Scientists at Duke and at Rutgers University have developed a mathematical framework they say will enable astronomers to test a new five-dimensional theory of gravity that competes with Einstein's General Theory of Relativity. Arlie O. Petters, professor of mathematics and physics at Duke, and Charles R. Keeton of Rutgers base their work on a recent theory called the type II Randall-Sundrum braneworld gravity model. The theory holds that the visible universe is a membrane (hence "braneworld") embedded within a larger universe, much like a strand of filmy seaweed floating in the ocean. The "braneworld universe" has five dimensions--four spatial dimensions, plus time--compared with the four dimensions--three spatial, plus time--laid out in the General Theory of Relativity. The scientists' findings appeared in the online edition of the journal Physical Review D. The framework Keeton and Petters developed predicts certain cosmological effects that, if observed, should help scientists validate the braneworld theory. The observations, they say, should be possible with satellites scheduled to launch in the next few years. If the braneworld theory proves to be true, it "would upset the applecart," Petters says. "It would confirm that there is a fourth dimension to space, which would create a philosophical shift in our understanding of the natural world." Keeton and Petters focused on one particular gravitational consequence of the braneworld theory that distinguishes it from Einstein's theory. The braneworld theory predicts that relatively small black holes created in the early universe have survived to the present. The black holes, with mass similar to a tiny asteroid, would be part of the "dark matter" in the universe. As the name suggests, dark matter does not emit or reflect light, but does exert a gravitational force. The General Theory of Relativity, on the other hand, predicts that such primordial black holes no longer exist, as they would have evaporated by now. "When we estimated how far braneworld black holes might be from Earth, we were surprised to find that the nearest ones would lie well inside Pluto's orbit," Keeton says. "If braneworld black holes form even 1 percent of the dark matter in our part of the galaxy," says Petters, "there should be several thousand braneworld black holes in our solar system." |

# Move Over, Einstein

August 1, 2006