Restoring Causality in Higher Curvature Gravity

TL;DR

This paper investigates whether a specific higher curvature gravity theory, GQG, can satisfy causality constraints by analyzing shock wave solutions and Shapiro time delays, showing it can remain causal within certain parameters.

Contribution

It demonstrates that GQG, a higher curvature gravity theory, can satisfy the CEMZ causality criterion by constructing exact shock wave solutions and analyzing time delays.

Findings

GQG can have positive Shapiro time delays within certain parameters.

GQG's graviton three-point coupling differs from GR.

The theory remains causal under the CEMZ criterion.

Abstract

Incorporating higher curvature terms into gravity theories modifies the classical field equations, potentially leading to theoretical issues like Shapiro time advancements that violate the Camanho, Edelstein, Maldacena, and Zhiboedov (CEMZ) causality criterion. We explore this criterion within the context of Generalised Quadratic Gravity (GQG), a higher curvature theory with a distinct graviton three-point coupling from General Relativity (GR). By constructing an exact shock wave solution of GQG and calculating the Shapiro time shift for a massless probe graviton, we demonstrate that it can remain strictly positive within a classically allowed parameter space of couplings, ensuring the theory's adherence to the CEMZ criterion. This finding indicates that GQG can offer a causal extension beyond GR, paving the way for further exploration into the consistency of classical higher curvature gravity theories.