Using A Brane to Probe the Bulk

Mercury, March/April 2006 Table of Contents

by Chad A. Middleton

Imagine by crude illustration that our universe is a slice of bread, one particular slice from a larger loaf. This odd imagining is not too far removed from how some cosmologists have begun to picture the Universe.

Now, in the parlance of cosmology, let us imagine our universe, the "slice," is a four-dimensional hypersurface-referred to as a 3-brane, "brane" being short for membrane-that resides in a higher dimensional "bulk" space, the "loaf." Further, consider that all forces and all matter (you, me, electrons, your iPod, etc.) are embedded in this 3-brane. Except for gravity, that is, which may be unique among the four fundamental forces in that it may roam freely both on the brane hypersurface and in the bulk space.

This general picture describes several braneworld models that physicists are studying in an attempt to understand the Universe, its origins, and its characteristics. For example, back in 1998 astronomers discovered that the rate of expansion of the Universe is increasing; since then a number of scientists have considered what might drive this acceleration—possibly an unseen though ever-present dark energy? One particular braneworld model, referred to in the scientific literature as the DGP model, stands alone in that it appears to offer an alternative explanation to dark energy as the cause of the universal acceleration. Sounds more like science fiction than science, right?

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