Bogeun Gwak

Cosmic Censorship of Rotating Anti-de Sitter Black Hole

We test the validity of cosmic censorship in the rotating anti-de Sitter black hole. For this purpose, we investigate whether the extremal black hole can be overspun by the particle absorption. The particle absorption will change the mass and angular momentum of the black hole, which is analyzed using the Hamilton-Jacobi equations consistent with the laws of thermodynamics. We have found that the mass of the extremal black hole increases more than the angular momentum. Therefore, the outer horizon of the black hole still exists, and cosmic censorship is valid.

Holographic D instanton liquid and chiral transition

We study the phase diagram of black D3 geometry with uniformly distributed D-instanton charge using the probe D7 brane. In the presence of uniform D-instanton charges, quarks can be confined, although gluons are not, because baryon vertices are allowed due to the net repulsive force on the on the probe D-branes. Since there is no scale in the geometry itself apart from the horizon size, there is no Hawking-Page transition. As a consequence, the D7 brane embedding can encode the effect of the finite temperature as well as finite baryon density even for low temperature. The probe D-brane embedding, however, undergoes a phase transition which can be interpreted as a chiral transition as we change temperature and density. We studied such phase transitions and calculated the constituent quark mass, chiral condensation, and the binding energy of baryons as function of the density. The baryon vertex melting is identified as the quark deconfinement. We draw the phase diagram according to these transitions.

Rotating black hole thermodynamics with a particle probe

The thermodynamics of Myers-Perry black holes in general dimensions are studied using a particle probe. When undergoing particle absorption, the changes of the entropy and irreducible mass are shown to be dependent on the particle radial momentum. The black hole thermodynamic behaviors are dependent on dimensionality for specific rotations. For a 4-dimensional Kerr black hole, its black hole properties are maintained for any particle absorption. 5-dimensional black holes can avoid a naked ring singularity by absorbing a particle in specific momenta ranges. Black holes over 6 dimensions become ultraspinning black holes through a specific form of particle absorption. The microscopical changes are interpreted in limited cases of Myers-Perry black holes using Kerr/CFT correspondence. We systematically describe the black hole properties changed by particle absorption in all dimensions.

Instability of Black Holes with a Gauss–Bonnet Term

We investigate the fragmentation instability of hairy black holes in the theory with a Gauss–Bonnet (GB) term in asymptotically flat spacetime. Our approach is through the non-perturbative fragmentation instability. By this approach, we investigate whether the initial black hole can be broken into two black holes by comparing the entropy of the initial black hole with the sum of those of two fragmented black holes. The relation between the black hole instability and the GB coupling with dilaton hair are presented. We describe the phase diagrams with respect to the mass of the black hole solutions and coupling constants. We find that a perturbatively stable black hole can be unstable under fragmentation.

Instability of rotating anti–de Sitter black holes

We study the instability of higher-dimensional rotating anti-de Sitter black holes through fragmentation. Fragmentation occurs when black holes rotate too fast to sustain their horizon, and then the black holes are broken into small pieces. Using this process, we test the stability of AdS black holes and discover the dynamical upper bounds of the angular momentum and the cosmological constant. We show that AdS black holes can exist stably within limited parameter ranges in the general dimensions. The parameter ranges are obtained numerically in terms of angular momentum and cosmological constant.

A particle probing thermodynamics in a three-dimensional black hole

We have shown the thermodynamic changes in a (2+1)-dimensional rotating black hole when it absorbs a particle. The microscopic changes that the black hole undergoes are interpreted using the AdS/CFT correspondence. Using the particle-absorption phenomenon, we formulate an irreducible mass and black hole entropy directly related to the particle momenta. We describe the black hole evolution that preserves the second law of thermodynamics. A number of microstates that evaluate the entropy are analyzed at each of the black hole evolution stages.

Particle probe of Hořava-Lifshitz gravity

In this work, the Kehagias-Sfetsos (KS) black hole in Hořava-Lifshitz gravity is probed through particle geodesics. It is found that the gravitational force of the KS black hole becomes weaker than that of Schwarzschild black hole around the horizon and interior space. Particles can be always scattered or trapped in new closed orbits, unlike those falling forever in Schwarzschild black hole. The properties of null and timelike geodesics are classified with values of coupling constants. The precession rates of the orbits are also evaluated. Furthermore, the time trajectories are classified under different values of coupling constants for both null and timelike geodesics. Physical phenomena that may be observable in our universe are discussed.

Local thermodynamics of KS black hole

We study the thermodynamics of the KS black hole which is an asymptotically flat solution of the HL gravity. In particular, we introduce a cavity to describe the thermodynamics at a finite isothermal surface on general ground and to get a well-defined thermodynamics. We show that there exists a locally stable small black hole which tunnels to the hot flat space below the critical temperature and to the large black hole above the critical temperature. Moreover, it turns out that the remnant decays into the vacuum through a quantum tunneling.

Geodesic Properties and Orbits in 5-Dimensional Hypercylindrical Spacetime

We investigate the geodesic motions of a massive particle and light ray in the hyperplane orthogonal to the symmetry axis in the 5-dimensional hypercylindrical spacetime. The class of the solutions depends on one constant a which is the ratio of string mass density and tension. There exist unstable orbits in null geodesic only in some range of a. The innermost stable circular orbits in timelike geodesic also exist only in a certain range of the parameter a. The capture cross section and the deflection angle of light ray are also computed.

Semiclassical strings in AdS(3) X S**2

In this paper, we investigate the semiclassical strings in AdS3 × S2, in which the string configuration of AdS3 is classified to three cases depending on the parameters. Each of these has a different anomalous dimension proportional to log S, S1/3 and S, where S is a angular momentum on AdS3. Further we generalize the dispersion relations for various string configuration on AdS3 × S2.