Abstract
Some exact, nonlinear, vacuum gravitational wave solutions are derived for certain polynomial gravities. We show that the boundaries of the gravitational domain of dependence, associated with events in polynomial gravity, are not null as they are in general relativity. The implication is that electromagnetic and gravitational causality separate into distinct notions in modified gravity, which may have observable astrophysical consequences. The linear theory predicts that tachyonic instabilities occur, when the quadratic coefficient of the Taylor expansion of is negative, while the exact, nonlinear, cylindrical wave solutions presented here can be superluminal for all values of . Anisotropic solutions are found, whose wave fronts trace out time- or spacelike hypersurfaces with complicated geometric properties. We show that the solutions exist in theories that are consistent with Solar System and pulsar timing experiments.
- Received 16 March 2017
DOI:https://doi.org/10.1103/PhysRevD.96.064032
© 2017 American Physical Society