Paul Steinhardt's universe is a lot like the workaday world of many people, a cycle of early vigor, spent energy, exhausted return, and new beginnings. However, in Steinhardt's universe, there is absolutely no end to the cycle.

The Princeton physicist and his colleague, Neil Turok of Cambridge University, have developed a whole new theory for how the universe came to be. Their proposal seeks to explain recently uncovered flaws in the scientifically accepted model for the origin and evolution of all known things. It describes a series of big bangs and equally significant crunches that form a never-ending cycle of rejuvenation and destruction.

In this universe -- our universe -- time never ends.

The current leading theory for the universe holds that it emerged from a single Big Bang sometime around 12 billion to 15 billion years ago, undergoing an early and rapid period of inflation. That much remains widely accepted.

"However, the standard model has some cracks," Steinhardt and Turok write in a paper published today in the online version of the journal Science.

Astronomers have in recent years learned that the universe is not just expanding, but is doing so at an ever-increasing pace. This can't be explained given the known matter and energy that exists. To account for the acceleration, theorists have conjured a product they call dark energy, which supposedly repels things rather than attracting, as gravity does.

No one has seen this dark energy, and scientists don't even know what it is. But they say it's all around us.

More important, it shouldn't be there.

"The recent discoveries of cosmic acceleration and gravitationally self-repulsive dark energy were not predicted and have no particular role in the standard model," Steinhardt and Turok argue. "Furthermore, the standard model does not explain the beginning of time,' the initial conditions of the universe, or what will happen in the long-term future."

So to patch some of the theoretical cracks, Steinhardt and Turok envision a universe based on perpetual expansion and contraction.

Here's how it works, and keep in mind we're jumping into the middle of the explanation: A big bang sends everything outward. Matter and radiation develop. Dark energy drives an expansion -- as is presently underway -- that lasts trillions of years. Finally, the matter, radiation, and even black holes are "diluted away," leaving the universe smooth, empty, and flat.

Then everything contracts in a so-called big crunch, and a fresh cycle begins.

"In this picture, space and time exist forever, Steinhardt says. "The big bang is not the beginning of time. Rather, it is a bridge to a pre-existing contracting era."

Curiously, the cyclic universe, as it is called, puts the origin of some present-day structures and events prior to the Big Bang.

While existing theory states that galaxies and large clusters of galaxies developed from lumps and filaments that formed in the otherwise smooth fabric of space and time shortly after the Big Bang, Steinhardt thinks the seeds of galaxy formation were created by instabilities that arose during the last contraction, before the crunch that led to "our" bang.

The new model "turns the conventional picture topsy-turvy," he says.

The cyclic universe has roots in even more complex thoughts like so-called superstring theory, which suggests there are as many as 10 spatial dimensions, not just the three we know of. The seemingly inexplicable physics of a big crunch and a big bang might be explained with the aid of these extra dimensions, which are otherwise invisible to us, several theorists believe.

In fact, Steinhardt, Turok and others proposed last year that our universe might have sprung from the collapse of an extra dimension, an idea they called the Ekpyrotic Universe. The cyclic universe builds on this former work but, Steinhardt says, does a better job explaining observations of our present universe.

Other theorists are not quick to give up their standard model, so the concept of a cyclic universe faces an uphill battle for prominence. Even Steinhardt acknowledges that the prospect of unseating a well established cosmological theory "would seem extremely dim."

Meanwhile, the new concept is not free of cracks, either: Even the cyclic universe does not address when the cycles began, so "the problem of explaining the beginning of time remains," the researchers say.
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