Various facets of phases in gravitational wave physics

This thesis contains two main results related to low-frequency gravitational waves. The first is a prescription for causality within low energy effective field theories (EFTs), specialised to gravitational theories. An EFT is a framework for parametrising macroscopic physics while remaining agnostic about the microscopic degrees of freedom. It turns out that the most generic naïvely local and covariant EFT action is not necessarily consistent with a physical UV theory, and thus imposing the causal propagation of waves can place non-trivial constraints on the EFT. Our criteria for “infrared causality” is that scattered waves do not experience a resolvable time advance relative to the geometry of the background. We apply this condition to the Gauss–Bonnet operator on black hole and pp-wave spacetimes and show that, within the EFT’s regime of validity, causality is respected. The second result relates to gravitational wave backgrounds (GWBs) within the standard cosmological model. We show that scalar perturbations to the background metric ruin any phase coherence in the GWB which may have been present at emission. The main consequence is that phase-coherent mapping methods have no foreseeable application to GWBs.Open Acces

Surfin' pp-waves with Good Vibrations: Causality in the presence of stacked shockwaves

Relativistic causality constrains the $S$-matrix both through its analyticity, and by imposing lower bounds on the scattering time delay. These bounds are easiest to determine for spacetimes which admit either a timelike or null Killing vector. We revisit a class of pp-wave spacetimes and carefully determine the scattering time delay for arbitrary incoming states in the eikonal, semi-classical, and Born approximations. We apply this to the EFT of gravity in arbitrary dimensions. It is well-known that higher-dimension operators such as the Gauss-Bonnet term, when treated perturbatively at low energies, can appear to make both positive and negative contributions to the time delays of the background geometry. We show that even when multiple shockwaves are stacked, the corrections to the scattering time delay relative to the background are generically unresolvable within the regime of validity of the effective field theory so long as the Wilson coefficients are of order unity. This is in agreement with previously derived positivity/bootstrap bounds and the requirement that infrared causality be maintained in consistent low-energy effective theories, irrespective of the UV completion.Comment: 68 pages, 3 figure