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Image: Cosmic branes
In the cyclic model, our three-dimensional universe is one of two surfaces, or "branes," separated by an extra dimension. The two branes bounce off each other to give rise to matter and radiation, and then expand and dissipate due to dark energy. Click for more information from Princeton cosmologist Paul Steinhardt, including a Flash animation.
By Alan Boyle Science editor
msnbc.com

How did the universe begin, and how will it end? Among cosmologists, the mainstream belief is that the universe began with a bang billions of years ago, and will fizzle out billions of years from now. But two theorists have just fired their latest volley at that belief, saying there could be a timeless cycle of expansion and contraction. It’s an idea as old as Hinduism, updated for the 21st century.

The “cyclic model,” developed by Princeton University’s Paul Steinhardt and Cambridge University’s Neil Turok, made its highest-profile appearance yet Thursday on Science Express, the Web site for the journal Science. But past incarnations of the idea have been hotly debated within the cosmological community for the past year — and Steinhardt acknowledges that he and Turok have an uphill battle on their hands.

“It will take people a while to get used to it,” Steinhardt told MSNBC.com. “This introduces a number of concepts that are quite unfamiliar, even to a cosmologist.”

Tinkering with the cosmos
Years ago, Steinhardt played a prominent role in formulating what is now the most widely accepted scientific picture of the universe’s beginnings, known as inflationary Big Bang theory: that a vanishingly small quantum fluctuation gave rise in an instant to an inflated region of space-time, kicking off an expansion that is now picking up speed.

The model has weathered repeated experimental tests, including studies of patterns in the microwave “afterglow” of the Big Bang.

“All the competing models were knocked off,” Steinhardt said. “So we had a situation where it looked as if we had converged on a single idea. But I was always disturbed by the idea that there were no competitors around.”

So Steinhardt, Turok and others began tinkering with alternate models. As successful as the inflationary theory was, there were some unexplained gaps: What sparked the universe in the first place? What is the role played by “dark energy,” a mysterious property that seems to be causing the universe’s expansion to accelerate? Is there any connection between the universe’s origins and a “theory of everything”?

Any alternate model should address such issues as well as explain the phenomena that the mainstream theory explains so well. The theorists came up with a doozy: Instead of one Big Bang, a succession of bangs could be sparked when rippling waves of space-time crash into each other in extra dimensions. It sounds like the cheesiest form of science fiction, but this scenario — dubbed the “ekpyrotic model,” which plays off the Greek word for “conflagration” — was put forward as the best rival to the mainstream model.

Responding to criticism
The theory touched off another kind of conflagration within the physics community. Some complained that the mathematical basis for the model was half-baked. Others said the theory depended on having just the right conditions to produce the desired results. Still others questioned why physicists needed to develop such a bizarre alternative.

“I was sharply critical of the ekpyrotic model,” said Andreas Albrecht, a cosmologist at the University of California at Davis. “I really do think it’s a step backwards. We could just declare that this is the state of the universe at the beginning. ... It’s a really hugely ambitious thing to do what inflation (theory) does, and it’s actually amazing that we can have something that can do that. So if we can do that, I don’t want to back off from that.”

Steinhardt and Turok took the criticism to heart, and their Science paper proposes a revised scenario called the cyclic model.

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“I think now we’ve gotten into a situation where the model is much more ambitious, even more ambitious than the standard model,” Steinhardt said.

The revised theory still thinks of our universe as one of two multidimensional surfaces, or “branes,” separated by an extra dimension. Over the course of trillions of years, the surfaces bounce off each other, sparking a Big Bang. But in the cyclic scenario, dark energy plays an essential role.

At first, matter and radiation are dominant in a newly spawned cosmos. However, the accelerating dark energy gradually drives the expansion of the universe to such an extent that the cosmos is virtually cleared out. Then a weak force starts bringing the branes back together in the extra dimension, setting the stage for another bounce, or “Big Crunch,” that touches off the next Big Bang.

“Even if the universe were disrupted from its periodic behavior, it would rapidly reconverge to the cyclic solution,” Steinhardt said, due to the clearing-out effect of dark energy. That means the cycles could continue without beginning or end.

The theorists acknowledge that their cyclic concept draws upon religious and scientific ideas going back for millennia — echoing the “oscillating universe” model that was in vogue in the 1930s, as well as the Hindu belief that the universe has no beginning or end, but follows a cosmic cycle of creation and dissolution.

“I didn’t start out liking this picture,” Steinhardt told MSNBC.com. “We’ve been led into this. All these concepts run into one another.”

Evolving theory
Albrecht admits that the idea of a bouncing universe is an improvement over the ekpyrotic model.

“The coolest thing about this idea is that they actually use the fact that the universe is accelerating today in an interesting way,” he said. “Today’s acceleration is the inflation for the next cycle.”

But he still thinks proponents of the cyclic model haven’t made their case.

“How do you actually make a collapsing universe bounce back? No one ever had a good idea about that,” Albrecht said. “What these guys realized was that if they got their wish for an ekpyrotic universe, then they could have the universe bounce back.”

Albrecht and other skeptics agree, however, that questioning the inflationary Big Bang model is a healthy process.

“Right now inflation has destroyed all its competition, so we need some competing ideas,” University of Chicago astrophysicist Michael Turner told MSNBC.com. “These guys have set for themselves a very bold goal, which is to explain how it all began and make connections to superstring theory. And that’s something we should be engaged in. But I don’t think we’ve made much progress yet.”

Steinhardt expects the debate to continue for years and perhaps decades to come. “I don’t expect a debate to be decisive,” he said. “I expect observations to be decisive.”

What kinds of observations could determine whether the cyclic model is correct or cracked? In their paper, Steinhardt and Turok say that if scientists detect gravitational waves left behind by the inflationary Big Bang, that would disprove their theory. But if such waves are missing, that would strengthen their case.

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