On Swift Gravitons

TL;DR

This paper investigates superluminal graviton propagation in higher curvature gravity theories using the method of characteristics in a pp-wave background, revealing conditions under which gravitons can travel faster than light.

Contribution

It applies the method of characteristics to identify superluminal graviton propagation in various higher curvature gravity theories on a pp-wave background.

Findings

Certain theories exhibit characteristic hypersurfaces outside the light-cone.

Superluminal propagation depends on graviton polarization.

Higher curvature terms can alter graviton causal structure.

Abstract

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