Holographic studies of quasi-topological gravity

TL;DR

This work introduces and analyzes quasi-topological gravity as a tractable holographic model for non-supersymmetric CFTs with curvature-cubed interactions. By constructing the AdS/CFT dictionary for central charges, energy-flux parameters, and three-point functions, it derives precise constraints on bulk couplings from positivity, energy flux, and causality considerations. It computes the holographic shear viscosity to entropy density ratio η/s, revealing a nonzero lower bound ~0.4140/(4π) within the allowed coupling region, and identifies regimes of plasma instabilities that limit the reliability of hydrodynamic results. The study highlights both the utility and the limitations of higher-derivative gravity as a toy model for strongly coupled conformal plasmas, suggesting further exploration in broader CFT parameter spaces and higher dimensions.

Abstract

Quasi-topological gravity is a new gravitational theory including curvature-cubed interactions and for which exact black hole solutions were constructed. In a holographic framework, classical quasi-topological gravity can be thought to be dual to the large $N_c$ limit of some non-supersymmetric but conformal gauge theory. We establish various elements of the AdS/CFT dictionary for this duality. This allows us to infer physical constraints on the couplings in the gravitational theory. Further we use holography to investigate hydrodynamic aspects of the dual gauge theory. In particular, we find that the minimum value of the shear-viscosity-to-entropy-density ratio for this model is $η/s \simeq 0.4140/(4π)$.