arXiv subjects

Gungwon Kang

Publications and source records attributed to Gungwon Kang.

35 records · Page 2Linked to original sources

arXiv2002

Classical Stability of Charged Black Branes and the Gubser-Mitra Conjecture

We have investigated the classical stability of magnetically charged black $p$-brane solutions for string theories that include the case studied by Gregory and Laflamme. It turns out that the stability behaves very differently depending on a coupling parameter between dilaton and gauge fields. In the case of Gregory and Laflamme, it has been known that the black brane instability decreases monotonically as the charge of black branes increases and finally disappears at the extremal point. For more general cases we found that, when the coupling parameter is small, black brane solutions become stable even before reaching to the extremal point. On the other hand, when the coupling parameter is large, black branes are always unstable and moreover the instability does not continue to decrease, but starts to increase again as they approach to the extremal point. However all extremal black branes are shown to be stable even in this case. It has also been shown that main features of the classical stability are in good agreement with the local thermodynamic behavior of the corresponding black hole system through the Gubser-Mitra conjecture. Some implications of our results are also discussed.

hep-th↗

arXiv2002

On the Stability of Black Strings/Branes

Some issues on the stability of black string or brane solutions are summarized briefly. The stability of dS/AdS-Schwarzschild black strings has been investigated. Interestingly, the AdS-Schwarzschild black strings turn out to be stable as the horizon size becomes larger than the AdS scale. It is also shown that BTZ black strings in four dimensions are stable regardless of the horizon size. Such stable feature seems to be common for several known black strings in dimensions lower than five. Some implications of our results on the role of non-uniformity in stable black string configurations are also discussed.

hep-th↗

arXiv2001

Holographic principle in the BDL brane cosmology

We study the holographic principle in the brane cosmology. Especially we describe how to accommodate the 5D anti de Sitter Schwarzschild (AdSS$_5$) black hole in the Binetruy-Deffayet-Langlois (BDL) approach of brane cosmology. It is easy to make a connection between a mass $M$ of the AdSS$_5$ black hole and a conformal field theory (CFT)-radiation dominated universe on the brane in the moving domain wall approach. But this is not established in the BDL approach. In this case we use two parameters $C_1, C_2$ in the Friedmann equation. These arise from integration and are really related to the choice of initial bulk matter. If one chooses a bulk energy density $ρ_B$ to account for a mass $M$ of the AdSS$_5$ black hole and the static fifth dimension, a CFT-radiation term with $ρ_{CFT} \sim M/a^{4}$ comes out from the bulk matter without introducing a localized matter distribution on the brane. This means that the holographic principle can be established in the BDL brane cosmology.

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arXiv2001

Stable Black Strings in Anti-de Sitter Space

In the five-dimensional Einstein gravity with negative cosmological constant in the presence/absence of a {\it non-fine-tuned} 3-brane, we have investigated the classical stability of black string solutions which are foliations of four-dimensional $AdS/dS$-Schwarzschild black holes. Such black strings are generically unstable as in the well-known Gregory-Laflamme instability. For $AdS$ black strings, however, it turns out that they become stable if the longitudinal size of horizon is larger than the order of the $AdS_4$ radius. Even in the case of unstable black strings, the $AdS$ black strings have a very different feature of string fragmentations from that in the flat brane world. Some implications of our results on the Gubser-Mitra conjecture are also discussed.

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arXiv2000

No Ghost State in the Brane World

We discuss the role of the trace part of metric fluctuations $h_{MN}$ in the Randall-Sundrum scenario of gravity. Without the matter, this field ($h=η^{MN}h_{MN}$) is a gauge-dependent term, and thus it can be gauged away. But, including the uniform source $\tilde{T}_{MN}$, this field satisfies the linearized equation $\Box_4 h =16πG_5 T^μ_μ$. This may correspond to the scalar $ξ^5$ in the bending of the brane due to the localized source. Considering the case of longitudinal perturbations ($h_{5μ} =h_{55}=0$), one finds the source relation $\tilde{T}^μ_μ=2\tilde{T}_{55}$, which leads to the ghost states in the massive modes. In addition, if one requires $T_{44}=2(T_{22}+T_{33})$, it is found that in the limit of $m^2_h \to 0$ we have the massless spin-2 propagation without the ghost state. This exactly corresponds to the same situation as in the intermediate scales of Gregory-Rubakov-Sibiryakov (GRS) model.

hep-th↗

arXiv2000

Randall-Sundrum Gauge in the Brane World

We discuss the Randall-Sundrum (RS) choice for $h_{MN}$ in the brane-world. We begin with the de Donder gauge (transverse-tracefree) including scalar($h_{55}$), vector($h_{5μ}$) and tensor($h_{μν}$) in five dimensions for comparison. One finds that $h_{55}=0$ and $h_{5μ}=0$. This leads to the RS choice. It appears that the RS choice is so restrictive for the five massless states, whereas it is unique for describing the massive states. Furthermore, one can establish the stability of the RS solution with the RS choice only.

