Seminar/Colloquium

Nemanja Kaloper

Out of Darkness

The Quest for $\Lambda$ Recent astronomical observations are forcing us to face the cosmological constant problem, which is perhaps the greatest challenge of modern fundamental physics. Solving it seems to require a paradigm shift in our thinking about nature. In this talk I will explain why the physics of the cosmological constant is so elusive, discuss the ideas oft seen in connection to it and specifically review why changing the gravitational sector of the theory may help.

Seminar titles + abstracts

Ruth Gregory

Karch Randall Black Strings and Holography

I will review the thorny issues of black holes on the Randall-Sundrum brane, and discuss recent results on Karch-Randall black strings and their impact on the holographic conjecture, i.e. that a classical bulk-brane black hole solution will correspond to a quantum corrected 4D black hole.

Toby Wiseman

Ricci flow as an algorithm to numerically construct black hole metrics

Ricci flow potentially provides a tool to allow explicit numerical construction of black hole metrics of interest in physics. Whilst in 4 dimensions stationary black holes of interest are known analytically, I will discuss how black holes in theories with extra dimensions are generally not known analytically - for example static black holes on branes. A numerical construction of such metrics is likely to be the only way to examine them. I will show that black holes (of interest) are actually unstable fixed points of Ricci flow and I will discuss how to use the flow in practice for this application.

Nino Flachi

Spinning black holes and LHC

We discuss possible collider signatures of TeV scale micro black holes.

Takahiro Tanaka

Classical black hole evaporation conjecture and floating black holes

In Randall-Sundrum single-brane (RS-II) model, it was conjectured that there is no static large black hole localized on the brane based on adS/CFT correspondence. Here we consider the phase diagram of black objects in the models extended from the RS-II model. We propose a scenario for the phase diagram consistent with the classical black hole evaporation conjecture. The proposed scenario indicates the existence of a rich variety of the families of black objects.

Marco Caldarelli

Gregory-Laframme on plasma tubes

For many years it has been known that black holes behave in many respects like fluids. For example, the membrane paradigm allows to successfully describe physics outside a black hole by a fluid membrane on the stretched horizon, and the Gregory-Laflamme instability of black strings has striking analogies with the Rayleigh-Plateau instability of fluids. These analogies become rigorous in presence of a negative cosmological constant: large black holes in anti-de Sitter (AdS) spacetime have recently been shown to be dual to lumps of plasma described by Navier-Stokes equations on the boundary of AdS. This allows us to get new insight into the physics of the Gregory-Laflamme instability and to explore the phases of AdS black holes. In this talk I will first review higher-dimensional black holes, their instabilities and their relation with fluid dynamics. Then, I will analyse the properties of lumps of plasma dual to AdS black holes, and obtain new results on the phases and stability properties of these black holes.

Sergey Solodukhin

Entanglement entropy, conformal invariance and extrinsic geometry

Lorenzo Sorbo

Locally localized shock waves

I will describe gravtational shock waves in the Karch- Randall (locally localized gravity) model, both in the case of a five- dimensional and of a four-dimensional bulk. Such shock wave solutions allow to study in the detail the behavior of locally localized gravitational fluctuations and to clarify the nature of the AdS_3 brane black holes. I will also discuss two new classes of exact solutions describing matter localized on the two-brane.

Tony Padilla

Braneworld isotropization=CFT particle production

We consider a magnetic Bianchi I braneworld, embedded in between two Schwarzschild-AdS spacetimes, boosted equal amounts in opposite directions and compare them to the analagous solution in four-dimensional General Relativity. The efficient dissipation of anisotropy on the brane is explicitly demonstrated, a process we dub braneworld isotropization. From the bulk point of view, we attribute this to anisotropic energy being carried into the bulk by hot gravitons leaving the brane. From the brane point of view this can be interpreted in terms of the production of particles in the dual CFT. We explain how this interpretation enables us to gain a better understanding of the behaviour of anisotropic branes already studied in the literature.

Simon Ross

Tachyons in BTZ

I will discuss the appearance and condensation of quasi-localised tachyons in black string spacetimes. I will describe the string spectrum on a BTZ black hole spacetime supported by NS-NS flux, which provides a calculable example where we would expect to find a quasi-localised tachyon. Such a tachyon appears in the spectrum, but is not localised in the region where the spatial circle is string scale. I will also discuss the relation to orbifolds of flat space.

Norihiro Tanahashi

Time-symmetric initial data of brane-localized black hole in RS-II model

Missing of the analytic solution of static brane-localized black hole in Randall-Sundrum II (RS-II) braneworld model has been a big problem for this model. In order to explain this missing of static brane-localized solution, it was conjectured that brane-localized static black hole larger than bulk curvature scale does not exist in RS-II model. In order to investigate the plausibility of this conjecture, we constructed time-symmetric initial data of brane-localized black holes and examined their properties such as their existence or stability. As a result, we obtained three-parameter family of initial data of brane-localized black hole, and found some evidences which implies that large brane-localized BH is unstable and small one is stable. We will explain this result and its extension to the Karch-Randall brane case, in which the spacetime on the brane is asymptotically AdS.

