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A new theory of the universe suggests that space and time may not have begun in a big bang, but may have always existed in an endless cycle of expansion and rebirth.
Princeton physicist Paul Steinhardt and Neil Turok of Cambridge University described their proposed theory in an article published April 25 in an online edition of Science.
The theory proposes that, in each cycle, the universe refills with hot, dense matter and radiation, which begins a period of expansion and cooling like the one of the standard big bang picture.
After 14 billion years, the expansion of the universe accelerates, as astronomers have recently observed. After trillions of years, the matter and radiation are almost completely dissipated and the expansion stalls. An energy field that pervades the universe then creates new matter and radiation, which restarts the cycle.
The new theory provides possible answers to several longstanding problems with the big bang model, which has dominated the field of cosmology for decades. It addresses, for example, the nagging question of what might have triggered or come "before" the beginning of time.
The idea also reproduces all the successful explanations provided by standard picture, but there is no direct evidence to say which is correct, said Steinhardt, a professor of physics.
"I do not eliminate either of them at this stage," he said. "To me, what's interesting is that we now have a second possibility that is poles apart from the standard picture in many respects, and we may have the capability to distinguish them experimentally during the coming years."
The big bang model of the universe, originally suggested over 60 years ago, has been developed to explain a wide range of observations about the cosmos. A major element of the current model, added in the 1980s, is the theory of "inflation," a period of hyperfast expansion that occurred within the first second after the big bang.
This inflationary period is critical for explaining the tremendous "smoothness" and homogeneity of the universe observed by astronomers, as well as for explaining tiny ripples in space that led to the formation galaxies.
Scientists also have been forced to augment the standard theory with a component called "dark energy" to account for the recent discovery that the expansion of the universe is accelerating.
The new model replaces inflation and dark energy with a single energy field that oscillates in such a way as to sometimes cause expansion and sometimes cause stagnation. At the same time, it continues to explain all the currently observed phenomena of the cosmos in the same detail as the big bang theory.
Because the new theory requires fewer components, and builds them in from the start, it is more "economical," said Steinhardt, who was one of the leaders in establishing the theory of inflation.
Another advantage of the new theory is that it automatically includes a prediction of the future course of the universe, because it goes through definite repeating cycles lasting perhaps trillions of years each.
The big bang/inflation model has no built-in prediction about the long-term future; in the same way that inflation and dark energy arose unpredictably, another effect could emerge that would alter the current course of expansion.
The cyclic model entails many new concepts that Turok and Steinhardt developed over the last few years with Justin Khoury, a graduate student at Princeton, Burt Ovrut of the University of Pennsylvania and Nathan Seiberg of the Institute for Advanced Study.
"This work by Paul Steinhardt and Neil Turok is extraordinarily exciting and represents the first new big idea in cosmology in over two decades," said Jeremiah Ostriker, professor of astrophysics at Princeton and the Plumian Professor of Astronomy and Experimental Philosophy at Cambridge.
"They have found a simple explanation for the observed fact the universe on large scales looks the same to us left and right, up and down -- a seemingly obvious and natural condition -- that in fact has defied explanation for decades."
Sir Martin Rees, Royal Society Research Fellow at Cambridge, noted that the physics concerning key properties of the expanding universe remain "conjectural, and still not rooted in experiment or observation."
"There have been many ideas over the last 20 years," said Rees.
"Steinhardt and Turok have injected an imaginative new speculation.
Their work emphasizes the extent to which we may need to jettison common sense concepts, and transcend normal ideas of space and time, in order to make real progress.
"This work adds to the growing body of speculative research which intimates that physical reality could encompass far more than just the aftermath of 'our' big bang."
The cyclic universe theory represents a combination of standard physical concepts and ideas from the emerging fields of string theory and M-theory, which are ambitious efforts to develop a unified theory of all physical forces and particles. Although these theories are rooted in complex mathematics, they offer a compelling graphic picture of the cyclic universe theory.
Under these theories, the universe would exist as two infinitely large parallel sheets, like two sheets of paper separated by a microscopic distance. This distance is a extra, or fifth dimension, that is not apparent us.
At our current phase in the history of the universe, the sheets are expanding in all directions, gradually spreading out and dispersing all the matter and energy they contain. After trillions of years, when they become essentially empty, they enter a "stagnant" period in which they stop stretching and, instead, begin to move toward each other as the fifth dimension undergoes a collapse.
The sheets meet and "bounce" off each other. The impact causes the sheets to be charged with the extraordinarily hot and dense matter that is commonly associated with the big bang. After the sheets move apart, they resume their expansion, spreading out the matter, which cools and coalesces into stars and galaxies as in our present universe.
The sheets, or branes, as physicists call them, are not parallel universes, but rather are facets of the same universe, with one containing all the ordinary matter we know and the other containing "we know not what," said Steinhardt.
It is conceivable, he said, that a material called dark matter, which is widely believed to make up a significant part of the universe, resides on this other brane. The two sheets interact only by gravity, with massive objects in one sheet exerting a tug on matter in the other, which is what dark matter does to ordinary matter.
The movements and properties of these sheets all arise naturally from the underlying mathematics of the model, noted Steinhardt. That is in contrast to the big bang model, in which dark energy has been added simply to explain current observations.
Steinhardt and Turok continue to refine the theory and are looking for theoretical or experimental ideas that might favor one idea over the other.
"These paradigms are as far apart as you can imagine in terms of the nature of time," said Steinhardt. "On the other hand, in terms of what they predict about the universe, they are as close as you can be up to what you can measure so far.
"Yet, we also know that, with more precise observations that may be possible in the next decade or so, you can distinguish them. That is the fascinating situation we find ourselves in. It's fun to debate which ones you like better, but I really think nature will be the final arbiter here."
