Theoretical Particle Physics

Joint seminars academic year: 2015-2016

Some lessons from the connection between asymptotic symmetries and soft theorems

Eduardo Conde (Seoul National University)
Wednesday, June 1st, 2016 13:00
KUL 200C 01.06 (Aud D)

This talk is devoted to exploring some aspects of the connection between asymptotic symmetries and soft theorems recently proposed by Strominger et al. In the first part I will deal with soft photons, where I will discuss the origin of the two terms appearing in the soft photon theorem from the perspective of symmetries at null infinity. The second part is reserved to soft gravitons, where I will argue for a UV-IR relation that allows to derive the complex-UV behavior of gravity amplitudes from multiple soft limits.

 

Relating Double Field Theory to N=2 gauged supergravity

Erik Plauschinn (LMU Munich)
Wednesday, June 1st, 2016 10:30
KUL 200C 01.06 (Aud D)

In this talk we start by giving a broader overview on T-duality and non-geometry in string theory. In the second part, we show that double field theory "compactified" on a Calabi-Yau three-fold in the presence of fluxes - geometric and non-geometric - gives rise to the scalar potential of N=2 gauged supergravity in four dimensions. [The latter part is based on the paper arXiv:1507.08059.]

 

Tunneling into Microstate Geometries

Daniel Mayerson (Michigan)
Wednesday, May 4th, 2016 13:00
ULB, Salle Solvay

In classical GR, collapsing shells form horizons and thus black holes. On the other hand, quantum information theory arguments (fuzzball/firewall) suggest the existence of additional structure at the black hole horizon, which can only form if the classical GR description breaks down before the collapsing shell reaches horizon size. I will discuss evidence that this is indeed the case: the tunneling amplitude of a collapsing shell of branes into (a specific class of) smooth horizonless microstate geometries indicates that the shell will tunnel into such horizonless configurations well before the horizon has a chance to form. These tunneling amplitudes further seem to imply that tunneling is preferred into microstate geometries with as many "centers", or topologically non-trivial cycles, as possible.

 

The Information Paradox revisited

Malcolm Perry (Cambridge)
Wednesday, May 4th, 2016 10:30
ULB, Solvay Room

We start with  a brief review of the information paradox. We move on to a discussion of the infrared problem in electrodynamics, the vacuum degeneracy and an infinity of conserved charges in electrodynamics. These charges are then shown to be associated with black holes. I then introduce supertranslation and and superrotation charges for black hole spacetimes. One is then led to ask if this is enough to resolve the information paradox.

 

A few algabraic surprises in Anti-de Sitter space

Oleg Evnin (Chulalongkorn University and VUB)
Wednesday, April 27th, 2016 13:00
KUL, 200C 1.06

Questions of nonlinear stability in global AdS space have recently received a significant amount of attention, both as an interesting problem in mathematical general relativity and nonlinear dynamics, and in relation to thermalization studies within the AdS/CFT paradigm. Working with nonlinear perturbation theory (the main technique available for analytic studies in this area) requires a thorough understanding of the properties of linearized AdS fields' mode functions, which are the fundamental building blocks in perturbative treatments of nonlinearities. While complicated explicit expressions for these mode functions are available in the literature, they hide a good deal of the elegant underlying structure dictated by the AdS symmetries. Extending the mode functions in the flat embedding space (in which the AdS space can be realized as a hyperboloid) results in families of homogeneous polynomials, on which the AdS isometries act in a straightforward manner. This suggests a simple proof of important selection rules in nonlinear perturbation theory.

Studies of multiplet structures of the mode functions furthermore reveal a relation to the Higgs oscillator, a well-known quantum-mechanical superintegrable system. This AdS connection leads to an explicit construction of the hidden symmetry generators for the Higgs oscillator, a long-standing problem in mathematical quantum mechanics.

