Gravity with higher-curvature terms and second-order field equations: $f(\mathcal{R})$ meets Gauss-Bonnet

TL;DR

This paper develops a new class of higher-curvature gravitational theories combining $f(\,\mathcal{R})$ and Gauss-Bonnet terms, leading to novel black hole solutions with preserved second-order field equations and insights into singularity behavior.

Contribution

It introduces a framework combining $f(\mathcal{R})$ and Gauss-Bonnet gravity, extending EdGB gravity to include arbitrary higher-curvature terms with second-order equations.

Findings

Black holes are modified by $f(\mathcal{R})$ terms.

Solutions retain features like minimum mass and multiple branches.

A mechanism suppresses Ricci scalar divergence inside black holes.

Abstract

General Relativity is expected to break down in the high-curvature regime. Beyond an effective field theory treatment with higher-order operators, it is important to identify consistent theories with higher-curvature terms at the nonperturbative level. Two well-studied examples are $f(\mathcal{R})$ gravity and Einstein-dilaton-Gauss-Bonnet (EdGB) gravity. The former shares the same vacuum solutions as General Relativity, including black holes, while the latter suffers from well-posedness issues due to quadratic curvature terms in the strong-coupling regime. We show that combining these two theories leads to genuinely new phenomena beyond their simple superposition. The resulting framework falls outside Horndeski's class, as it can be recast as a gravitational theory involving two nonminimally coupled scalar fields with nontrivial mutual interactions. This construction naturally extends EdGB gravity to include arbitrary higher-curvature terms, providing a versatile setting to address fundamental questions. Focusing on quadratic and quartic corrections, we find that: (i) black holes are modified by $f(\mathcal{R})$ terms, unlike the case without Gauss-Bonnet interactions; (ii) the resulting solutions retain the qualitative nonperturbative features of EdGB black holes with certain couplings, such as a minimum mass and multiple branches; (iii) a nontrivial mechanism suppresses the divergence of the Ricci scalar in the black-hole interior; (iv) still, even with quartic corrections, the singularity structure and elliptic regions inside the horizon remain similar to those of pure EdGB gravity. This suggests that, at the nonperturbative level, the theory's ill-posedness cannot be resolved by adding individual higher-order terms. This conjecture could be tested by studying the nonlinear dynamics, which remains governed by second-order field equations.