# Joint seminars academic year: 2016-2017

### 3D/4D gravity as the dimensional reduction of a theory of differential forms in 6D/7D

Speaker: Kirill Krasnov (University of Nottingham)

Time: Wednesday Februari 15th 2017 - 1:00pm

Location: ULB, Solvay Room

I will show how 3D gravity arises as the dimensional reduction (on S^3) of a certain theory of 2 and 3-forms in 6 dimensions. Similarly, 4D gravity arises as the reduction of a certain theory of 3-forms in 7D.

### On Singularities, Quantum Noise and holographic Complexity

Speaker: Eliezer Rabinovici (Hebrew University of Jerusalem)

Time: Wednesday Februari 15th 2017 - 10:30am

Location: ULB, Solvay Room

Various properties of some space like singularities in string theory will be discussed in the AdS/CFT holographic framework. Attempts to apply the diagnostic tools of long time correlations and complexity, to these singularities, will be described.

### Holography Meeting

Time: Friday Jan 27th, 2017 at 9am

Location: KU Leuven, Aula Arenbergkasteel (01.07), Kasteelpark Arenberg 1, Heverlee

*String Fluctuations in Non-Conformal Holography (Konstantin Zarembo- Nordita, Stockholm)*

Abstract: Quantum fluctuations of the string worldsheet lead to important effects in gauge-string duality. One example is the Lüscher term is the QQ-bar potential in QCD, important for matching the lattice data. I will discuss non-conformal holography of the N=2* theory which provides a controllable setup where the effects of string fluctuations are explicitly calculable in field theory, using localization, even though the problem is intrinsically strongly-coupled. The results obtained by direct quantization of the dual string theory perfectly match the field-theory predictions, after subtle effects like dilaton coupling to the string worldsheet are properly taken into account.

*One fish, two fish, red fish, blue fish” (Christopher Herzog - SYNT, Stony Brook)*

Abstract: Based on arXiv:1605.01404 - Konstantin Zarembo (Nordita, Stockholm)

*Black Holes, Matrix Models and Large D (Frank Ferrari - ULB, Brussels)*

Abstract: I plan to cover the following topics: 1) what kind of quantum mechanical models are expected to describe black holes, as suggested by holography and string theory; 2) how, surprisingly, completely different-looking ``SYK’' models, more familiar to condensed matter physicists, have been shown to also posses some (but not all) of the relevant properties to describe black holes; 3) how the link between 1) and 2) can be made, through a new large D limit of matrix models inspired in part from studies of the large dimension limit of general relativity and in part from work in tensor model diagrammatics, and what the consequences could be.

*BPS Particles and Local Operators (Clay Cordova - IAS, Princeton)*

Abstract: I will describe several recently posed conjectures that relate the BPS particle spectrum in 4d N=2 theories to supersymmetric local operators. I will apply these ideas to compute the superconformal indices of strongly coupled N=2 CFTs and explain how the results interplay with recent results relating 2d chiral algebras to 4d CFTs. Time permitting, I will also explain how to determine the spectrum of local operators bound to a defect using similar ideas.

### String theory on AdS_3 and Higher Spins

Speaker: Juan Jottar (ETH Zurich)

Time: Wednesday December 7th 2016 - 1:00pm

Location: ULB, Solvay Room

It has been recently shown that the free symmetric product orbifold CFT, describing the IR (near-horizon) limit of the D1-D5 system, contains a closed subsector controlled by a 2d CFT with small N=4 supersymmetry and conserved currents of spin s for all integer values of s. This result lends credibility to the idea that tensionless string theory on AdS_3 contains a Vasiliev-like 3d higher spin theory. We discuss whether these findings can be understood directly from the worldsheet perspective, and the extent to which one can identify the string excitations corresponding to the Vasiliev fields. To this end, we study the spectrum of type IIB string theory on the AdS_3 x S^3 x T^4 background with pure NS-NS flux, related to the D1-D5 system by S-duality. In particular, we focus on the meaning of the tensionless limit in this configuration, and whether the latter is enough to guarantee the emergence of higher spin symmetry.

### Deforming the web of generalized free CFTs

Speaker: Anastasios Petkou (Aristotle University of Thessaloniki)

Time: Wednesday December 7th, 2016 - 10:30am

Location: ULB, Solvay Room

Generalized free CFTs can be defined in arbitrary dimensions. Their OPE structure exhibits an intriguing analytic structure for dimensions d>6 when one enters into the regions of non-unitarity. Nevertheless, the analytic structure of the free theories can be used as a tool to explore the huge web of deformed generalized free CFTs. Along the way I present a unified method to calculate leading order results for the couplings and anomalous dimensions for unitary and non-unitary nontrivial CFTs in arbitrary dimensions.