 

Poles in three-point functions

Joseph Minahan (Uppsala)
Wednesday, April 27th, 2016 10:30
KUL, 200C 1.06

Three-point functions for primary operators in conformal field theories should be nonsingular.  Yet for certain operators in N=4 super Yang-Mills one seems to find poles in these correlators in the strong coupling regime.  By considering a simple field theory model I will explain the presence of these poles.

 

Higher-Spin Symmetries: from quantum gravity to boiling water

Evgeny Skvortsov (LMU Munich)
Wednesday, April 20th, 2016 13:00
UMons, Building Pentagone, OA.11

Higher-spin symmetries are infinite-dimensional extensions of the space-time symmetries. They are global symmetries of free CFT's and gauge symmetries of higher-spin theories. Moreover, when broken in a smart way higher-spin symmetry seems to be present in the physically interesting systems like the 3d Ising model. After reviewing various properties and implications of higher-spin symmetries on both sides of the AdS/CFT duality I will show how broken higher-spin symmetry determines to the lowest order the critical indices of the Wilson-Fisher CFT's via the multiplet recombination.

 

Defects and universality in applications of AdS/CFT to condensed matter physics

Johanna Erdmenger (MPI Munich)
Wednesday, April 20th, 2016 10:30
UMons, Building Pentagone, OA.11

We present a recent holographic model of a magnetic impurity interacting with a strongly coupled system. This corresponds to a Kondo model in which the electrons are strongly correlated. The holographic model involves an RG flow from a UV to an IR fixed point.
We calculate the entanglement entropy in this model and show that it agrees with previous field theory results in the large N limit. We also study time dependence and quantum quenches. The response of the system to a quantum quench is determined by the quasinormal modes.
Moreover, we show recent results on universality in AdS/CFT applications in relation to the conductivity in systems with broken translation invariance.

 

Bootstrapping the S-matrix

Miguel Paulos (CERN)
Wednesday, April 13th, 2016 13:00
ULB, solvay Room

Bootstrap methods have been used to derive detailed predictions for critical exponents in several conformal field theories. On the other hand, integrability techniques can be used to completely solve certain massive 2d quantum field theories. In this talk we will show that there is a non-trivial overlap between these two approaches. By bootstrapping 2d CFTs dual to massive quantum field theories in AdS space, we are able to bound the S-matrices of the latter. These bounds are saturated by known integrable theories such as the sin Gordon model.

 

A goldstino at the bottom of the cascade

Matteo Bertolini (SISS, Trieste)
Wednesday, April 13th, 2016 10:30
ULB, Solvay Room

In this talk I will discuss the holographic description of supersymmetry breaking vacua in N=1 quiver gauge theories arising from D-branes at CY singularities, focusing on the conifold theory as a protoype. There exists a two-parameter family of supersymmetry breaking solutions with the asymptotic of the supersymmetric Klebanov-Strassler background. Within this family, we show that those (and only those) solutions related to antiD-branes at the tip of the conifold correspond to dual field theory vacua where a goldstino mode is present and supercurrent Ward identities hold. Our findings do not depend on the IR singularity of the dual backgrounds, nor on its resolution. As such, they constitute a check for the existence of supersymmetry breaking vacua in the conifold cascading gauge theory, and in a large class of N=1 quiver gauge theories.

 

Resolved Gravity Duals of N=4 Quiver Field Theories in 2+ 1 d

Willian Cottrell (Wisconsin)
Wednesday, March 16th, 2016 13:00
ULB, solvay Room

 In this talk, we will describe the IIA / M-Theoretic dual of a general N=4, 2+1d gauge theory with U(N) gauge groups. Due to the presence of non-compact cycles, rules for charge quantization and the mapping to field theory are rather subtle. We will provide this mapping in detail and then explain an apparent discrepancy between our expectations for susy breaking in field theory and in gravity.

