Conformal Gravity Redux: Ghost-turned-Tachyon
  We analyze conformal gravity in translationally invariant approximation, where the metric is taken to depend on time but not on spatial coordinates. We find that the field mode which in perturbation theory has a ghostlike kinetic term, turns into a tachyon when nonlinear interaction is accounted for. The kinetic term and potential for this mode have opposite signs. Solutions of nonlinear classical equations of motion develop a singularity in finite time determined by the initial conditions. 
Ibrahim B. Ilhan and Alex Kovner
hep-th gr-qc hep-ph