Shadows, Signals, and Stability in Einsteinian Cubic Gravity

TL;DR

This paper explores the phenomenology of Einsteinian cubic gravity, deriving an approximate black hole solution, analyzing test particle orbits, and comparing black hole shadows to general relativity, revealing small but potentially observable deviations.

Contribution

It provides the first analytic approximation for black hole solutions in Einsteinian cubic gravity and studies their observational signatures.

Findings

The approximate black hole solution is valid outside the horizon.

ECG black holes have larger shadows than GR counterparts.

Departures from GR are small but potentially distinguishable in observations.

Abstract

We conduct a preliminary investigation into the phenomenological implications of Einsteinian cubic gravity (ECG), a 4-dimensional theory of gravity cubic in curvature of interest for its unique formulation and properties. We find an analytic approximation for a spherically symmetric black hole solution to this theory using a continued fraction ansatz. This approximate solution is valid everywhere outside of the horizon and we use it to study the orbit of massive test bodies near a black hole, specifically computing the innermost stable circular orbit. We compute constraints on the ECG coupling parameter imposed by Shapiro time delay. We then compute the shadow of an ECG black hole and find it to be larger than its Einsteinian counterpart in general relativity for the same value of the mass. Applying our results to Sgr A*, we find that departures from general relativity are small but in principle distinguishable.