 

Old and new scaling laws in quantum quench

Sumit Das (Kentucky)
Wednesday, March 16th, 2016 10:30
ULB, Solvay Room

Systems with time dependent couplings which interpolate between constant values and involve critical points are expected to display universal features. Recently, holographic methods have been used to understand some key aspects of such quantum quenches, which have applications to many areas of physics. This has led to insight into key aspects of well known scaling behaviour for slow quenches which cross or approach critical points, viz. Kibble-Zurek scaling. In the opposite regime of fast quench, holographic methods have uncovered new scaling laws which have been subsequently shown to hold in general quantum field theories. This talk will review some of these exciting developments.

 

Scrambling of locally pertubed thermal states

Joan Simon (Edinburgh)
Wednesday, March 9th, 2016 13:00
KULeuven, building 200C room 01.06 (auditorium D)

We compute the evolution of quantum entanglement in locally perturbed thermal states in 2d CFTs at large c. We derive the scrambling time scale from the condition of vanishing mutual information and match these results from holographic calculations.

 

Correlation Functions of Coulomb Branch Operators

Zohar Komargodski
Wednesday, March 9th, 2016 10:30
KULeuven, building 200C room 01.06 (auditorium D)

We discuss the correlation function of chiral operators in N=2 supersymmetric four-dimensional gauge theories. We show that the problem can be reduced to certain determinants, which are exactly computable using localisation techniques. Hence, we determine the non-perturbative chiral ring in N=2 theories. This leads to new connections with integrability, resurgence theory, and the Bootstrap program.

 

Field Theory Description of Topological States of Matter

Andrea Cappelli (INFN Florence)
Wednesday, March 2nd, 2016 13:00
ULB, Solvay Room

I introduce the topological states of matter, starting from the example of the quantum Hall effect. Then I discuss the massless edge states and their field theory description. Finally, I compare the ten-fold classification of non-interacting states with the characterization in terms of field theory anomalies.

 

A Monotonicity Theorem for Two-dimensional Boundaries and Defects

Andy O'Bannon (Southhampton)
Wednesday, March 2nd, 2016 10:30
ULB, Solvay Room

I will present a proof for a monotonicity theorem, or c-theorem, for a three-dimensional Conformal Field Theory (CFT) on a space with a boundary, and for a higher-dimensional CFT with a two-dimensional defect. The proof is applicable only to renormalization group flows that preserve locality, reflection positivity, and Euclidean invariance along the boundary or defect, and that are localized at the boundary or defect, such that the bulk theory remains conformal along the flow. The method of proof is a generalization of Komargodski’s proof of Zamolodchikov’s c-theorem. The key ingredient is an external “dilaton” field introduced to match Weyl anomalies between the ultra-violet (UV) and infra-red (IR) fixed points. Reflection positivity in the dilaton’s effective action guarantees that a certain coefficient in the boundary/defect Weyl anomaly must take a value in the UV that is larger than (or equal to) the value in the IR. This boundary/defect c-theorem may have important implications for many theoretical and experimental systems, ranging from graphene to branes in string theory and M-theory.

 

Black resonators & geons: evading black hole theorems and the path to cosmic censorship violation

Oscar Dias (Southhampton)
Wednesday, February 24th, 2016 13:00
ULB, Solvay Room

Superradiance is the wave analogue of the Penrose process whereby energy and angular momentum can be extracted from a black hole. In a confined background - for example AdS - the reflective asymptotic boundary conditions lead to an instability. The zero-modes of this instability signal a bifurcation to novel black holes that are not time independent neither axisymmetric. Instead they are time-periodic. Their single helical isometry evades the assumptions of Hawking's rigidity theorem, that would otherwise rule-out their existence. We construct these 'black resonators' within Einstein-AdS gravity, thus proving that Kerr-AdS is not the unique family of stationary solutions of the theory. The zero horizon radius limit of these resonators can be a 'geon', i.e. a time-periodic gravitational soliton that describes the back-reaction of a gravitational wave in AdS. Furthermore, geons and their collisions are key players of the weakly turbulent nonlinear instability of AdS. From the properties of the black resonators and geons we then argue and conjecture that the time evolution of the superradiant instability leads to a violation of the cosmic censorship.

