$tring Theory and an Accelerating Universe

Abstract

An accelerating Universe can be accommodated naturally within non-critical string theory, in which scattering is described by a superscattering matrix $ that does not factorize as a product of $S$- and $S^{†}$-matrix elements and time evolution is described by a modified Liouville equation characteristic of open quantum-mechanical systems. We describe briefly alternative representations in terms of the stochastic Ito and Fokker-Planck equations. The link between the vacuum energy and the departure from criticality is stressed. We give an explicit example in which non-marginal $tring couplings cause a departure from criticality, and the corresponding cosmological vacuum energy relaxes to zero à la quintessence.

11 pages LATEX, one eps figure incorporated