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## Bhw2013.wdfiles.com

Ballon Bayona, Alfonso, Universidade do Porto,Title: Magnetic effect on a holographic deconfinement transition,Abstract: In this talk I will describe the effect of a magnetic field on anAdS/QCD deconfinement transition. The AdS/QCD approach can be thoughtas a holographic description of Quantum Chromodynamics (QCD) in the large-N limit, where N is the number of colors. I will focus on the Sakai-Sugimotomodel, which describes confinement and chiral symmetry breaking and mapsthe deconfinement transition to a Hawking-Page transition.

Bertolami, Orfeu, Universidade do Porto,Title: Phase-Space Noncommutative Black Holes,Abstract: The remarkable quantum properties of phase-space noncommutativeblack holes are described and discussed.

Bolejko, Krzysztof, The University of Sydney,Title: Apparent and absolute horizons in the Szekeres models,Abstract: The apparent horizon can be defined as the outer envelope of a regionin which every bundle of null geodesics is converging. A similar approach can betaken to define an absolute apparent horizon, i.e. as an envelope of a region inwhich every bundle of null accelerated rays (hence non-geodesics) is converging.

In my talk I will describe both these horizons in relation to the Szekeres model,which is an exact, non-stationary, non-symmetrical, and inhomogeneous solutionof the Einstein equations. I will discuss differences between these two horizons,which only in the limit of spherical symmetry coincide. I will show how theyform in the case of anisotropic collapse of an astrophysical object.

Title: Massive gravitons on black hole spacetimes: Instabilities and hairy blackholes,Abstract: In this talk we develop the study of massive spin-2 fluctuationsaround GR black holes. First, we show that the Schwarzschild geometry islinearly unstable for small tensor masses, against a spherically symmetric mode.

Second, we provide solid evidence that the Kerr geometry is also genericallyunstable, both against the spherical mode and against long-lived superradiantmodes. We end up by showing that hairy solutions exist in massive gravity andare, most likely, the generic end state of the spherically symmetric instability ofthe Schwarzschild black holes.

Bronnikov, Kirill, VNIIMS, Moscow; PFUR, Moscow,Title: Dilaton gravity, black holes and quasi-black holes,Abstract: We consider static electrically charged dust configurations in theframework of Einstein-Maxwell-dilaton gravity with the interaction P (χ)F 2where P (χ) is an arbitrary function of the dilaton χ, generalizing the Majumar-Papapetrou setup. The necessity of introducing a scalar charge density is re-vealed. Examples of (quasi-) black hole solutions are given, including thosewhere the energy densities of matter and the scalar field are positive.

Title: Indefinite theta functions and black hole partition functions,Abstract: We explore various aspects of supersymmetric black hole partitionfunctions in four-dimensional toroidally compactified heterotic string theory.

These functions suffer from divergences owing to the hyperbolic nature of thecharge lattice in this theory, which prevents them from having well-defined mod-ular transformation properties. In order to rectify this, we regularize these func-tions by converting the divergent series into indefinite theta functions, therebyobtaining fully regulated single-centered black hole partition functions.

Title: Precision gravitational-wave physics,Abstract: I discuss possible systematic effects in gravitational wave detection,and how they could hamper our ability to test GR. I will focus on astrophysically”dirty” black holes.

Title: Strong cosmic censorship with a cosmological constant II.

Abstract: We discuss new results concerning strong cosmic censorship for thespherically symmetric Einstein-Maxwell-scalar field system with a cosmologicalconstant.

