Brane World Solution to the Cosmological Constant Problem

We consider a model with two parallel (positive tension) 3-branes separated by a distance $L$ in five-dimensional spacetime. If the interbrane space is anti–de Sitter, or is not precisely anti–de Sitter but contains no event horizons, the effective four-dimensional cosmological constant seen by observers on one of the branes (chosen to be the visible brane) becomes exponentially small as $L$ grows large.