String theory and quintessence

We discuss the obstacles for defining a set of observable quantities analogous to an S-matrix which are needed to formulate string theory in an accelerating universe. We show that the quintessence models with the equations of state -1 < w < -1/3 have future horizons and may be no better suited to an S-matrix or S-vector description. We also show that in a class of theories with a stable supersymmetric vacuum, a system cannot relax into a zero-energy supersymmetric vacuum while accelerating if the evolution is dominated by a single scalar field with a stable potential. Thus describing an eternally accelerating universe may be a challenge for string theory as presently defined.