Czinner, Viktor G., Universidade do Minho,Title: Thermodynamics of Schwarzschild black holes from the R´
formula,Abstract: Black hole thermodynamics has been actively investigated recentlyin a non-extensive framework. As a novel approach to the problem, in this talkI will present a model where the Bekenstein-Hawking entropy of Schwarzschildblack holes is regarded as a non-extensive Tsallis entropy, and its formal loga-rithm, the R´
enyi entropy, is considered for the canonical analysis. As a result,
the temperature - horizon radius relation has the same form as the one ob-tained from a (3+1)-dimensional black hole in anti-de Sitter space using theoriginal Boltzmann entropy formula. In both cases the temperature has a min-imum. A semi-classical estimate of the horizon radius at this minimum leadsto a Bekenstein bound for the q-parameter in the R´
holes, which is surprisingly close to other q-parameter fits from very distant ar-eas of physics, like cosmic ray spectra measurements, or power-law distributionof quarks coalescing to hadrons in high energy accelerator experiments.

Diaz-Polo, Jacobo, Universidade do Minho,Title: Black hole entropy in loop quantum gravity,Abstract: I will review how black holes are described within the loop quantumgravity framework and present some recent results for the computation of theirentropy.

Ferreira, Hugo, University of Nottingham,Title: Warped AdS3 black holes: are they classically stable?Abstract: (2+1)-dimensional gravity allows us to study aspects of classical andquantum gravity in a simpler technical setting which retains much of the con-ceptual complexity of the standard (3+1)-dimensional gravity. However, pureEinstein gravity lacks propagating degrees of freedom in 2+1 dimensions. Topo-logically Massive Gravity is a deformation of GR which includes propagatingdegrees of freedom. Besides the famous BTZ black hole solution, this theoryhas a whole new class of black hole solutions – the warped AdS3 black holes –which can be viewed as deformed BTZ solutions but which, counterintuitively,are not asymptotically AdS – they are actually (almost) asymptotically flat! Inthis talk, I will describe the classical features of this interesting set of solutions.

First, I will show that, in contrast with the BTZ solution, classical superradi-ance of a massive scalar field on the background of a warped AdS3 black hole isalways present. Despite this fact, I then show that the black hole is classicallystable to scalar perturbations, namely there are no superradiant instabilities,even if it is enclosed by a stationary mirror with Dirichlet boundary conditions.

This is a surprising result in view of the similarity between the causal struc-ture of the warped AdS3 black hole and the Kerr spacetime in 3+1 dimensions,which is classically unstable due to superradiant instabilities.

This work has been published in Phys. Rev. D 87, 124013 (2013).

Title: On gravity, black holes and confinement,Abstract: We discuss the interplay between gravity and confinement and itsrelevance for black hole evaporation.

Title: Non-perturbative effects to the quantum entropy,Abstract: In this talk I will review recent developments in the computation ofquantum corrections to the entropy of supersymmetric black holes in the contextof the AdS2/CFT1 correspondence. I will show how we can use supersymmetriclocalisation of supergravity on AdS2 backgrounds to compute quantum correc-tions to the Bekenstein-Hawking’s area formula. We look in particular to 1/8BPS black holes in four dimensional N = 8 string theory and compare thegravity computation with the well known microscopic answer. The leading an-swer, given by a Bessel function of the first kind is reproduced entirely fromsupergravity while non-perturbative contributions given in part by number the-oretic Kloosterman sums arise by considering AdS2 orbifolds. These results gowell beyond the large charge approximation and may be seen as a first examplewhere holography works exactly.

Herdeiro, Carlos A. R., Universidade de Aveiro,Title: Interaction of charged scalar fields with charged black holes,Abstract: I shall discuss quasi-bound states of charged scalar fields in thebackground of charged black holes. In particular I shall discuss:
- How a particularly simple behaviour emerges for the imaginary part of the
frequencies as a double limit is taken, in which both the scalar field and thebackground black hole become extremal.

- The superradiant modes that occur when the scalar field is enclosed in a
cavity and show the growth rate of these superradiant modes can be much largerthan in the analogous setup for rotating black holes.

