  We consider plane-symmetric spacetimes satisfying Einstein's field equations
with positive cosmological constant, when the matter is a fluid whose pressure
is equal to its mass-energy density (i.e. a so-called stiff fluid). We study
the initial-value problem for the associated Einstein equations and establish a
global existence result. The late-time asymptotics of solutions is also
rigorously derived, and we conclude that the spacetime approaches the de Sitter
spacetime while the matter disperses asymptotically. A technical difficulty
dealt with here lies in the fact that solutions may contain vacuum states as
well as velocities approaching the speed of light, both possibilities leading
to singular behavior in the evolution equations.
