before it's in the papers"
August 03, 2010
TO THE WORLD SCIENCE HOME PAGE
Another universe preceded ours, researchers claim
and World Science staff
Three physicists say they have done calculations showing before the birth of our universe, which is expanding, there was an earlier universe that was shrinking.
The results stem from a theory that holds that the fabric of space and time is made up of minuscule, indivisible bits, much as matter is.
Cosmologists believe our universe began in a sort of explosion called the Big Bang. At that time, everything that exists—which just previously had been packed into one infinitely dense point—burst outward, creating space and time. The cosmos is still expanding, according to this view, because it was born expanding.
According to some proposals, the Big Bang is a repeating cycle. Universes would expand, then shrink back to a point, then expand again endlessly. Thus the “bang” would be really more like a “bounce.”
The idea is appealing in some ways, but cosmologists have found it far from easy to test. Einstein’s General Theory of Relativity, which helps describe the evolution of the cosmos, is silent on what might have happened before the Big Bang.
“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 at 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 space and time at vast scales.
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 space-time, 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 to account for different different possible values of some numbers. But the results continued pointing to a “Big 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 description that systematically establishes its existence and deduces properties its space-time geometry.
The notion that space-time has a geometry involves the idea that it can be curved or flat. A “flat” space-time is one that behaves the way we normally expect; for example, parallel lines never meet. But Einstein found that material objects can deform this flatness, introducing curvature, depending on how heavy they are.
At the vastest scales, space-time can theoretically also be either curved or flat. Recent experiments have determined that it is in fact flat, or nearly so.
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. The two are otherwise contradictory in some ways.
Loop quantum gravity, which was pioneered at Ashtekar’s institute, proposes that space-time 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 similar ideas, 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 proposed a Bounce of sorts, also, 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 space, called a “braneworld”—the word comes from the fact that it’s considered 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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