We consider effects of non-uniformity of quasi-spherical small black hole horizons on scattering massless spineless particles in the long-wave approximation. Focusing on 4D flat and AdS neutral black hole backgrounds with conformally spherical geometry of the horizon, we observe the notable differences in compare to the scattering process on the spherically-symmetric black holes. In particular, the absorption cross-section becomes dependent on both, polar and azimuthal, spherical angles, projections of the angular momentum do not keep anymore and the angular momentum operator by itself, though remains quantised, is not quantised in integers. However, within the long-wave approximation, the main conclusion of previously obtained results on scattering on the spherically-symmetric black holes remains the same: the total absorption cross-section is proportional to the area of the black hole. The proportionality coefficient does not depend on the scalar wave frequency in the flat space black hole background, and is dependent on the admissible from the unitarity requirement frequencies in the background of AdS black hole. As a by-product of our studies we establish a quasi-spherical non-static Vaidya-type black hole solutions and outline the relation between real solutions to the elliptic Liouville equation on 2D plane and on two-dimensional sphere.

We consider the membrane viewpoint a l\`a Parikh-Wilczek on the Kerr solution for a rotating black hole. Computing the stress-energy tensor of a close-to-the-horizon stretched membrane and comparing it to the stress-tensor of a viscous fluid, we recover transport coefficients in terms of the Kerr geometry. Viscosities of the dual fluid remain constant, while the rest of the transport coefficients become complex functions of radial and angle coordinates. We study the qualitative behavior of the pressure, expansion, and energy/momentum densities for two specific black holes: the slowly rotating black hole, with the angular momentum of one percent of the black hole mass squared, and the extremal Kerr black hole. For the Kerr solution in the Boyer-Lindquist coordinates, these transport coefficients generally have poles at different values of the radial coordinate in the range between the horizon and the Schwarzschild radius of the black hole, in dependence on the fixed angle direction. We briefly discuss our findings in the context of a relation between the Membrane Paradigm and the AdS/CFT correspondence, the KSS bound violation, the coordinate choice, and a non-stationary extension of the Kerr solution.

We consider subtleties of the horizon (null-hypersurface) limit in the Parikh-Wilczek Membrane Approach to Black Holes. Specifically, we refine the correspondence between the projected Einstein equations of gravity with matter and the Raychaudhuri-Damour-Navier-Stokes (RDNS) equations of relativistic hydrodynamics. For a general configuration of gravity with matter we obtain additional terms in the hydrodynamic equations, which include very specific combinations of the contracted logarithmic derivatives of a parameter (the regularization function) determining the proximity of a stretched membrane to the black hole horizon. Nevertheless, direct computations of the new terms for exact (Schwarzschild and Kerr) black hole solutions prompt the standard form of the RDNS equations, due to the non-expanding horizon property of these solutions. Therefore, the reduction of the extended RDNS equations to their classical form may be viewed as an additional consistency condition in the exact black hole solutions hydrodynamics, and may serve as a non-trivial test for various viable approximations of spacetime metrics. We compare in detail the Parikh-Wilczek Membrane Approach with the Gourgoulhon-Jaramillo method of a null-hypersurface description, as well as give the link of the obtained results to our previous work on the Kerr black holes.

Supersymmetry is one of the most important and indispensable ingredients of modern theoretical physics. However, the absence, at least at the time of publishing this review, of experimental verification of supersymmetry in elementary particle/high-energy physics casts doubt on the viability of this concept at the energies achievable at the LHC. The more unexpected are either already experimentally verified or proposed for verification manifestations of supersymmetry at the level of low (condensed matter physics, quantum optics) and medium (nuclear physics) energies where standard quantum mechanics works. Using examples of various systems from completely different areas of physics, we discuss the isospectrality of quantum Hamiltonians, hidden and explicit supersymmetry, the advantages of the supersymmetric quantum mechanics approach and its role in quantum engineering.

We construct series of solutions for the Kerr-type rotating black hole with non-trivial matter in flat and (A)dS backgrounds. Symmetry arguments and singularity analysis in the proposed black hole models fix the free parameters of the solutions, and the study of popular energy conditions makes it possible to impose constraints on configuration and field content of external matter. The resulted geometry of spacetimes is featured by a special type singularity in the north and south pole directions, inducing the bipolar outflow of particles from black holes. As a step toward the construction of a non-stationary rotating black hole solution in the presence of matter, we explore the zero angular momentum limit of the constructed metrics. The use of the Eddington-Finkelstein coordinates allows us to find a generalization of the proposed construction to the Vaidya-type black hole. Finally, employing the Newman-Janis algorithm, we find the corresponding generalization of the Kerr-Vaidya black hole solution.

We establish solutions corresponding to AdS4 static charged black holes with inhomogeneous two-dimensional horizon surfaces of constant curvature. Depending on the choice of the 2D constant curvature space, the metric potential of the internal geometry of the horizon satisfies the elliptic wave/elliptic Liouville equations. We calculate the charge diffusion and transport coefficients in the hydrodynamic limit of gauge/gravity duality and observe the exponential suppression in the diffusion coefficient and in the shear viscosity-per-entropy density ratio in the presence of an inhomogeneity on black hole horizons with planar, spherical, and hyperbolic geometry. We discuss the subtleties of the approach developed for a planar black hole with inhomogeneity distribution on the horizon surface in more detail and find, among others, a trial distribution function, which generates values of the shear viscosity-per-entropy density ratio falling within the experimentally relevant range. The solutions obtained are also extended to higher-dimensional AdS space. We observe two different DC conductivities in 4D and higher-dimensional effective strongly coupled dual media and formulate conditions under which the appropriate ratio of different conductivities is qualitatively the same as that observed in an anisotropic strongly coupled fluid. We briefly discuss ways of how the Liouville field could appear in condensed matter physics and outline prospects of further employing the gauge/gravity duality in CMP problems.