Can the Universe escape eternal acceleration ?

Recent astronomical observations of distant supernovae lightcurves, [1]-[4], suggest that the expansion of the universe has recently begun to accelerate. Acceleration is created by an antigravitational repulsive stress, like that produced by a positive cosmological constant, or universal vacuum energy. It creates a rather bleak eschatological picture. An ever-expanding universe’s future appears to be increasingly dominated by its constant vacuum energy. A universe doomed to accelerate forever will produce a state of growing uniformity and cosmic loneliness. Structures participating in the cosmological expansion will ultimately leave each others’ horizons and information-processing must eventually die out[5]. Here, we examine whether this picture is the only interpretation of the observations. We find that in many well-motivated scenarios the observed spell of vacuum domination is only a transient phenomenon. Soon after acceleration starts, the vacuum energy’s anti-gravitational properties are reversed, and a matter-dominated decelerating cosmic expansion resumes. Thus, contrary to general expectations, once an accelerating universe does not mean always an accelerating universe. The observed cosmic acceleration, if due to vacuum repulsive stresses, is very puzzling. In contrast to conventional forms of matter, like pressureless dust or radiation, the cosmological energy densities contributed by these