  We demonstrate that if k-essence can solve the coincidence problem and play
the role of dark energy in the universe, the fluctuations of the field have to
propagate superluminally at some stage. We argue that this implies that
successful k-essence models violate causality. It is not possible to define a
time ordered succession of events in a Lorentz invariant way. Therefore,
k-essence cannot arise as low energy effective field theory of a causal,
consistent high energy theory.