- Compare the evolution of superradiant modes in the frequency and time
Title: Entropy of extremal black holes from the entropy of matter shells: a so-lution to the debate,Abstract: Black hole entropy S is one of the most fascinating issues in con-temporary physics, as one does not yet know what are the degrees of freedomat the fundamental microlevel. In addition, there are two mutually inconsistentresults for extremal black holes. There is the usual S = A/4 value, where A isthe horizon radius, obtained from string theory, and there is the prescriptionS = 0 obtained from the fact that for extremal black holes the period of theEuclidean time is not fixed in a classical calculation of the action. In order tobetter understand this problem, we exploit a framework set up by Martinez anduse a thermodynamic approach for an electrically charged thin shell. We thentake the shell radius into its horizon limit. This limit is called a quasi-blackhole. We show that
(i) for a non-extremal shell the horizon limit yields S = A/4, and(ii) for an extremal shell the horizon limit gives an entropy which is a func-
tion of the horizon radius alone, but the precise functional form depends on howwe set the initial shell.

This formalism clearly shows that non-extremal and extremal black holes aredifferent objects.

Title: Buchdahl limit for stars with a Schwarzschild interior and a small elec-trical charge: Analytical approach based on Misner’s method to solve the TOVequation,Abstract: Schwarzschild’s interior solution (1916) for a star with constant en-ergy density, can be neatly found from the Tolman-Oppenheimer-Volkoff (TOV)equation as first presented by Misner (Brandeis School 1969). From it one candeduce at once the Buchdahl limit (PR 1959). This is the minimum radius R astar with a given fixed mass M can have. It is a model-independent bound andit is achieved when the central pressure goes to infinity. For perfect fluids thelimit is R/M = 9/4, which is stronger than the bound R/M = 2 for excludingtrapped surfaces.

By adding a small charge Q to the interior Schwarzschild solution, and
matching it to a Reissner-Nordstrom exterior, one can study analytically, by
an approximation method valid to the corresponding Buchdahl limit. We findthat the limit in this case is in accord with Andreasson’s results (CMP 2009) forthin charged shells and the numerical results of Arbanil, Lemos, Zanchin (PRD2013).

Title: Black holes and wormholes in nonrelativistic theories of gravitation,Abstract: When many alternative theories to general relativity are being sug-gested, and the gravitational field is being theoretically and experimentally putto test, it is interesting to try an alternative to Newton’s gravitation throughan unexpected but simple modification of it. The idea is to have Newtoniangravitation, not in flat space, as we are used to, but in curved spaced. Thisintriguing possibility has been suggested by Abramowicz (2012). Noting thatNewtonian gravitation in curved space, and the corresponding Newtonian dy-namics in a circle distinguishes three radii, namely, the geodesic radius (whichgives the distance from the center to its perimeter), the circumferential radius(which is given by the perimeter divided by 2π), and the curvature radius (whichis Frenet’s radius of curvature of a curve, in this case a circle), he calculated theperihelion advance formula for a planet in an elliptical orbit in the gravitationalfield of a star in spherical space in curved Newtonian gravitation.

One can further consider solutions with matter, in which case Poisson’s
equation for Newtonian gravitation in curved space has to be used. Moreover,an equation that connects the curvature to the matter, i.e, connects the Ricciscalar with the density, can be sought for, giving an enhanced Newtonian theoryof gravitation as proposed by Abramowicz, Ellis, Horak, and Wielgus (2013).

Now this enhanced Newtonian gravitation can have wormhole solutions. We
present here a Newtonian wormhole in the enhanced Newtonian gravitation incurved space. We also comment on black hole solutions.

e, Javier, Universidad de Santiago de Compostela,
Title: Holographic Entanglement Entropy on Time Dependent Backgrounds,Abstract: The evolution of entanglement after a quench is a subject of muchinterest. Holographically it has been addressed in a number of papers where thetime dependent background is a collapsing metric of the Vaydia type. Goingbeyond that case needs numerical methods and demanding computation. I willreview the state of the art and report on work in progress in our group.

Minamitsuji, Masato, Instituto Superior T´
Title: Solutions in the scalar-tensor theory with nonminimal derivative cou-pling,Abstract: We present black hole type solutions in the scalar-tensor theorywith nonminimal derivative coupling to the Einstein tensor. The effects of thenonminimal derivative coupling appear in the large scales, while the solutionsapproach those in the Einstein gravity in the small scales.

