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Bouncing back.
This computer representation shows the contraction of a previous universe (downward curve) resulting in the creation and expansion of the current universe.

Martin Bojowald/Pennsylvania State University

Time Before Time

A cosmologist has created a mathematical model that he says shows space-time, contrary to common wisdom, did not begin with the Big Bang. Instead, the model suggests a universe pretty much like the one we live in today existed before the event, except it was contracting instead of expanding. If ever proven, the idea could force a complete rethinking of the origins of the cosmos and perhaps even open a doorway to an endless future.

The Big Bang--the sudden and extremely rapid expansion of space-time that began 13.7 billion years ago--is generally accepted among scientists as the beginning of the universe. However, they have long puzzled over a paradox that the event caused in the mathematical calculations of Einstein's Theory of General Relativity. At the moment of the Big Bang, everything was thought to be crammed into a singularity--a space with no dimensions--that also contained infinite density. Einstein couldn't explain how such a state could give rise to a universe of finite density and possibly finite dimensions. Theoretical physicist Sean Carroll of the California Institute of Technology in Pasadena put it more succinctly: "Everyone's calculations show the universe started from a singularity," he says, "but no one believes it."

Most cosmologists have come to think that quantum mechanics--something unknown during Einstein's time--could hold the key to this conundrum. According to quantum mechanics, random activity on an extremely tiny scale can affect the outcome of events vast distances away and involving gigantic masses. For example, adherents of this theory believe that the current universe turned out so lumpy--with clusters of galaxies in some areas and nearly empty space in others--because of quantum fluctuations at the moment of the Big Bang.

But so far quantum mechanics has not been able to explain where the universe came from in the first place. Although most cosmologists think it sprang forth from nothingness along with the forces of nature, theoretical physicist Martin Bojowald of Pennsylvania State University in University Park thinks his mathematical model points to something even stranger: The cosmos is a leftover from a previous manifestation of existence.

Bojowald's model is based on a new and developing line of theoretical reasoning called Loop Quantum Gravity, which attempts to reconcile Einstein's theory with quantum mechanics. As Bojowald reported online in the 1 July Nature Physics, the model shows that at the moment of the Big Bang, the current universe had a minimum volume that was not zero and carried a huge but finite amount of energy. Furthermore, the calculations strongly suggest that the current universe actually received a kick–start from the dying epoch of a previous, contracting universe. They do so by showing that the previous universe could not have compacted itself into a singularity, as general relativity predicts, because at extremes of temperature and pressure, gravity becomes repulsive instead of attractive. As a result, gravitational energy grew so large as the previous universe contracted that it created a "Big Bounce," as Bojowald calls it, which ignited the current expansion. With some luck, he says, it might be possible to find clues about what the previous universe was like from more detailed observations and models of quantum mechanics.

Far-fetched or not, Carroll thinks Bojowald's model represents a "good thing to be doing," because somehow cosmologists are going to have to resolve the singularity dilemma, and although his conclusions might not provide the correct answer, it's "not premature to be asking the questions."

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