  The presently accelerating universe may keep accelerating forever, eventually
run into the event horizon problem, and thus be in conflict with the
superstring idea. In the other way around, the current accelerating phase as
well as the fate of the universe may be swayed by a negative cosmological
constant, which dictates a big crunch. Based on the current observational data,
in this paper we investigate how large the magnitude of a negative cosmological
constant is allowed to be. In addition, for distinguishing the sign of the
cosmological constant via observations, we point out that a measure of the
evolution of the dark energy equation of state may be a good discriminator.
Hopefully future observations will provide much more detailed information about
dark energy and thereby indicates the sign of the cosmological constant as well
as the fate of the presently accelerating universe.