Title: Strong cosmic censorship with a cosmological constant I.

Abstract: We discuss the existing results on strong cosmic censorship for spher-ically symmetric scalar fields, and how the inclusion of a positive cosmologicalconstant may change the mass inflation picture.

Title: Collapse of self-interacting fundamental fields in asymptotic spacetimes,Abstract: It was recently pointed out that anti-de Sitter(AdS) spacetime isunstable against gravitational collapse. The pertubation in AdS does not sim-ply decay away and can be reflected by AdS boundary to nonlinearly interactwith one another. Confinement would play an important role in the nonlin-ear, turbulent instability. On the other hand, massive fundamental fields canalso provide low-frequency confinement. We revisit an old problem on collapseof massive fields and will show the collapse after successive reflections by thepotential wall.

Title: Black holes and Matrix Quantum Mechanics,Abstract: In this talk, I will quickly review the duality between supersymmet-ric matrix quantum mechanics and 11 dimensional supergravity, that followsfrom the decoupling limit of D0-branes. I will then discuss the thermodynamicsof the system at finite temperature and under a massive deformation. In par-ticular, I shall explain how to construct the black hole geometry that describesthe deconfined phase of the plane-wave matrix model.

Title: Thermodynamics and entropy of shells in 2+1 dimensional AdS space-times and the BTZ black hole limit,Abstract:We study the thermodynamics of 3-dimensional thin matter shells inthe context of general relativity. We start with the calculation of the necessarypressure and energy of the thin shell in order for it to be static which allows thecalculation of the entropy as well as a thermodynamic stability analysis. Wethen take the shells to their gravitational radius, where it is shown that theypossess the entropy of BTZ black holes.

Title: Holographic collisions in confining theories,Abstract: High energy collisions in non-abelian gauge theories is currently asubject of great interest. In heavy ion collisions performed at RHIC and LHCthe formation of a quark-gluon plasma is observed. The physics involved is pre-sumably described by Quantum Chromodynamics (QCD), a non-conformal fieldtheory that exhibits confinement. Although the dual of QCD is not known, theanalogous process in gauge theories with a gravity dual can be described via thecollision of two objects that form a black hole in an asymptotically AdS space-time. This is a challenging problem that requires solving Einstein’s equations ina dynamical setting. Nevertheless, various aspects of collision processes in con-
formal field theories have been explored recently by employing the AdS/CFTcorrespondence.

I will describe a first step towards extending this program to gravitational
duals of confining gauge theories. We consider a linearized approach in whichtwo point particles traveling in an AdS-soliton background suddenly collide toform an object at rest. Our results include some universal features that areexpected also for non-linear collisions. The importance of phenomenologicalcutoffs to regularize the emission spectrum will also be addressed.

Title: Superradiant instabilities in warped geometries,Abstract: We study the development of superradiant instabilities for rotatingblack strings in warped extra-dimensions, focusing on 5-dimensional Randall-Sundrum scenarios with either a compact or infinite extra-dimension. We con-sider, in particular, the spectrum of Kaluza-Klein bound states in the simplestrotating black string geometry and show that it approaches a continuum in thedecompactification limit, rendering the black string unstable for any mass andspin. We then discuss the potential implications of this result in view of otherknown instabilities and properties of these solutions.

Rubiera-Garcia, Diego, Universidade Federal da Para´ıba,Title: Semiclassical Palatini geons,Abstract: We present a explicit realization of Wheeler’s geon concept intro-duced in 1955. These geons arise in an extension of General Relativity con-taining (Planck-supressed) quadratic terms and formulated within a first-order(Palatini) formalism. We describe how the Reissner-Nordstr¨
formed by non-perturbative effects close to the central-point singularity, whichis generically replaced by a wormhole. We consider the dynamical process bywhich such geons may be generated, and discuss their potential applications asdark matter candidates and objects at particle accelerators.

