  The mystery of the cosmological constant is probably the most pressing
obstacle to significantly improving the models of elementary particle physics
derived from string theory. The problem arises because in the standard
framework of low energy physics, there appears to be no natural explanation for
vanishing or extreme smallness of the vacuum energy, while on the other hand it
is very difficult to modify this framework in a sensible way. In seeking to
resolve this problem, one naturally wonders if the real world can somehow be
interpreted in terms of a vacuum state with unbroken supersymmetry.
