scenario would slightly modify the long-established law of gravity. Such a minute difference could be picked up, however, only at distances comparable to or smaller than the bulk thickness—that is, at one millimeter or less. Since at the time they made the prediction, gravitational measuring instruments had not yet probed such tiny ranges, the researchers felt free to make such an assertion. Within a few years, however, Eric Adelberger of the Eöt-Wash group used ultrasensitive torsion balance experiments to rule out such deviations down to scales much smaller than a millimeter. These investigations placed sharp restrictions on the model, and theorists await the results of further testing.

One variation of the large extra dimension scenario, called the “manyfold universe model,” offers an intriguing possible explanation for dark matter. Developed by the ADD group, along with Stanford researcher Nemanja Kaloper, it posits that the visible universe resides on a single brane, folded up like an accordion. In between each crease, slivers of bulk preclude light from passing through. Photons, after all, are open strings and must cling forever to the brane. Gravitons, on the other hand, can freely jump from one fold to the next. Thus, in this model, gravitation reaches beyond where luminous radiation can penetrate. For example, imagine that a star or galaxy is located on the next fold over from ours. Because its light rays would need to travel a long distance along the brane to reach us, it would appear extremely remote—or, perhaps, not even visible at all. Yet its gravitons could jump across the thin layer of bulk and influence a part of space much closer to us—the Milky Way’s halo, for example. They could slightly warp that region, leading to a gravitational lensing effect. The result would be the phenomena we associate with dark matter. Hence, according to the theorists, what we call dark matter could well be visible material situated on another fold of our brane.

As we’ve found with many theories, in battling one cosmological mystery, it’s tempting to try to vanquish them all. While attempting to exorcise the dark-matter demon, the ADD group tried to slay the



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