  It has been suggested that a scalar field with negative kinetic energy, or
``ghost,'' could be the source of the observed late-time cosmological
acceleration. Naively, such theories should be ruled out by the catastrophic
quantum instability of the vacuum. We derive phenomenological bounds on the
Lorentz-violating ultraviolet cutoff Lambda which must apply to low-energy
effective theories of ghosts, in order to keep the instability at unobservable
levels. Assuming only that ghosts interact at least gravitationally, we show
that Lambda < 3 MeV for consistency with the cosmic gamma ray background. We
also show that theories of ghosts with a Lorentz-conserving cutoff are
completely excluded.
