Phantom dark energy with tachyonic instability: Metric perturbations

We study the evolution of metric perturbations in a model of UV-stable phantom dark energy. The model has tachyon instabilities at long wavelengths. We find that like the tachyon modes themselves, metric perturbations grow exponentially in time, starting from very small values determined by vacuum fluctuations, and may become sizeable at late times. We derive a constraint on the model parameters that follows from requiring that the metric perturbation amplitudes not be exceedingly large today.