### Segmented strings

Speaker: David Vegh (Utrecht)

Time: Wednesday November 30th 2016 - 1:00pm

Location: KULeuven, 200N.004

Segmented strings in flat space are piecewise linear solutions. Kinks between the segments move with the speed of light and their worldlines form a lattice on the worldsheet. This description can be generalized to anti-de Sitter spacetime where the string motion is non-linear. In this talk, I will review the properties of segmented strings and relate them to discrete-time Toda-type lattices.

### The primordial universe as a particle detector

Speaker: Guilherme Pimantel (Amsterdam)

Time: Wednesday November 30th, 2016 - 10:30am

Location: KULeuven, 200N.004

After a review of the theory of inflationary perturbations, in particular non-Gaussianity and tensor fluctuations, I will explain how future measurements will allow us to learn more about the particle content and the dynamics of inflation. I also emphasize the role of conformal symmetry and its breaking in determining both the shape and size of inflationary perturbations. If time permits, I will make some comments about the wavefunction of the universe, the Wheeler-deWitt equation, and soft graviton theorems.

### Classicalization and Dual Theory

Speaker: Cesar Gomez (Madrid)

Time: Wednesday November 23rd, 2016 - 1:00pm

Location: ULB, Solvay Room

We discuss classicalization in quantum gravity and its connection with dual theories i.e. string theory.

### M5-branes on an elliptic fibration

Speaker: Benjamin Assel (CERN)

Time: Wednesday November 23rd, 2016 - 10:30am

Location: ULB, Solvay Room

Four-dimensional N=4 SYM theory has an SL(2,Z) group of electro-magnetic dualities, which can be realized geometrically by regarding the theory as the dimensional reduction of the six-dimensional (2,0) SCFT, or M5 brane theory, on a torus.

In this talk I will generalize this setup by allowing the M5 brane theory to live on a non-trivial elliptic fibration. The reduction on the elliptic fiber will lead to the N=4 SYM theory with varying coupling constant and surface defects, around which the theory undergoes duality transformations. I will also discuss interesting aspects of the twisting procedure which allows to preserve supersymmetries.

### Relative entropy of excited states in conformal field theories

Speaker: Gabor Sarosi (VUB)

Time: Wednesday November 16th, 2016 - 1:00pm

Location: KULeuven, building 200C, room 01.06

We study the relative entropy between the reduced density matrices obtained from globally excited states in conformal field theories of arbitrary dimensions. We find a general formula in the small subsystem size limit. When one of the states is the vacuum of the CFT, our result matches with the holographic entanglement entropy computations in the corresponding bulk geometries, including AdS black branes. We also discuss the first asymmetric part of the relative entropy and comment on some implications of the results on the distinguishability of black hole microstates in AdS/CFT.

### Out of equilibrium steady states in conformal field theories

Speaker: Amos Yarom (Technion)

Time: Wednesday November 16th, 2016 - 10:30am

Location: KULeuven, building 200C, room 01.06

I will consider properties of a non equilibrium steady state generated by placing two initial heat baths in contact with each other. The dynamics of the system under consideration is governed by a conformal field theory. When the number of spacetime dimensions is very large the equations of motion for the system simplify. The ``phase diagram'' associated with the steady state, the dual, dynamical, black hole description of this problem, and its relation to the fluid/gravity correspondence will be discussed.

### Codimension-2 Microstates of Black Holes

Speaker: Masaki Shigemori (Queen Mary)

Time: Wednesday November 9th, 2016 - 1:00pm

Location: Solvay Room, ULB

We study string-theory configurations representing microstates of four-dimensional supersymmetric black holes. These configurations are generalizations of the Bates-Denef/Gutowski Gauntlett/Bena-Warner multi-center solutions with codimension-3 centers to include codimension-2 ones. In general, codimension-2 centers have non-trivial duality monodromies around them and can be regarded as non-Abelian anyons. We will discus the relevance of such configurations for the black microstate (non-)geometry program.

### Black hole instabilities and the formation of singularities in general relativity

Speaker: Pau Figueras (Queen Mary)

Time: Wednesday November 9th, 2016 - 10:30am

Location: Solvay Room, ULB

Black holes play a central role in our understanding of general relativity, the currently accepted classical theory of gravity. The recent discovery of gravitational waves by the Ligo-Virgo collaboration confirmed their existence in Nature. But the importance of four dimensional black holes to describe some of the observed compact dark objects in the Universe hinges on their dynamical stability under small perturbations. In recent years, motivated by more fundamental theories of gravity such as string theory, there has been a lot of interest in the community to study black holes in spacetimes with more than four spacetime dimensions. In this talk I will show that in dimensions higher than four, asymptotically flat black holes can be dynamically unstable. Furthermore, I will present results of numerical simulations of these instabilities in full non-linear regime that suggest that unstable horizons develop a fractal structure that leads to a naked singularity in finite time.

Moreover, I will argue that the mechanism behind these instabilities is universal. These results provide evidence that the weak cosmic censorship conjecture, which is the greatest unsolved problem in general relativity, is false in spacetimes with more than four dimensions.