 

Supersymmetric AdS backgrounds and their moduli spaces

Jan Louis (DESY)
Wednesday, February 24th, 2016 10:30
ULB, Solvay Room

We determine fully supersymmetric AdS backgrounds of various supergravities, discuss their moduli spaces and their role in the AdS/CFT correspondence.

 

Resumming instantons in N = 2SCFT

Marco Billo (INFN Turin)
Wednesday, February 17th, 2016 13:00
KULeuven 200N room 00.01

The validity of the modular anomaly equation satisfied by the the prepotential of 4-dimensional N = 2* theories is related to S-duality. The recursion relations that follow from the modular anomaly equation allow one to write the prepotential in terms of (quasi)-modular forms, thus resumming the instanton contributions. These results can be checked against the microscopic multi-instanton calculus in the case of classical algebras, but are valid also for the exceptional E 6,7,8 , F 4 and G 2 algebras, where direct computations are not available. We briefly discuss, from this point of view, the Nf = 2Nc theories.

 

Non-relativistic Hydrodynamics and Lifshitz Black Branes

Jelle Hartog (ULB)
Wednesday, February 17th, 2016 10:30
KULeuven 200N room 00.01

Like a relativistic field theory couples to Riemannian geometry, the natural geometry to which a non-relativistic field theory (Galilean, Lifshitz, etc) couples is called torsional Newton-Cartan (TNC) geometry. This is also the geometry that one finds on the boundary of a Lifshitz space-time. I will explain what TNC geometry is and then study fluids on it, mostly at the level of perfect fluids. I will show that one can obtain Lifshitz perfect fluids by a twisted null reduction of scale invariant Galilean (Schroedinger) perfect fluids. In the last part of the talk I will discuss a holographic realization of such a fluid in terms of Lifshitz black branes with momentum in a theory with a broken U(1) symmetry.

 

A topological gauge theory for entropy and the emergence of hydrodynamics

Felix Haehl (Durham)
Wednesday, February 10th, 2016 13:00
KULeuven 200N room 00.01

Systematically understanding effective field theory of systems in mixed states (or systems with dissipation) is of crucial importance to many problems (such as off-equilibrium statistical physics, hydrodynamics, entanglement, gravity with horizons etc). It turns out that the framework of hydrodynamics is constraining enough to make progress on this problem. On the other hand, it is complex enough to unveil a remarkable universal structure underlying such systems. In this talk, I focus on the question how the general theory of hydrodynamics emerges as a low-energy effective field theory from Schwinger-Keldysh path integrals. In particular, I will describe how unitarity of Schwinger-Keldysh description generically leads to a topological (super)symmetry. Further, I will argue that for near-thermal systems the macroscopic entropy current can be understood as the symmetry current of an emergent U(1) gauge invariance, which implements microscopic KMS relations. I will also outline how understanding hydrodynamics as a twisted supersymmetric sigma-model is expected to lead to a very simple effective action, which describes all of hydrodynamic transport (including dissipation) consistent with the Second Law. Via AdS/CFT these structures are conjectured to underlie the universal low-energy sector of an effective open string theory living at the black hole horizon.

 

Gauged Supergravity for AdS and Flat Space

Alessandra Gnecchi (KUL)
Wednesday, February 10th, 2016 13:00
KULeuven 200N room 00.01

In this talk I will underline aspects of gauged Supergravity solutions in four dimensional Anti de Sitter and flat space, as Supergravity theories with gauging admit flat as well as curved-space vacua. I will present recent studies on black holes in Anti de Sitter, both in the supersymmetric as well as non-extremal case, with particular interest in their string-theory origin and holographic interpretation. I will then consider flat-gaugings, for which the theory still possesses a Minkowski vacuum, and explain how they affect black hole physics, in particular their microscopic interpretation, according to the Rholography construction.

