horizon problem as well—without applying the broad sword of inflation. The idea is that the universe, once twisted up into a tight space, has since opened up (along at least one dimension)—like origami restored to its original sheets—thereby separating regions that were originally in close contact, leading to large-scale uniformity. As the group wrote: “The folded universe picture permits apparently superluminal communication between different segments of the brane through the bulk. This could give a non-inflationary solution to the horizon problem, if the brane was originally crumpled in a small higher-dimensional box and later unfolded.”
Competing with the ADD scenario and its “manyfold” variation are other brane-world types with fundamentally different properties; chief among them are the models (mentioned earlier) of Lisa Randall and Raman Sundrum. While one of these types has the standard two branes delineated by M-theory, another possesses but a single brane with a warped infinite extra dimension. The warping refers to the bulk having a nonflat five-dimensional geometry—namely, anti-de Sitter space, which serves as a trough, focusing the gravitons in a region close to our brane. Hence, gravity is weak but not too weak.
The Randall-Sundrum model can be pictured as an endless desert with a giant rock in the middle—akin to Uluru (Ayers rock) standing tall in the Australian wilderness. Uluru’s location represents our brane; the desert stands for the bulk around it; and the rock itself, all matter and energy besides gravitons. Naturally, the rock remains fixed to the site, like conventional matter on the brane.
Now suppose that a desert spirit suddenly transforms the rock into a giant block of ice, like a glacier. This picture represents gravitons. If the desert is completely flat, the ice would quickly melt, then spread out over an extremely large area. This thin coating of water would rapidly evaporate. By analogy, gravitons exuding into a flat, infinite bulk would have absolutely no strength. But if the desert around Uhuru were slanted toward the center, it would collect the water into a substantial pond. Similarly, a warped brane would