arXiv subjects

Hongsu Kim

Publications and source records attributed to Hongsu Kim.

48 records · Page 3Linked to original sources

arXiv1999

New Black Hole Solutions in Brans-Dicke Theory of Gravity

Existence check of non-trivial, stationary axisymmetric black hole solutions in Brans-Dicke theory of gravity in different direction from those of Penrose, Thorne and Dykla, and Hawking is performed. Namely, working directly with the known explicit spacetime solutions in Brans-Dicke theory, it is found that non-trivial Kerr-Newman-type black hole solutions different from general relativistic solutions could occur for the generic Brans-Dicke parameter values -5/2\leq ω<-3/2. Finally, issues like whether these new black holes carry scalar hair and can really arise in nature and if they can, what the associated physical implications would be are discussed carefully.

gr-qc↗

arXiv1998

Spinning BTZ Black Hole versus Kerr Black Hole : A Closer Look

By applying Newman's algorithm, the AdS_3 rotating black hole solution is ``derived'' from the nonrotating black hole solution of Banados, Teitelboim, and Zanelli (BTZ). The rotating BTZ solution derived in this fashion is given in ``Boyer-Lindquist-type'' coordinates whereas the form of the solution originally given by BTZ is given in a kind of an ``unfamiliar'' coordinates which are related to each other by a transformation of time coordinate alone. The relative physical meaning between these two time coordinates is carefully studied. Since the Kerr-type and Boyer-Lindquist-type coordinates for rotating BTZ solution are newly found via Newman's algorithm, next, the transformation to Kerr-Schild-type coordinates is looked for. Indeed, such transformation is found to exist. And in this Kerr-Schild-type coordinates, truely maximal extension of its global structure by analytically continuing to ``antigravity universe'' region is carried out.

gr-qc↗

arXiv1998

Classical and Quantum Anisotropic Wormholes in Pure General Relativity

In the homogeneous and isotropic Friedmann-Robertson-Walker minisuperspace model, it is known that there are no Euclidean wormhole solutions in the pure gravity system. Here it is demonstrated explicitly that in Taub cosmology, which is one of the simplest anisotropic cosmology models, wormhole solutions do exist in pure general relativity in both classical and quantum contexts.

gr-qc↗

arXiv1998

Classical and Quantum Instantons in Yang-Mills Theory in the Background of de Sitter Spacetime

Instantons and their quantisation in pure Yang-Mills theory formulated in the background of de Sitter spacetime represented by spatially-closed ($k = 1$) Friedmann-Robertson-Walker metric are discussed. As for the classical treatment of the instanton physics, first, explicit instanton solutions are found and next, quantities like Pontryagin index and the semiclassical approximation to the inter-vacua tunnelling amplitude are evaluated. The Atiyah-Patodi-Singer index theorem is checked as well by constructing explicitly the normalizable fermion zero modes in this de Sitter spacetime instanton background. Finally, following the kink quantisation scheme originally proposed by Dashen, Hasslacher and Neveu, the quantisation of our instanton is performed. Of particular interest is the estimate of the lowest quantum correction to the inter-vacua tunnelling amplitude arising from the quantisation of the instanton. It turns out that the inter-vacua tunnelling amplitude gets enhanced upon quantising the instanton.

hep-th↗

arXiv1997

Thermodynamics of Black Holes in Brans-Dicke Gravity

It has recently been argued that non-trivial Brans-Dicke black hole solutions different from the usual Schwarzschild solution could exist. We attemt here to ``censor'' these non-trivial Brans-Dicke black hole solutions by examining their thermodynamic properties. Quantities like Hawking temperature and entropy of the black holes are computed. Analysis of the behaviors of these thermodynamic quantities appears to show that even in Brans-Dicke gravity, the usual Schwarzschild spacetime turns out to be the only physically relevant uncharged static black hole solution.

gr-qc↗

arXiv1997

Exotic Solutions in Einstein-Antisymmetric Tensor Theory

Classical solutions of the self-interacting, non-abelian antisymmetric tensor gauge theory of Freedman and Townsend coupled to Einstein gravity is discussed. Particularly, it is demonstrated that the theory admits a classical metric solution which, depending on the value of the gauge coupling parameter of the theory, exhibits a black hole with an exotic non-abelian hair or a spacetime showing the ``violation of the cosmic censorship hypothesis'' which should be distinguished from white holes.