 

Localization on twisted spheres and supersymmetric GLSM in 2d

Cyril Closset (SUNY Stony Brook)
Wednesday, December 9th, 2015 13:00
KULeuven 200C room 01.27

I will revisit the A-twisted gauged linear sigma model (GLSM) in the case of (2,2) supersymmetry in two dimensions, and its Omega-background deformation. Exact results for correlation functions on the sphere can be obtained in terms of Jeffrey-Kirwan residues on the Coulomb branch, which has a number of interesting applications. I will also explain a generalization to (0,2) supersymmetric GLSMs with a (2,2) locus.

 

Supersymmetric and Non-Supersymmetric Black Hole Microstates

David Turton (IPhT, Saclay)
Wednesday, December 9th, 2015 10:30
KULeuven 200C room 01.27

The study of black hole microstates in string theory is an important problem, which has implications for the black hole information paradox. Of particular interest are BPS and near-BPS D1-D5-P bound states in string theory, which can be studied holographically. I will first review the information paradox, and the status of our knowledge of three-charge black hole microstates. I will then present work which identifies the CFT description of a class of BPS and non-BPS D1-D5-P supergravity solutions, including both regular solutions and solutions with conical defects. The CFT states exhibit fractionation of the momentum charge. I will then describe work in progress on constructing more general classes of black hole microstate geometries which are dual to CFT states involving fractionated momentum. I will close with a discussion of the physics of an observer falling into a black hole.

 

SL(2) \times R^+ exceptional field theory and F-theory

 

Chris Blair (VUB)
Wednesday, December 2nd, 2015 13:00
ULB, Solvay Room

I will discuss attempts to geometrise the duality symmetries of string theory and M-theory. I will concentrate on exceptional field theory, where one reformulates supergravity on an extended spacetime on which the U-duality group acts geometrically, and on F-theory, where an auxiliary torus is introduced to geometrise the SL(2) S-duality of type IIB. In particular, I will describe the construction of the exceptional field theory which makes manifest the U-duality group SL(2) \times R^+ in 9 dimensions by introducing a three-dimensional extended space, and discuss the relationship of this to F-theory.

 

Bulk quartic vertices from boundary four-point correlators

Xavier Bekaert (Tours)
Wednesday, December 2nd, 2015 10:30
ULB, Solvay Room

Clarifying the locality properties of higher-spin gravity is a pressing task, but notoriously difficult due to the absence of a simultaneously weakly-coupled and weakly-curved regime. An explicit toy example where this question can be addressed is the quartic self-interaction of the AdS scalar field present in the higher-spin multiplet. We investigate this issue in the context of the holographic duality between the minimal bosonic higher-spin theory on AdS4 and the free O(N) vector model in three dimensions. he exact explicit form of the derivative expansion of the bulk scalar quartic vertex is obtained from the field theory four-point function of the dual single-trace operator.

 

Large Field Inflation - String Theory & Phenomenology

Alexander Westphal (DESY, Hamburg)
Wednesday, November 25th, 2015 13:00
KULeuven, building 200C room 01.27

The recent combined the results from the Planck satellite and the Keck/BICEP2  telescopes have left a visibly reduced but still sizable window for large-field inflation. We will discuss its status in the context of string theory, looking both at the underlying structure visible in recent progress, and the more generic phenomenological consequences. We will also comment on recent progress towards explicit local constructions for the more 'rigid' versions of axion monodromy in warped throats, and comment on the discussion around the weak gravity conjecture.

 

Integrability and Exact results in N=2 gauge theory

Elli Ponomi (DESY and National Technical University of Athens)
Wednesday, November 25th, 2015 10:30
KULeuven, building 200C room 01.27

Any N=2 gauge theory in four dimensions contains a set of local operators made only out of fields in the N=2 vector multiplet that is closed under renormalization to all loops, with SU(2,1|2) symmetry. We present a diagrammatic argument that for any planar N=2 theory the SU(2,1|2) Hamiltonian acting on infinite spin chains is identical to all loops to that of N=4 SYM, up to a redefinition of the coupling constant g^2 → f(g^2). Thus, this sector is integrable and anomalous dimensions can be read off from the N=4 ones up to this redefinition. The functions f(g^2) dubbed as effective couplings encode the relative, finite renormalization between the N=2 and the N=4 gluon propagator and thus can be computed in perturbation theory using Feynman diagrams. For each N=2 theory exact effective couplings can be obtained by computing different exact results for localizable observables such as Wilson loops and the Bremsstrahlung function and by comparing them with their N = 4 counterparts.

