  Observational evidence suggests that our universe is presently dominated by a
dark energy component and undergoing accelerated expansion. We recently
introduced a model, motivated by string theory for short-distance physics, for
explaining dark energy without appealing to any fine-tuning. The idea of the
transplanckian dark energy (TDE) was based on the freeze-out mechanism of the
ultralow frequency modes, $\omega(k)$ of very short distances, by the expansion
of the background universe, $\omega(k) \leq H$. In this paper we address the
issue of the stress-energy tensor for the nonlinear short-distance physics and
explain the need to modify Einstein equations in this regime. From the modified
Einstein equations we then derive the equation of state for the TDE model,
which has the distinctive feature of being continually time-dependent. The
explanation of the coincidence puzzle relies entirely on the intrinsic
time-evolution of the TDE equation of state.
