Einstein-Gauss-Bonnet Black Strings at Large DD

We study the black string solutions in the Einstein-Gauss-Bonnet(EGB) theory at large $D$. By using the $1/D$ expansion in the near horizon region we derive the effective equations that describe the dynamics of the EGB black strings. The uniform and non-uniform black strings are obtained as the static solutions of the effective equations. From the perturbation analysis of the effective equations, we find that thin EGB black strings suffer from the Gregory-Laflamme instablity and the GB term weakens the instability when the GB coefficient is small, however, when the GB coefficient is large the GB term enhances the instability. Furthermore, we numerically solve the effective equations to study the non-linear instability. It turns out that the thin black strings are unstable to developing inhomogeneities along their length, and at late times they asymptote to the stable non-uniform black strings. The behavior is qualitatively similar to the case in the Einstein gravity. Compared with the black string instability in the Einstein gravity at large D, when the GB coefficient is small the time needed to reach to final state increases, but when the GB coefficient is large the time to reach to final state decreases. Starting from the point of view in which the effective equations can be interpreted as the equations for the dynamical fluid, we evaluate the transport coefficients and find that the ratio of the shear viscosity and the entropy density agrees with that obtained previously in the membrane paradigm after taking the large $D$ limit.Comment: 22 pages, 8 figures, some errors corrected, references adde

Charged Scalar Perturbations around Garfinkle-Horowitz-Strominger Black Holes

We examine the stability of the Garfinkle-Horowitz-Strominger (GHS) black hole under charged scalar perturbations. We find that different from the neutral scalar field perturbations, only two numerical methods, such as the continued fraction method and the asymptotic iteration method, can keep high efficiency and accuracy requirements in the frequency domain computations. The comparisons of the efficiency between these two methods have also been done. Employing the appropriate numerical method, we show that the GHS black hole is always stable against charged scalar perturbations. This is different from the result obtained in the de Sitter and Anti-de Sitter black holes. Furthermore we argue that in the GHS black hole background there is no amplification of the incident charged scalar wave to cause the superradiance, so that the superradiant instability cannot exist in this spacetime.Comment: 24 pages, 5 figure

Superradiant instability of Kerr-de Sitter black holes in scalar-tensor theory

We investigate in detail the mechanism of superradiance to render the instability of Kerr-de Sitter black holes in scalar-tensor gravity. Our results provide more clues to examine the scalar-tensor gravity in the astrophysical black holes in the universe with cosmological constant. We also discuss the spontaneous scalarization in the de Sitter background and find that this instability can also happen in the spherical de Sitter configuration in a special style.Comment: (v2)21 pages, 21 figures; Sec. V revised; This version has been accepted for publication by JHE