Transient Accelerating Scalar Models with Exponential Potential
  We study a known class of scalar dark energy models in which the potential is
of exponential type and the current accelerating era is transient. We find
that, although a decelerating era will return in future, however, when
extrapolating the model back to the earlier stages ($z \gtrsim 4$), the scalar
dark energy becomes dominant over the matter. So these models do not have the
desired tracking behavior, and the predicted transient period of acceleration
can not be adopted into the standard scenario of Big Bang cosmology. When
couplings between the scalar field and the matter are introduced, the models
still has the problem, only the time of deceleration return will be varied. To
achieve re-deceleration, one has to turn to alternative models that are
consistent with the standard Big Bang scenario.