Joan Camps

Generating Stationary Axisymmetric Black Hole solutions: Black Rings in Taub-NUT

In this talk I will review the class of Stationary and Axisymmetric solutions to Einstein's Equations. Some of the existing methods for the generation of such solutions will be outlined. As an application I will explain how to construct a solution representing a rotating Black Ring in Taub-NUT, which is the SUGRA approximation of the eleven dimensional picture of a system containing D0 and D6 branes in IIA.

Oriol Pujolas

Strongly Coupled Radiation from Accelerated Mirrors

We discuss the particle production by a (uniformly) accelerated mirror in asymptotically Anti de Sitter space. We consider the configurations where the mirror is in equilibrium with the radiation that it produces and determine the amount of radiation for ${\cal N}=4$ SYM both at weak and strong coupling using the AdS/CFT correspondence. This case can be solved exactly and the vacuum state is uniquely defined, once the boundary conditions at infinity are specified. In particular, there is always a state for which there is no radiation. However, we argue that this is not the relevant state because it corresponds to unphysical boundary conditions. Instead, in the natural vacuum state some radiation is present. Furthermore, in this state the energy density in the radiation at strong coupling is of the same order as at weak coupling. We discuss the possible implications of these results for the Brane Black Holes.

Barak Kol

Classical Effective Field Theory (CLEFT) and the delocalized effective degrees of freedom of a black hole at low frequencies

The talk will start by describing the CLEFT framework which replaces small but extended objects such as black holes by point particles, and employing deep field theoretic tools including regularization and renormalization. The talk will proceed to the issue of black hole degrees of freedom. Identifying the fundamental ones poses a long-standing puzzle. In hep-th/0511133 <http://arxiv.org/abs/hep-th/0511133> Goldberger and Rothstein forwarded a theory of the low frequency degrees of freedom within the effective field theory approach, where they are relevancy-ordered but of unclear physical origin. Here these degrees of freedom will be identified with near-horizon but non-compact gravitational perturbations which are decomposed into delocalized multipoles. Their world-line (kinetic) action will be presented and their interactions discussed. The case of the long-wavelength scattering of a scalar wave off a Schwarzschild black hole will be treated in some detail, interpreting within the CLEFT approach the equality of the leading absorption cross section with the area.

Oscar Dias

Microscopic theory of black hole superradiance

We study how black hole superradiance appears in string microscopic models of rotating black holes. In order to disentangle superradiance from finite-temperature effects, we consider an extremal, rotating D1-D5-P black hole that has an ergosphere and is not supersymmetric. We explain how the microscopic dual accounts for the superradiant ergosphere of this black hole. The bound 0< omega < m Omega_H on superradiant mode frequencies is argued to be a consequence of Fermi-Dirac statistics for the spin-carrying degrees of freedom in the dual CFT. We also compute the superradiant emission rates from both sides of the correspondence, and show their agreement.

Renaud Parentani

Beyond the semiclassical description of black hole evaporation

In the semi-classical treatment, i.e. in a classical black hole geometry, Hawking quanta emerge from trans-Planckian configurations because of scale invariance. There is indeed no scale to stop the blue-shifting effect encountered in the backward propagation towards the event horizon. On the contrary, when taking into account the gravitational interactions neglected in the semi-classical treatment, a UV scale stopping the blue-shift could be dynamically engendered. This result can be also obtained by considering light propagation in a stochastic ensemble of metrics.

David Langlois

Cosmological perturbations in multi-field DBI inflation

The Dirac-Born-Infeld (DBI) inflation scenario, related to the motion of a brane in a higher-dimensional warped background, represents an interesting alternative to slow-roll inflation. If the brane moves in the angular directions, as well as in the radial direction, then DBI inflation is described by a multi-field action with non-standard kinetic terms. This talk will present the analysis of cosmological perturbations in this type of scenario, taking into account the presence of entropy modes.

Christos Charmousis

Codimension 2 braneworlds

We will examine maximally symmetric braneworlds of codimension 2 in the context of Lovelock theory. We will argue that certain set-ups present self-acceleration and/or self-tuning properties. In passing we will present solutions describing Lovelock regular instantons and we will briefly describe the quantum tunneling process they describe.

Alessandro Fabbri

Holographic description of evaporating black holes

The holographic conjecture for black holes localized on a 3-brane in RS2 models predicts the existence of a classical 5D time-dependent solution dual to a 4D evaporating black hole. Attempts are made to describe features of evaporating black hole solutions (apparent and event horizons splitting, tunneling as a mechanism for the emission) in terms of classical bulk physics. We also comment on difficulties in holographically relating the (classical) GL instability with the (quantum) Hawking effect.