  There are two important examples of physical systems which violate the strong
energy condition : Universes (like, it would seem, our own) with a positive
cosmological constant, and wormholes. We suggest that a positive cosmological
constant can be reconciled with string theory by considering wormholes in
string backgrounds. This is argued in two directions : first, we show that
brane-worlds with positive cosmological constants give rise to bulk
singularities which are best resolved by embedding the brane-world in an
AdS/CFT wormhole; and second, for the simplest kind of wormhole in an
asymptotically AdS space, we show that the IR stability of the matter needed to
keep the wormhole open requires the presence of a brane-world. UV stability
conditions then forbid a negative cosmological constant on the brane-world.