### BGG sequences, metric projective geometry and partially massless gauge theories

Speaker: Emanuele Latini (University of Bologna)

Time: Wednesday October 26th, 2016 - 1:00pm

Location: Pentagone Building, Room 0A11, UMons

BGG complexes are projective resolutions of certain representations of a Lie algebra. Within the context of Cartan and in particular parabolic geometry they have a nice interpretation in terms of sequences of invariant differential operators. In this talk we will focus on the special case of metric projective geometry that is the interaction of projective and pseudo-Riemannian geometry. We show how, in this case, the BGG machinery combines with structures known as Yang-Mills detour complexes to produce a general tool for generating PM gauge theories.

### Higher Spins and Strings

Speaker: Matthias Gaberdiel (ETH Zurich)

Time: Wednesday October 26th, 2016 - 10:30am

Location: Pentagone Building, Room 0A11, UMons

The conjectured relation between higher spin theories on anti de-Sitter (AdS) spaces and weakly coupled conformal field theories is reviewed. I shall then outline the evidence in favour of a concrete duality of this kind, relating a specific higher spin theory on AdS3 to a family of 2d minimal model CFTs, and show how this duality fits into the framework of the familiar stringy AdS/CFT correspondence. Finally, I shall explain how Yangian symmetries appear in this context, hinting at an underlying integrable symmetry.

### Black holes at large D: Things we’ve learned so far

Speaker: Roberto Emparan (Barcelona)

Time: Wednesday October 19th, 2016 - 1:00pm

Location: Building 200N, room 00.04, KULeuven

The dynamics of black holes and black branes greatly simplifies in the limit in which the number of spacetime dimensions D grows very large. I will give an overview of some of the main things we have learned and of results we have achieved so far in the study of this large-D expansion.

### Hexagons and Three-Point Functions

Speaker: Benjamin Basso (ENS Paris)

Time: Wednesday October 19th, 2016 - 10:30am

Location: Building 200N, room 00.04, KULeuven

I will present a framework for computing correlators of three single trace operators in planar N=4 SYM theory that uses hexagonal patches as building blocks. This approach allows one to exploit the integrability of the theory and derive all loop predictions for its structure constants. After presenting the main ideas and results, I will discuss recent perturbative tests and open problems.

### T-branes and 3d Mirror Symmetry

Speaker: Roberto Valandro (Trieste)

Time: Wednesday October 12th, 2016 - 1:00pm

Location: Building 200N, Room 00.004, ULB

T-branes are exotic bound states of D-branes, characterised by mutually non-commuting vacuum expectation values for the worldvolume scalars. In this seminar, I will show how this effect can be felt by a probe M2 brane on the dual M-theory side. The main tool to understand this is 3d mirror symmetry. Through this language, a very concrete framework is developed for understanding T-branes in M-theory. This led us to uncover a new class of N=2 quiver gauge theories, whose Higgs branches mimic those of membranes at ADE singularities, but whose Coulomb branches differ from their N=4 counterparts.

### Hydrodynamics and its destruction

Speaker: Saso Grozdanov (Leiden)

Time: Wednesday October 12th, 2016 - 10:30am

Location: Building 200N, Room 00.004, ULB

Hydrodynamics provides a versatile effective description of phenomena across length scales. I will begin this talk by presenting various aspects of hydrodynamics from the point of view of, first, field theory, and then, holography. In field theory, the focus will be a systematic construction of magnetohydrodynamics using the concept of generalised global symmetries. In holography, I will discuss transport at intermediate ’t Hooft coupling, universality and how one can use hydrodynamic insight to prove bounds on conductivities in strongly disordered materials. I will then move beyond hydrodynamics and discuss what happens to spectra and far-from-equilibrium properties of holographic theories as the ’t Hooft coupling is tuned from infinite to weak coupling. In particular, I will show how (weakly coupled) kinetic theory results emerge, how hydrodynamics becomes applicable over smaller ranges of energies and how hydrodynamic modes are eventually destroyed.

### The N=2 superconformal bootstrap

Speaker Pedro Liendo (DESY)

Time: Wednesday October 5th, 2016 - 1:00pm

Location: Forum E, ULB

In this talk we will present an overview of the N=2 superconformal bootstrap program. We will introduce a protected subsector present in any 4d N=2 SCFT whose correlation functions are captured by a two-dimensional chiral algebra. Thanks to a subtle interplay between 4d and 2d quantities, it is possible to obtain analytic constraints that significantly reduce the allowed theory space. Once an overall picture of the landscape is obtained, one can then use modern numerical techniques and attempt to solve individual theories numerically.

### The Power of Supergravity Solutions

Speaker: Bert Vercnocke (KU Leuven)

Time: Wednesday October 5th, 2016 - 10:30am

Location: Forum E, ULB

In this talk I want to stress how useful solutions of supergravity have been in uncovering new features about quantum gravity and string theory. In that context I will discuss my recent work on black holes, horizonless supergravity solutions and supersymmetry breaking with anti-branes.