Shock, Jonathan, University of Cape Town,Title: The open string membrane paradigm with external electromagnetic fields,Abstract: We study the effective geometry felt by the fluctuations of openstrings living on the worldvolume of probe D-branes in the presence of back-ground electromagnetic fields. This is captured by an effective action consistingof a Maxwell term and a topological term, with the role of the metric played bythe open string metric. Studying generalized Eddington-Finkelstein coordinatesfor stationary but non-static manifolds, we consider an open string membraneparadigm to obtain a generic formula for the DC transport coefficients, includ-ing the effect of external electromagnetic fields present on the worldvolume ofthe probe branes. We show that the previously studied singular shell, presentwhen a critical electric field strength is turned on, behaves as a horizon for theopen string degrees of freedom. The results of this analysis can be used to definea membrane paradigm for a very general class of spacetimes with non-diagonalmetrics.

Tavakoli, Yaser, Universidade da Beira Interior,Title: Quantum gravity inspired homogeneous dust collapse,Abstract: The evolution of the gravitational system is described by Einstein’sfield equation which can be solved for a general spherically symmetric spacetimein vacuum without the assumption of staticity. According to Birkhoff’s theoremin general relativity, all spherically symmetric solutions of Einstein’s equationsin vacuum must be static and asymptotically flat (in the absence of cosmologicalconstant). It shows that the Schwarzschild solution remains the only solutionof this more general system of equations, that is, all spherically symmetricspacetimes, with vanishing Ricci tensor, are static. Several attempts have beendone in order to establish a Birkhoff-like theorem in quantum gravity. In thistalk, we study whether or not loop quantum gravity will share such property ofEinstein’s gravity, investigating if the exterior vacuum solutions in gravitationalcollapse with LQG are static.

Vitagliano, Vincenzo, Instituto Superior T´
Title: Horizon thermodynamics and spacetime mappings,Abstract: When black holes are dynamical, event horizons are replaced byapparent and trapping horizons. Conformal and Kerr-Schild transformations arewidely used in relation with dynamical black holes and we study the behaviourunder such transformations of quantities related to the thermodynamics of thesehorizons. The transformation properties are not those expected on the basis ofnaive arguments.

Title: Gravitational Physics from the embedding point of view,Abstract: The formulation of GR in terms of embedding space variables in 10or more dimensions is reconsidered. We argue that questions of local existenceand uniqueness are not as straightforward, and questions of global existence anduniqueness not as hopeless, as one may expect.

Zaslavskii, Oleg, Kharkov V. N. Karazin University,Title: Ultra-high energy collisions near black holes: new developments,Abstract: We suggest brief review of the effect of acceleration of particles byrotating and charged black holes to unbound energies in the centre of massframe. Simple and general explanations of the effect are given:
(i) the kinematic one based on the behaviour of relative velocity of colliding
(ii) the geometric one, based on properties of particles’ four-velocities with
respect to a local light cone near the horizon.

The similar effect near the inner black hole horizon is also discussed and therole of the bifurcation point is revealed. We also consider which energies can bedetected by an observer at infinity. The possible role of gravitational radiationis discussed.

ao, Miguel, Rochester Institute of Technology,
Title: Warped dynamical grids for accreting binary black holes,Abstract: We present a warped gridding scheme adapted to simulate gas dy-namics in binary black hole spacetimes. The grid concentrates grid points inthe vicinity of each black hole to resolve the smaller scale structures there, andrarefies grid points away from the binary to keep the overall problem size toa practical level. Though general, this construction is intended to be used incircumbinary accretion disk numerical evolutions. In this talk we will introducethe field of general relativistic magnetohydrodynamics (GRMHD), motivate andshow the construction of our warped gridding scheme and show preliminary re-sults of its usefulness in evolving a circumbinary disk surrounding an equal-massnon-spinning black hole binary.

Source: http://bhw2013.wdfiles.com/local--files/talks/abstracts.pdf

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