Causality in gravitational theories with second order equations of motion

Understand

This paper considers diffeomorphism invariant theories of gravity coupled to matter, with second order equations of motion.

• This includes Einstein-Maxwell and Einstein-scalar field theory with (after field redefinitions) the most general parity-symmetric four-derivative effective field theory corrections.
• A gauge-invariant approach is used to study the characteristics associated to the physical degrees of freedom in an arbitrary background solution.
• The symmetries of the principal symbol arising from diffeomorphism invariance and the action principle are determined.