Applicability of standard unitarity and causality bounds to gravitational scattering

Determine whether standard unitarity and causality constraints on scattering amplitudes, such as partial-wave unitarity bounds used in non-gravitational quantum field theories, apply straightforwardly to gravitational theories, and clarify how these constraints should be formulated when asymptotic states are ill-defined in curved spacetimes.

Background

The paper discusses partial-wave amplitudes for scalar scattering mediated by gravity and notes that, in non-gravitational quantum field theories, amplitudes must satisfy properties including unitarity and causality. However, the authors caution that even defining asymptotic states in curved spacetime can be challenging or impossible depending on the asymptotic structure, making the straightforward application of standard bounds uncertain in gravitational contexts.

For the purposes of the present calculation, the authors assume standard notions like unitarity and causality hold in flat backgrounds, but they explicitly flag the broader question of whether and how these constraints apply in quantum gravity as unresolved.