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arXiv2000

S-wave absorption of scalars by noncommutative D3-branes

On the supergravity side, we study the propagation of the RR scalar and the dilaton in the D3-branes with NS $B$-field. To obtain the noncommutative effect, we consider the case of $B\to \infty(θ\toπ/2)$. We approximate this as the smeared D1-brane background with $F_5=H=0$. In this background, the RR scalar induces an instability of the near-horizon geometry. However, it turns out that the RR scalar is nonpropagating, while the dilaton is a physically propagating mode. We calculate the s-wave absorption cross section of the dilaton. One finds $σ_0^ϕ|_{B\to\infty} \sim (\tilde ω\tilde R_{π\over 2})^{8.9} / ω^5$ in the leading-order while $σ_0^ϕ|_{B=0} \sim (\tilde ωR_0)^{8}/ω^5$ in the D3-branes without $B$-field. This means that although the dilaton belongs to a minimally coupled scalar in the absence of $B$-field, it becomes a sort of fixed scalar in the limit of $B \to \infty$.

hep-th↗

arXiv2000

Greybody factor for D3-branes in B field

We calculate the effect of noncommutative spacetime on the greybody factor on the supergravity side. For this purpose we introduce a system of D3-branes with a constant NS $B$-field along their world volume directions ($x_2, x_3$). Considering the propagation of minimally coupled scalar with non-zero momentum along($x_2, x_3$), we derive an exact form of the greybody factor in $B$ field. It turns out that $σ^{B\ne0}_l > σ^{B=0}_l$. This means that the presence of $B$-field (the noncommutativity) suppresses the potential barrier surrounding the black hole. As a result, it comes out the increase of greybody factor.

hep-th↗

arXiv2000

Comment on "Metric Fluctuations in Brane Worlds"

Recently, Ivanov and Volovich (hep-th/9912242) claimed that the perturbation of $h_{μν}$ with nonvanishing transverse components $h_{5μ}$ is not localized on the brane because $h_{μν}$ depends on the fifth coordinate $z$ linearly. Consequently, it may indicate that the effective theory is unstable. However, we point out that such linear dependence on $z$ can be {\it gauged away}. Hence the solution does not belong to the physical one. Therefore, even if one includes $h_{5μ}$, Randall and Sundrum's argument for the localized gravity on the brane remains correct.

hep-th↗

arXiv1999

Superradiance and the Statistical-Mechanical Entropy of Rotating BTZ Black Holes

We have considered the divergence structure in the brick-wall model for the statistical mechanical entropy of a quantum field in thermal equilibrium with a black hole which {\it rotates}. Especially, the contribution to entropy from superradiant modes is carefully incorporated, leading to a result for this contribution which corrects some previous errors in the literature. It turns out that the previous errors were due to an incorrect quantization of the superradiant modes. Some of main results for the case of rotating BTZ black holes are that the entropy contribution from superradiant modes is positive rather than negative and also has a leading order divergence as that from nonsuperradiant modes. The total entropy, however, can still be identified with the Bekenstein-Hawking entropy of the rotating black hole by introducing a universal brick-wall cutoff. Our correct treatment of superradiant modes in the ``angular-momentum modified canonical ensemble'' also removes unnecessary introductions of regulating cutoff numbers as well as ill-defined expressions in the literature.

gr-qc↗

arXiv1997

Quantum Ergoregion Instability

We have shown that, as in the case of black holes, an ergosphere itself with no event horizon inside can evaporate spontaneously, giving energy radiation to spatial infinity until the ergoregion disappears. However, the feature of his quantum ergoregion instability is very much different from black hole radiation. It is rather analogous to a laser amplification. This analysis is based on the canonical quantization of a neutral scalar field in the presence of unstable modes characterized by complex frequencies in a simple model for a rapidly rotating star.

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arXiv1997

Quantum Aspects of Ergoregion Instability

It has been known classically that a star with an ergoregion but no event horizon is unstable to the emission of scalar, electromagnetic and gravitational waves. This classical ergoregion instability is characterized by complex frequency modes. We show how to canonically quantize a neutral scalar field in the presence of such unstable modes by considering a simple model for a rapidly rotating star. Some of interesting results is that there exists a physically meaningful mode decomposition including unstable normal mode solutions whose representation turns out to be a non-Fock-like Hilbert space. A ``particle" detector model placed in the in-vacuum state also shows that stars with ergoregions give rise to a spontaneous energy radiation to spatial infinity until ergoregions disappear.

