If Penn Physics Professor Burt Ovrut is correct, our universe may be the aftermath of a cosmic hit-and-run. According to Ovrut's new theory, the universe was formed after the collision of two membranes, or "branes" in scientific jargon. It is being hailed as the biggest new cosmological idea since the "Big Bang" theory was introduced 20 years ago. "Our world lives as a big membrane in this five-dimensional space, and this is not so far-fetched," Ovrut said. "What happens is really pretty amazing.... We asked ourselves what the physics of that would be." The new scenario, dubbed the "Ekpyrotic Universe" by co-creator Paul Steinhardt of Princeton University, takes its name from the Greek word for "conflagration." Steinhardt worked on the theory with Ovrut, Neil Turok of Cambridge University and Justin Khoury, a graduate student at Princeton. The Big Bang theory could not adequately explain the uniformity of the universe's large-scale structure, as there is no way for any exchange of matter or energy over such large distances. To address this discrepancy, in the early 1980s, Massachusetts Institute of Technology physicist Alan Guth theorized a period of "inflation," or very rapid expansion of the early universe. Ovrut said that he found the inflation theory "a little unsettling." "It always struck me as using a sledgehammer to crack a peanut," he said. "We're able to explain things [with the Ekpyrotic model] that were mysteries for many years." "Basically, what we wanted to achieve was to find some alternative to inflation," Khoury said. "The idea of branes had been around for quite a while. It has gained interest in the nineties from string theory." Other cosmologists have generally received the new theory with cautious enthusiasm, as it is promising but has not yet been tested. "It's certainly not completely crazy," said Edward Wright, an astrophysicist from the University of California at Los Angeles. "It seems to fit in with ideas of string theory. It's certainly a possibility, but I can't really say that it's more than that." "Basically, the predictions aren't definite enough to really test very easily, though one prediction they have made should be testable in a few years," Wright said. "It'd be interesting to see how it comes out, but currently it's within the realm of possibility but not observationally confirmed." The predictions to which Wright referred concern the cosmic microwave background, the nearly uniform background radiation widely thought to be the residual glow of the Big Bang. Most inflationary theories predict a red-shifted CMB, that is, lower frequencies have higher amplitudes. The Ekpyrotic theory predicts a blue-shifted CMB. Current measurements of this radiation are not yet good enough to determine whether the CMB is red- or blue-shifted. Khoury made the point that since some inflationary theories predict a blue-shifted CMB, gravitational waves would be more useful in validating the Ekpyrotic theory. "There is one particular prediction that is different in either case, and that has to do with the spectrum of gravitational waves," Khoury said. "Inflation predicts that it is possible to observe some spectrum of gravitational waves, due to inflation, whereas our model predicts that this spectrum is basically unobservable. It is there, but it is very, very faint."
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