Superluminal graviton propagation

We use the method of characteristics to study superluminal graviton (which we call the swift graviton) propagation in theories of higher-curvature gravity of the form (Riemann)2, (Riemann)3, 2(Riemann)2 and (Riemann)4. We consider a pp-wave background. When probed by gravitons with an appropriate polarization, several of the gravitational theories under consideration exhibit characteristic hypersurfaces outside the flat spacetime light cone.