Xing-Hui Feng

Black hole entropy and viscosity bound in Horndeski gravity

A bstractHorndeski gravities are theories of gravity coupled to a scalar field, in which the action contains an additional non-minimal quadratic coupling of the scalar, through its first derivative, to the Einstein tensor or the analogous higher-derivative tensors coming from the variation of Gauss-Bonnet or Lovelock terms. In this paper we study the thermodynamics of the static black hole solutions in n dimensions, in the simplest case of a Horndeski coupling to the Einstein tensor. We apply the Wald formalism to calculate the entropy of the black holes, and show that there is an additional contribution over and above those that come from the standard Wald entropy formula. The extra contribution can be attributed to unusual features in the behaviour of the scalar field. We also show that a conventional regularisation to calculate the Euclidean action leads to an expression for the entropy that disagrees with the Wald results. This seems likely to be due to ambiguities in the subtraction procedure. We also calculate the viscosity in the dual CFT, and show that the viscosity/entropy ratio can violate the η/S ≥ 1/(4π) bound for appropriate choices of the parameters.

Scalar hairy black holes in general dimensions

We obtain a class of asymptotic flat or (anti)--de Sitter [(A)dS] hairy black holes in

Thermodynamics of Charged Black Holes in Einstein-Horndeski-Maxwell Theory

D

Butterfly velocity bound and reverse isoperimetric inequality

-dimensional Einstein gravity coupled to a scalar with a certain scalar potential. For a given mass, the theory admits both the Schwarzschild-Tangherlini and the hairy black holes with different temperature and entropy, but satisfying the same first law of thermodynamics. For some appropriate choice of parameters, the scalar potential can be expressed in terms of a superpotential, and it can arise in gauged supergravities. In this case, the solutions develop a naked curvature singularity and become the spherical domain walls. Uplifting the solutions to

Holographic complexity and two identities of action growth

D=11

Non-Abelian (Hyperscaling Violating) Lifshitz Black Holes in General Dimensions

or 10, we obtain solutions that can be viewed as spherical M-branes or D3-branes. We also add electric charges to these hairy black holes. All these solutions contain no scalar charges in that the first law of thermodynamics is unmodified. We also try to construct new AdS black holes carrying scalar charges, with some moderate success in that the charges are pre-fixed in the theory instead of being some continuous integration constants.

Bounce Universe and Black Holes from Critical Einsteinian Cubic Gravity

We extend an earlier investigation of the thermodynamics of static black holes in an Einstein-Horndeski theory of gravity coupled to a scalar field, by including now an elec- tromagnetic field as well. By studying the two-parameter families of charged static black holes, we obtain much more powerful constraints on the thermodynamics since, unlike in the uncharged one-parameter case, now the right-hand side of the first law is not automatically integrable. In fact, this allows us to demonstrate that there must be an additional contribution in the first law, over and above the usual terms expected for charged black holes. The origin of the extra contribution can be attributed to the behaviour of the scalar field on the horizon of the black hole. We carry out the calculations in four dimensions and also in general dimensions. We also derive the ratio of viscosity to entropy for the dual boundary field theory, showing that the usual viscosity bound for isotropic solutions can be violated, with the ratio depending on the mass and charge.

Horndeski gravity and the violation of reverse isoperimetric inequality

We study the butterfly effect of the AdS planar black holes in the framework of Einsteins general relativity. We find that the butterfly velocities can be expressed by a universal formula

Gödel universe from string theory

v_{\rm B}^2 = TS/(2V_{\rm th} P)

Higher-Derivative Gravity with Non-minimally Coupled Maxwell Field

. In doing so, we come upon a near-horizon geometrical formula for the thermodynamical volume