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A. J. M. Medved

Publications and source records attributed to A. J. M. Medved.

At least 73 records · Page 4Linked to original sources
arXiv2008

The ratio of shear viscosity to entropy density in generalized theories of gravity

Near the horizon of a black brane solution in Anti-de Sitter space, the long-wavelength fluctuations of the metric exhibit hydrodynamic behaviour. For Einstein's theory, the ratio of the shear viscosity of near-horizon metric fluctuations eta to the entropy per unit of transverse volume s is eta/s=1/4 pi. We propose that, in generalized theories of gravity, this ratio is given by the ratio of two effective gravitational couplings and can be different than 1/4 pi. Our proposal implies that eta/s is equal for any pair of gravity theories that can be transformed into each other by a field redefinition. In particular, the ratio is 1/4 pi for any theory that can be transformed into Einstein's theory; such as F(R) gravity. Our proposal also implies that matter interactions -- except those including explicit or implicit factors of the Riemann tensor -- will not modify eta/s. The proposed formula reproduces, in a very simple manner, some recently found results for Gauss-Bonnet gravity. We also make a prediction for eta/s in Lovelock theories of any order or dimensionality.

hep-th↗
arXiv2008

On the Kerr Quantum Area Spectrum

Suppose that there is a quantum operator that describes the horizon area of a black hole. Then what would be the form of the ensuing quantum spectrum? In this regard, it has been conjectured that the characteristic frequencies of the black hole oscillations can be used to calibrate the spacing between the spectral levels. The current article begins with a brief review of this conjecture and some of its subsequent developments. We then suggest a simple but vital modification to a recent treatment on the Kerr (or rotating black hole) spectrum. As a consequence of this refinement, we are able to rectify a prior inconsistency (as was found between two distinct calculations) and to establish, unambiguously, a universal form for the Kerr and Schwarzschild spectra.

gr-qc↗
arXiv2008

A Brief Editorial on de Sitter Radiation via Tunneling

Very recently (by which we mean two days ago), Y. Sekiwa submitted a paper [arXiv:0802.3266] that dismisses some previous calculations of the de Sitter radiative temperature. Significantly, these calculations employed the (so-called) tunneling methodology, which the stated author then goes on to correspondingly revise. After briefly commenting on what the tunneling framework can be claimed to say and what it cannot, we respond to the author's criticisms and proposed revision.

hep-th↗
arXiv2008

A Comment or two on Holographic Dark Energy

It has, quite recently, become fashionable to study a certain class of holographic-inspired models for the dark energy. These investigations have, indeed, managed to make some significant advances towards explaining the empirical data. Nonetheless, surprisingly little thought has been given to conceptual issues such as the composition and the very nature of the implicated energy source. In the current discourse, we attempt to fill this gap by the way of some speculative yet logically self-consistent arguments. Our construction takes us along a path that begins with an entanglement entropy and ends up at a Hubble-sized gas of exotic particles. Moreover, our interpretation of the dark energy turns out to be suggestive of a natural resolution to the cosmic-coincidence problem.

hep-th↗
arXiv2008

A Commentary on Ruppeiner Metrics for Black Holes

There has been some recent controversy regarding the Ruppeiner metrics that are induced by Reissner-Nordstrom (and Reissner-Nordstrom-like) black holes. Most infamously, why does this family of metrics turn out to be flat, how is this outcome to be physically understood, and can/should the formalism be suitably modified to induce curvature? In the current paper, we provide a novel interpretation of this debate. For the sake of maximal analytic clarity and tractability, some supporting calculations are carried out for the relatively simple model of a rotating BTZ black hole.

gr-qc↗
arXiv2006

An Implication of "Gravity as the Weakest Force"

The negative specific heat of a radiating black hole is indicative of a cataclysmic endpoint to the evaporation process. In this letter, we suggest a simple mechanism for circumventing such a dramatic outcome. The basis for our argument is a conjecture that was recently proposed by Arkani-Hamed and collaborators. To put it another way, we use their notion of ``Gravity as the Weakest Force'' as a means of inhibiting the process of black hole evaporation.

hep-th↗
arXiv2005

Hawking Radiation as Tunneling through the Quantum Horizon

Planck-scale corrections to the black-hole radiation spectrum in the Parikh-Wilczek tunneling framework are calculated. The corrective terms arise from modifications in the expression of the surface gravity in terms of the mass-energy of the black hole-emitted particle system. The form of the new spectrum is discussed together with the possible consequences for the fate of black holes in the late stages of evaporation.

hep-th↗
arXiv2005

On Hawking Radiation as Tunneling with Logarithmic Corrections

There has been recent speculation that the tunneling paradigm for Hawking radiation could -- after quantum-gravitational effects have suitably been incorporated -- provide a means for resolving the (black hole) information loss paradox. A prospective quantum-gravitational effect is the logarithmic-order correction to the Bekenstein-Hawking entropy/area law. In this letter, it is demonstrated that, even with the inclusion of the logarithmic correction (or, indeed, the quantum correction up to any perturbative order), the tunneling formalism is still unable to resolve the stated paradox. Moreover, we go on to show that the tunneling framework effectively constrains the coefficient of this logarithmic term to be non-negative. Significantly, the latter observation implies the necessity for including the canonical corrections in the quantum formulation of the black hole entropy.