hep-th↗

arXiv1997

Classical and Quantum Wormholes in Einstein-Yang-Mills Theory

Wormhole spacetimes may be responsible for the possible loss of quantum coherence and the introduction of additional fundamental quantum indeterminancy of the values of constants of nature. As a system which is known to admit such classical wormhole solutions, Einstein-Yang-Mills (EYM) theory is revisited. Since the classical wormhole instanton solution in this theory has been studied extensively thus far, in the present work, ``quantum wormholes'' are explored. Namely in the context of a minisuperspace quantum cosmology model based on this EYM theory, ``quantum wormhole'', defined as a state represented by a solution to the Wheeler-DeWitt equation satisfying an appropriate wormhole boundary condition, is discussed. Finally, it is proposed that the minisuperspace model based on this theory in the presence of the cosmological constant may serve as a simple yet interesting system displaying an overall picture of entire universe's history from the deep quantum domain all the way to the classical domain.

hep-th↗

arXiv1997

Axionic Wormholes : More on their Classical and Quantum Aspects

As a system which is known to admit classical wormhole instanton solutions, Einstein-Kalb-Ramond (KR) antisymmetric tensor theory is revisited. As an untouched issue, the existence of fermionic zero modes in the background of classical axionic wormhole spacetime and its physical implications is addressed. In particular, in the context of a minisuperspace quantum cosmology model based on this Einstein-KR antisymmetric tensor theory, ``quantum wormhole'', defined as a state represented by a solution to the Wheeler-DeWitt equation satisfying an appropriate wormhole boundary condition, is discussed. An exact, analytic wave function for quantum wormholes is actually found. Finally, it is proposed that the minisuperspace model based on this theory in the presence of the cosmological constant may serve as an interesting simple system displaying an overall picture of entire universe's history from the deep quantum domain all the way to the classical domain.

hep-th↗

arXiv1997

Toward a Complete Analysis of the Global Structure of Kerr-Newman Spacetime

An attempt is made to supplement Carter's partial investigation of the global structure of Kerr-Newman spacetime on the symmetry axis. Namely, the global structure of θ= const. timelike submanifolds of Kerr-Newman metric starting from the symmetry axis all the way down to the equatorial plane are studied by introducing a new time coordinate slightly different from the usual Boyer-Lindquist time coordinate. It turns out that the maximal anaytic extension of θ= θ_0 (0 \leq θ_0 < π/2) submanifolds is the same as that of the symmetry axis first studied by Carter whereas θ= π/2 equatorial plane has the topology identical to that of the Reissner-Nordstrom spacetime. General applicability of this method to Kerr-Newman-type black hole solutions in other gravity theories is discussed as well.

gr-qc↗

arXiv1997

Notes on Spinning AdS_3 Black Hole Solution

By applying Newman's method, the AdS_3 rotating black hole solution is "derived" from the nonrotating black hole solution of Banados, Teitelboim and Zanelli (BTZ). The rotating BTZ solution derived in this fashion is given in "Boyer-Lindquist-type" coordinates whereas the form of the solution originally given by BTZ is given in a kind of an "unfamiliar" coordinates which are related to each other by a transformation of time coordinate alone. The relative physical meaning between these two coordinates is carefully studied by evaluating angular momentum per unit mass, angular velocity, surface gravity and area of the event horizon in two alternative coordinates respectively. The result of this study leads us to the conclusion that the BTZ time coordinate must be the time coordinate of an observer who rotates around the axis of the spinning hole in opposite direction to that of the hole outside its static limit.

hep-th↗

arXiv1997

The Absence of Fermionic Superradiance (A Simple Demonstration)

Superradiant scattering, which can be thought of as the wave analogue of the Penrose process is revisited. As is well-known, boson fields display superradiance provided they have frequency in a certain range whereas fermion fields do not. A succinct superradiance-checking algorithm employing particle number or energy current is formally reviewed and then applied to the case of fermion field. The demonstrations of the absence of fermionic superradiance in terms of the particle number current exist in the literature but they are in the context of two-component SL(2,C) spinor formalism for massive spinor and SO(3,1) Dirac spinor formalism for massless spinor. Here we present an alternative demonstration in terms of both particle number and energy current but in a different context of local SO(3,1) Dirac spinor formalism for both massless and massive spinors. It appears that our presentation confirms the absence of fermionic superradiance in a more simple and systematic manner.

hep-th↗

arXiv1997

Quantum Brans-Dicke Gravity in Euclidean Path Integral Formulation

The conformal structure of Brans-Dicke gravity action is carefully studied. It is discussed that Brans-Dicke gravity action has definitely no conformal invariance. It is shown, however, that this lack of conformal invariance enables us to demonstrate that Brans-Dicke theory appears to have a better short-distance behavior than Einstein gravity as far as Euclidean path integral formulation for quantum gravity is concerned.

hep-th↗