"Cyclic-universe models were popular in the 1920s and '30s," Steinhardt says. "But they were based on the idea of a Big Bang followed by a Big Crunch followed by another Big Bang." In these models, the same matter is endlessly recycled, so the entropy of the universe—its tendency toward disorder over time—increases from one cycle to the next. "The result is that each subsequent cycle gets longer," Steinhardt says. "And if you go back into the past, each cycle gets shorter. Ultimately, you still have to have a beginning." In principle, scientists shouldn't care. In practice, most have a very human tendency to abhor the idea of a beginning to time. And most find the prospect of a universe that will end someday to be rather grim. In this new cyclic model, the universe starts essentially empty each time. That means virtually no matter gets recycled. So entropy doesn't increase, and there is no beginning or end to time.
The model works so well that one might expect cosmologists to embrace it wholeheartedly. Actually, the reception has been lukewarm. One reason is that at the moment of collision, the extra dimension separating the two branes goes from vanishingly small to literally zero. That creates what physicists call a singularity, a point at which the laws of physics break down. Although superstring theory might help explain what happens in a singularity, it hasn't done so yet. "The problem is very difficult," Turok admits.
That, say some physicists, is an understatement. "I don't think Paul and Neil come close to proving their case," says Alan Guth, a cosmologist at MIT who is a founding father of inflation theory. "But their ideas are certainly worth looking at." Nathan Seiberg, a string theorist at the Institute for Advanced Study in Princeton, is also cautious. "I don't know whether their model is right or wrong," he says.
Joel Primack, a physicist and cosmologist at the University of California at Santa Cruz, isn't even all that interested in whether it's right or wrong. "I think it's silly to make much of a production about this stuff," he says. "I'd much rather spend my time working on the really important questions observational cosmology has been handing us about dark matter and dark energy. The ideas in these papers are essentially untestable."
Steinhardt and Turok respond that their theory could gain credence from LISA, a proposed space probe that would look for gravity waves from the early universe. Gravity waves are ripples in the fabric of space-time that were predicted by Albert Einstein. So far, they are theoretical. But by 2020, the LISA experiment—pairs of free-flying satellites that would move apart and together with each passing wave—could either find confirming evidence of inflation or find nothing and thus tip the scales toward ekpyrosis. Inflation theory posits that the entire mass of the universe accelerated to many times the speed of light in a fraction of a second and should have set the entire cosmos ringing with gravity waves. Ekpyrosis, by contrast, which involves a very slow collision between universes, wouldn't generate observable waves. "If we're right," says Steinhardt, "it will be terrifically exciting. If we turn out to be wrong, that'll be disappointing, of course, but it's still important to challenge inflation with alternate theories so we can see how robust it really is."
David Spergel, a Princeton astrophysicist and a member of the WMAP satellite research team, agrees. "Cosmology has to be tied in with superstring theory sooner or later," he says. "There are several ideas out there competing with inflation, and they may all turn out to be wrong. But I'd say this one has the best chance of being right." If it is, we need to rethink our place in the universe—in fact, we need to rethink the universe itself. In the ekpyrotic view of reality, everything that astronomers have ever observed is just a speck within the higher dimensions, and all of history since the Big Bang is but an instant in the infinity of time. This view of creation is far grander than the universe of traditional cosmology or the universe of the Bible.
So far, the pope hasn't weighed in.