Quintessence as a runaway dilaton

We consider a late-time cosmological model based on a recent proposal that the infinite-bare-coupling limit of superstring or M theory exists and has good phenomenological properties, including a vanishing cosmological constant, and a massless, decoupled dilaton. As it runs away to +∞, the dilaton can play the role of the quintessence field recently advocated to drive the late-time accelerated expansion of the Universe. If, as suggested by some string theory examples, appreciable deviations from general relativity persist even today in the dark matter sector, the Universe may smoothly evolve from an initial “focusing” stage, lasting until radiation-matter equality, to a “dragging” regime, which eventually gives rise to an accelerated expansion with frozen Ω(dark energy)/Ω(dark matter).