  We show that a massless canonical scalar field minimally coupled to general
relativity can become a tachyonic ghost at low energies around a background in
which the scalar's gradient is spacelike. By performing a canonical
transformation we demonstrate that this low energy ghost can be recast, at the
level of the action, in a form of a fluid that undergoes a Jeans-like
instability affecting only modes with large wavelength. This illustrates that
low-energy tachyonic ghosts do not lead to a catastrophic quantum vacuum
instability, unlike the usual high-energy ghost degrees of freedom.