gr-qc↗
arXiv2005

On Hawking Radiation as Tunneling with Back-Reaction

Recently, Angheben et.al. [hep-th/0503081] have presented a refined method for calculating the (tree-level) black hole temperature by way of the tunneling paradigm. In the current letter, we demonstrate how their formalism can be suitably adapted to accommodate the (higher-order) effects of the gravitational back-reaction.

gr-qc↗
arXiv2005

A not so brief commentary on cosmological entropy bounds

There has been, quite recently, a discussion on how holographic-inspired bounds might be used to encompass the present-day dark energy and early-universe inflation into a single paradigm. In the current treatment, we point out an inconsistency in the proposed framework and then provide a viable resolution. We also elaborate on some of the implications of this framework and further motivate the proposed holographic connection. The manuscript ends with a more speculative note on cosmic time as an emergent (holographically induced) construct.

hep-th↗
arXiv2004

When conceptual worlds collide: The GUP and the BH entropy

Recently, there has been much attention devoted to resolving the quantum corrections to the Bekenstein--Hawking (black hole) entropy. In particular, many researchers have expressed a vested interest in fixing the coefficient of the sub-leading logarithmic term. In the current paper, we are able to make some substantial progress in this direction by utilizing the generalized uncertainty principle (GUP). Notably, the GUP reduces to the conventional Heisenberg relation in situations of weak gravity but transcends it when gravitational effects can no longer be ignored. Ultimately, we formulate the quantum-corrected entropy in terms of an expansion that is consistent with all previous findings. Moreover, we demonstrate that the logarithmic prefactor (indeed, any coefficient of the expansion) can be expressed in terms of a single parameter that should be determinable via the fundamental theory.

hep-th↗
arXiv2004

A comment on black hole entropy or does Nature abhor a logarithm?

There has been substantial interest, as of late, in the quantum-corrected form of the Bekenstein-Hawking black hole entropy. The consensus viewpoint is that the leading-order correction should be a logarithm of the horizon area; however, the value of the logarithmic prefactor remains a point of notable controversy. Very recently, Hod has employed statistical arguments that constrain this prefactor to be a non-negative integer. In the current paper, we invoke some independent considerations to argue that the "best guess" for the prefactor might simply be zero. Significantly, this value complies with the prior prediction and, moreover, seems suggestive of some fundamental symmetry.

gr-qc↗
arXiv2004

A brief commentary on black hole entropy

It is commonplace, in the literature, to find that the Bekenstein-Hawking entropy has been endowed with having an explicit statistical interpretation. In the following essay, we discuss why such a viewpoint warrants a certain degree of caution.

gr-qc↗
arXiv2004

Symmetries at stationary Killing horizons

It has often been suggested (especially by Carlip) that spacetime symmetries in the neighborhood of a black hole horizon may be relevant to a statistical understanding of the Bekenstein-Hawking entropy. A prime candidate for this type of symmetry is that which is exhibited by the Einstein tensor. More precisely, it is now known that this tensor takes on a strongly constrained (block-diagonal) form as it approaches any stationary, non-extremal Killing horizon. Presently, exploiting the geometrical properties of such horizons, we provide a particularly elegant argument that substantiates this highly symmetric form for the Einstein tensor. It is, however, duly noted that, on account of a "loophole", the argument does fall just short of attaining the status of a rigorous proof.

gr-qc↗
arXiv2004

Quasinormal modes for single horizon black holes in generic 2-d dilaton gravity

There has been some recent speculation that a connection may exist between the quasinormal-mode spectra of highly damped black holes and the fundamental theory of quantum gravity. This notion follows from a conjecture by Hod that the real part of the highly damped mode frequencies can be used to calibrate the semi-classical level spacing in the black hole quantum area spectrum. However, even if the level spacing can be fixed in this manner, it still remains unclear whether this implies a physically significant "duality" or merely a numerical coincidence. This tapestry of ideas serves as the motivation for the current paper. We utilize the "monodromy approach" to calculate the quasinormal-mode spectra for a generic class of black holes in two-dimensional dilatonic gravity. Our results agree with the prior literature whenever a direct comparison is possible and provide the analysis of a much more diverse class of black hole models than previously considered.

gr-qc↗
arXiv2004

Anatomy of a Bounce

Holographic considerations are used in the scrutiny of a special class of brane-world cosmologies. Inherently to this class, the brane typically bounces, at a finite size, as a consequence of a charged black hole in the bulk. Whereas a prior treatment [hep-th/0301010] emphasized a brane that is void of standard-model matter, the analysis is now extended to include an intrinsic (radiation-dominated) matter source. An interesting feature of this generalized model is that a bounce is no longer guaranteed but, rather, depends on the initial conditions. Ultimately, we demonstrate that compliance with an appropriate holographic bound is a sufficient prerequisite for a bounce to occur.

hep-th↗
arXiv2004

Holography: 2-D or not 2-D?

As was recently pointed out by Cadoni, a certain class of two-dimensional gravitational theories will exhibit (black hole) thermodynamic behavior that is reminiscent of a free field theory. In the current letter, a direct correspondence is established between these two-dimensional models and the strongly curved regime of (arbitrary-dimensional) anti-de Sitter gravity. On this basis, we go on to speculatively argue that two-dimensional gravity may ultimatley be utilized for identifying and perhaps even understanding holographic dualities.

gr-qc↗