Images: after animation by Paul Steinhardt

A UNIVERSAL CYCLE of birth and rebirth occurs every trillion years or so, according to one new cosmology. Big bangs result when two 10-dimensional "branes" collide (1) and expand (2) and then collide again (4). In this scenario, our universe (3) marks just one phase in this infinite cycle.

Some questions are disquieting because they can be answered in only one of two equally mind-boggling ways. For instance, are we the sole intelligent beings in the universe, or will we find others? Another discomforting doozy is this: did the universe begin at some remote time in the past, or was it always here?

The big bang clearly marks some kind of first. That fearsome flash of energy and expansion of space set in motion everything our eyes and telescopes can see today. But on its own, the big bang theory would leave us in a curved universe where matter and energy aren't well mixed. In fact, we now know that spacetime is flat and that galaxies and radiation are evenly distributed throughout. To shore up the big bang theory, cosmologists proposed that the universe began with a burst of exponential expansion from a single uniform patch of space, whose stamp remains on the cosmos to this day. Such inflationary cosmologies have worked so well they've crowded out all the competition.

During this past year, however, one group of researchers has started to challenge that idea's preeminence, though the field of cosmology has yet to be completely taken with the new approach. Drawing on some cutting-edge but unproved notions in particle physics, the challengers interpret the big bang as a violent clash between higher-dimensional objects. In the latest installment to the saga, the authors of this interpretation have found a way to turn that single clash into a never-ending struggle that rears its fiery head every trillion years or so, making our universe just one phase in an infinite cycle of birth and rebirth.

Such cyclic ideas are not new. In the 1930s, the late Richard Tolman of the California Institute of Technology wondered what would happen if a closed universe¿in which all matter and energy are ultimately compacted in a big crunch¿were to survive its closure and burst forth again. Unfortunately, as Tolman realized, the universe would gather entropy during each new cycle; to compensate, it would have to grow every time like a runaway snowball. And just as a snowball has to begin at some point in time, so, too, would such a universe.

Then in the 1960s, physicists proved that a big crunch, too, must culminate in a singularity¿a point stuffed with infinite matter and heat¿where general relativity breaks down. The laws of physics are thus up for grabs. "The idea of a cyclic universe has been around for a long time," says Andreas Albrecht of the University of California at Davis, a co-inventor of inflation, "and it has always been plagued by a fundamental problem: what physics causes the collapsing universe to bounce back into the expanding phase?"

String-ularity

FIRST STRING. String theory has spawned more than one attempt to do away with the big bang singularity.

One potential way of getting around that problem is by supposing that elementary particles such as electrons, photons and quarks are really just manifestations of tiny strings of energy jiggling in higher dimensions. The thing is, such a string theory requires the universe to have at least 10 dimensions, as opposed to the usual three in space and one in time that we perceive. "In string theory you learn one thing¿you are in higher dimensions," says string theorist Burt Ovrut of the University of Pennsylvania. "Then the question is, where does our real world come from? That's a damn good question."