provide the ultimate reality check on his model. "There are many beautiful, insightful ideas that turn out not to be the way nature works. The cyclical universe may be one of them," Kolb says. "I don't think this is the way nature works. Maybe there'll be an application for it someday. Or some of the ideas may be used in some other way."

Steinhardt hasn't tossed in the towel on inflation, either: "I'm just hedging my bets."

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