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arXiv1996

Black Hole Area in Brans-Dicke Theory

We have shown that the dynamics of the scalar field $ϕ(x)= ``G^{-1}(x)"$ in Brans-Dicke theories of gravity makes the surface area of the black hole horizon {\it oscillatory} during its dynamical evolution. It explicitly explains why the area theorem does not hold in Brans-Dicke theory. However, we show that there exists a certain non-decreasing quantity defined on the event horizon which is proportional to the black hole entropy for the case of stationary solutions in Brans-Dicke theory. Some numerical simulations have been demonstrated for Oppenheimer-Snyder collapse in Brans-Dicke theory.

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arXiv1996

Quantization of Scalar Field in the Presence of Imaginary Frequency Modes

Complex frequency modes occur for a scalar field near a rapidly rotating star {\it with ergoregion but no event horizon}. Such complex frequency modes must be included in the quantization of the field. As a model for this system, we have investigated a real scalar field with mass $μ$ in a one-dimensional square-well potential. If the depth of the potential is greater than $μ^2$, then there exist imaginary frequency modes. It is possible to quantize this simple system, but the mode operators for imaginary frequencies satisfy unusual commutation relations and do not admit a Fock-like representation or a ground state. Similar properties have been discussed already by Fulling for a complex scalar field interacting with an external electrostatic potential. We are interested in the field dynamics in the physical case where the initial state of the quantum field is specified before the complex frequency modes develop. As a model for this, we investigated a free scalar field whose ``mass" is normal in the past and becomes ``tachyonic" in the future. A particle detector in the far future placed in the in-vacuum state shows non-vanishing excitations related to the imaginary frequency modes as well. Implications of these results for the question of vacuum stability near rapidly rotating stars and possible applications to other fields in physics are discussed briefly.

hep-th↗

arXiv1995

Increase of Black Hole Entropy in Higher Curvature Gravity

We examine the Zeroth Law and the Second Law of black hole thermodynamics within the context of effective gravitational actions including higher curvature interactions. We show that entropy can never decrease for quasi-stationary processes in which a black hole accretes positive energy matter, independent of the details of the gravitational action. Within a class of higher curvature theories where the Lagrangian consists of a polynomial in the Ricci scalar, we use a conformally equivalent theory to establish that stationary black hole solutions with a Killing horizon satisfy the Zeroth Law, and that the Second Law holds in general for any dynamical process. We also introduce a new method for establishing the Second Law based on a generalization of the area theorem, which may prove useful for a wider class of Lagrangians. Finally, we show how one can infer the form of the black hole entropy, at least for the Ricci polynomial theories, by integrating the changes of mass and angular momentum in a quasistationary accretion process.

gr-qc↗

arXiv1994

On Black Hole Entropy

Two techniques for computing black hole entropy in generally covariant gravity theories including arbitrary higher derivative interactions are studied. The techniques are Wald's Noether charge approach introduced recently, and a field redefinition method developed in this paper. Wald's results are extended by establishing that his local geometric expression for the black hole entropy gives the same result when evaluated on an arbitrary cross-section of a Killing horizon (rather than just the bifurcation surface). Further, we show that his expression for the entropy is not affected by ambiguities which arise in the Noether construction. Using the Noether charge expression, the entropy is evaluated explicitly for black holes in a wide class of generally covariant theories. Further, it is shown that the Killing horizon and surface gravity of a stationary black hole metric are invariant under field redefinitions of the metric of the form $\bar{g}_{ab}\equiv g_{ab} + Δ_{ab}$, where $Δ_{ab}$ is a tensor field constructed out of stationary fields. Using this result, a technique is developed for evaluating the black hole entropy in a given theory in terms of that of another theory related by field redefinitions. Remarkably, it is established that certain perturbative, first order, results obtained with this method are in fact {\it exact}. The possible significance of these results for the problem of finding the statistical origin of black hole entropy is discussed.}

gr-qc↗

arXiv1993

Conformal Invariance of Black Hole Temperature

It is shown that the surface gravity and temperature of a stationary black hole are invariant under conformal transformations of the metric that are the identity at infinity. More precisely, we find a conformal invariant definition of the surface gravity of a conformal Killing horizon that agrees with the usual definition(s) for a true Killing horizon and is proportional to the temperature as defined by Hawking radiation. This result is reconciled with the intimate relation between the trace anomaly and the Hawking effect, despite the {\it non}invariance of the trace anomaly under conformal transformations.

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