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May 15, 2006

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Another universe may have preceded ours, study finds

May 14, 2006
Courtesy Penn State University
and World Science staff

Three physicists say they have done calculations suggesting that before the birth of our universe, which is expanding, there was an earlier universe that was shrinking. 

The results stem from a theory that claims the fabric of space and time is made up of minuscule, indivisible bits, much as matter is.

The diagram represents our expanding universe as the right branch of the arc. The present time is at the right edge. According to Ashtekar's calculations, when looking backward throughout the history of the universe, 'time' does not go to the point of the Big Bang but bounces to the left branch of the drawing, which describes a contracting universe. High resolution version here.

Scientists believe our cosmos began in a sort of explosion called the Big Bang, when everything that exists—which just previously had been packed into one infinitely dense point—burst outward.

The universe is still expanding, according to this view, because it was born expanding.

According to some proposals, the Big Bang is a repeating cycle. Universes might expand, then shrink back to a point, then expand again. Thus the “bang” would be really more like a bounce.

The idea is appealing in some ways, but scientists have found it far from easy to test. Einstein’s General Theory of Relativity, a key basis for the Big Bang theory, is silent on what happened before that event.

“General relativity can be used to describe the universe back to a point at which matter becomes so dense that its equations don’t hold up,” said Abhay Ashtekar, director of the Institute for Gravitational Physics and Geometry at Penn State University in University Park, Penn.

To go further, physicists must use tools Einstein didn’t have, he added. Ashtekar and two post-doctoral researchers developed such tools through a combination of quantum physics—the science of subatomic particles—and general relativity, which describes the large-scale structure of space and time. 

They found that before the Big Bang, there was a contracting universe. Other than the fact that it was shrinking, they added, it was similar to ours in terms of the geometry of its space and time, or spacetime, as cosmologists call it since Einstein found the two are interwoven.

“In place of a classical Big Bang there is in fact a quantum bounce,” said Ashtekar. “We were so surprised by the finding,” he added, that the team repeated the calculations for months to include different different possible values of some numbers representing the current universe. But the results kept pointing to a bounce.

The findings appear in the current issue of the research journal Physical Review Letters.

While the general idea of another, pre-Big Bang universe isn’t new, Ashtekar said, this is the first mathematical study that systematically establishes its existence and deduces properties of its spacetime geometry.

The notion that spacetime has a geometry involves the idea that it can be curved or flat. A “flat” spacetime is one in which geometry works as we normally expect; for example, parallel lines never meet. But Einstein found that material objects deform this flatness, introducing curvature.

To arrive at their pre-existing universe finding, Ashtekar’s group used loop quantum gravity, a theory that seeks to reconcile General Relativity with quantum physics. These two seemingly fundamental theories are otherwise contradictory in some ways.

Loop quantum gravity, which was pioneered at Ashtekar’s institute, proposes that spacetime has a discrete “atomic” structure, as opposed to being a continuous sheet, as Einstein, along with most us, assumed. 

In loop quantum gravity, space is thought of as woven from one-dimensional “threads.” The continuum picture remains mostly valid as an approximation. But near the Big Bang, this fabric is violently torn so that its discrete, or quantum, nature becomes important. One outcome of this is that gravity becomes repulsive instead of attractive, Ashtekar argued; the result is the Big Bounce.

Paul Steinhardt of Princeton University, a cosmologist who has explored some related concepts, wrote in an email that the new research “supports, in a general way, the idea that the Big Bang need not be the beginning of space and time.”

The universe “may have undergone one or more bangs in its past history,” he added.

Steinhardt and colleagues have also proposed a bounce of sorts, but it’s different. It could turn out that the two scenarios are equivalent at some deep level, but that’s not known, he added. Steinhardt’s scenario makes use of string theory, another attempt to reconcile General Relativity with quantum physics. 

Some versions of string theory portray our visible universe as a three-dimensional space embedded in an invisible space having more dimensions. Our zone, called a braneworld—the word comes from its similarity to a sort of membrane—could periodically bounce into another, parallel braneworld.

Such an event might look to us, stuck in a few dimensions as we are, as a Big Bang. “I don’t know if [Ashtekar’s] case translates into a bounce between braneworlds like we are describing,” Steinhardt wrote. But by his estimate, this cataclysm won’t take place for another roughly 300 billion years—so there is hopefully plenty of time to answer the question.

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