Accelerating universe without event horizon

It is shown that if a small negative cosmological constant is added to quintessence models with equation of state $p=\omega\rho$ on the range $-1 <\omega < -1/3$, the resulting scenarios could not contain any future event horizons. Therefore, such cosmological accelerating scenarios could not present the kind of obstacles recently pointed out to define a set of observable quantities analogous to an S-matrix needed by predictive M and string theories in an accelerating universe driven by dark energy. These scenarios become accelerating only at early times after coincidence time, and lead finally to re-collapse.