Universe Reborn Endlessly in New Model of the Cosmos
for National Geographic News
April 25, 2002
It could be a time-honored philosophy of Eastern gurus—the view that time has neither a beginning nor an end, and that the universe is locked in a perpetual cycle of formation and dissipation. But it's the latest scientific model of the cosmos, and it comes from top theorists in Princeton, New Jersey, and Cambridge, England.
This narrow, deep view of the universe reveals a plethora of galaxies as seen in visible and infrared light by NASA's Hubble Space Telescope. Photograph by R. Williams (Space Telescope Science Institute), the HDF-South team, and NASA
This new, cyclic model of the universe offers an appealing alternative to the prevailing theory, according to Paul J. Steinhardt, a theoretical physicist at Princeton University. "It predicts all the features of the standard model, using fewer ingredients," he said.
Steinhardt and his colleague Neil Turok of Cambridge University proposed the new model in a report posted April 25th on the Science Express Web site of the journal Science.
In the most widely accepted cosmological model, called the inflationary model, the universe was born in an instantaneous creation of matter and energy known as the Big Bang. As the universe has inflated since that event, matter and energy have spread out in clumps. The spreading could potentially continue forever.
"The inflation idea has been tremendously influential," noted Robert P. Kirshner, an astrophysicist at Harvard University. "No observation's been found that proves it wrong." But, he added, "that does not, of course, mean that it's right."
Nevertheless, the inflationary theory has survived since it was introduced in the late 1970s, while cosmologists have discarded competing ideas one by one.
New Theoretical Competition
Steinhardt was one of the theorists responsible for devising the inflationary model more than 20 years ago. Yet he shrugs off suggestions that he's trying to corner the cosmological market. "Having more than one theory is very important for motivating new experiments," he said.
Although he's excited by the possible implications of the new model, Steinhardt declined to bet on whether it or the conventional model is more representative of the nature of the universe.
"The conventional model has come out spectacularly well," he said, adding that he has nevertheless long wondered whether a different model might explain the universe equally well—or perhaps better. "That's what started us on this adventure," he said.
Steinhardt said several features of the cosmos can be better explained by the cyclic model, including the geometry of the universe, its overall uniformity, and, in particular, the existence of a phenomenon known as acceleration.
Recently gathered data from exploding, dying stars called supernovae have revealed that the universe is not only expanding, as predicted, but that its rate of expansion is accelerating. The only force that could explain such cosmic acceleration is a source of energy, not visible or yet identified by scientists, that permeates the entire universe. Physicists have dubbed the mysterious force "dark energy."
The discovery several years ago of acceleration and the underlying dark energy came as a surprise to scientists because the standard model did not predict such features.
The new model offers a streamlined alternative. It treats the Big Bang not as the true moment of creation, but as a transition between two cycles in an endless process of cosmological rebirth.
According to the model, the Big Bang is followed by a period of slow expansion and gradual accumulation of dark energy. As dark energy becomes dominant, it stimulates cosmic acceleration. The current era is near the transition between these stages, Steinhardt said.
As accelerated expansion proceeds over trillions of years, matter and energy are gradually stretched thin across the universe.
Eventually, matter, radiation, and even black holes become so stretched out that they are dissipated to almost nothing, leaving behind a massive universe that is virtually empty, Steinhardt explained.
At this point in the cycle, particles of matter are so far apart—and moving away from each other so rapidly—that they cannot interact and are effectively separated into distinct universes.
Steinhardt and Turok call this vacuum-like stage the "big crunch." The vacuum triggers dark energy to materialize into matter and radiation in another Big Bang, refreshing the cycle of expansion.
Other scientists are intrigued by the new model, but it hasn't won them over yet.
Kirshner credits Steinhardt and Turok with assembling the new model to be consistent with what is known about the universe. "They've been careful to account for the known facts," he said.
The new model, Kirshner said, "is highly speculative, but it's not unthinkable."
Rigorously testing the two theories against each other will take some time. Steinhardt already has some ideas about how it could be done.
For example, gravitational waves, a feature of the universe predicted by general relativity, would take a different form in these two models. There would not be long-wavelength gravitational waves in a cyclic universe, whereas there would be in an inflationary universe.
Efforts are underway to measure and characterize gravitational waves, but it will likely take at least several years to gather useful data. The Planck satellite scheduled to be launched by the European Space Agency about 2008 may help settle the question, Steinhardt said.