The Classical Stability Of The Ghost Condensate
  We discuss classical stability of the cosmological mechanism which is responsible for the ghost condensation. We show that the simplest general covariant effective action which includes two covariant derivatives may lead to an overshoot of the condensation point into the regime, where the model becomes classically unstable. The model exhibits this behaviour for the de Sitter and matter dominated universes when the initial values taken to be in the region of validity of the low-energy effective theory. In the matter dominated case there is a finite time (which can be large enough to exceed the present age of the Universe) during which the model remains in the stable region. In inflationary regime the system does not experience an overshoot only if the Hubble parameter is larger than the characteristic energy scale of the effective field theory (EFT). This latter regime is, however, inconsistent with the EFT description at the condensation point. We discuss the limitations of the use of the truncated action and point out how one can avoid problems due to the overshoot in order to have a viable ghost condensation mechanism. 
Alexey Anisimov, Alexander Vikman