 

D-brane moduli stabilisation and large field inflation

Fernando Marchesano (Universidad Autónoma de Madrid)
Wednesday, November 18th, 2015 13:00
ULB, Solvay Room

We revisit the standard counting of D-brane moduli in 4d string compactifications. We discuss some previously overlooked stabilisation mechanism, which in particular fixes D-brane Wilson lines. At low energies such stabilisation mechanism translates into a bilinear open-closed superpotential, similar to the ones used in the supergravity literature to build models of large field inflation. We then study to what extent string compactifications including this superpotential resemble such supergravity models of inflation.

 

Entanglement dynamics in 2d CFT

Alice Bernamonti (KULeuven)
Wednesday, November 18th, 2015 10:30
ULB, Solvay Room

I will present recent results on the spreading of entanglement following a quantum quench to 2d CFT. In particular, I will show that entanglement scrambling occurs generically in these theories and discuss how this depends on the value of the central charge.

 

Universal entanglement of singular surfaces

Pablo Bueno (KULeuven)
Wednesday, November 4th, 2015 13:00
VUB, room 1G003

The entanglement entropy in three-dimensional conformal field theories (CFTs) receives a logarithmic contribution characterized by a regulator-independent function a(θ) when the entangling surface contains a sharp corner with opening angle θ. In the smooth-surface limit, (θ → π), this corner contribution vanishes as a(θ) = σ(θ−π)^2. I will review our recent conjecture that for any three-dimensional CFT, this corner coefficient σ is determined by C_T, the coefficient appearing in the two-point function of the stress tensor through the relation σ/CT = π^2/24. I will also describe the extension of this conjecture to general Rényi entropies and higher-dimensional CFTs, and comment on the implications of our findings.

 

A quantum mechanical model for Holography

Troels Hamark (Niels Bohr Institure, Copenhagen)
Wednesday, November 4th, 2015 10:30
VUB, room 1G003

We introduce a new type of quantum mechanical theory called Spin Matrix theory. It is a generalization of nearest-neighbor spin chain theories. We show that Spin Matrix theory arise from N=4 super Yang-Mills theory near certain zero-temperature critical points. We find that Spin Matrix theory contains a variety of phases that mimics that of the AdS/CFT correspondence, and hence gives a quantum mechanical model of the AdS/CFT correspondence. Finally, we sugges that Spin Matrix theory by itself can describe a Holographic correspondence.

 

Cold holographic matter and a color group-breaking instability

Javier Tarrio (ULB)
Wednesday, October 28th, 2015 13:00
KULeuven 200C, room 01.27

I will describe holographic models, obtained from string theory, of strongly coupled super Yang-Mills theories at zero temperature with a finite density of matter. This region of the phase diagram of YM theories is little understood due to the lack of reliable QFT computations. In the gravity side the technical challenge lies in the backreaction of the matter sector. I will also comment on some signals of instabilities towards the dynamical breaking of the YM gauge group, leading to color superconductivity.

 

Geometry, Modularity and Entanglement in WCFT

Diego Hofman (University of Amsterdam)
Wednesday, October 28th, 2015 10:30
KULeuven 200C, room 01.27

Warped Conformal Theories (WCFTs) are 2 dimensional non relativistic theories with enough structure to obtain exact results of comparable power to usual 2 dimensional CFTs. As they are non-relativistic they do not couple naturally to Riemannian geometry but, instead, to a particular version of Newton-Cartan geometry. The study of these geometric features is of great importance as it sheds light on the meaning of Holography away from AdS spaces and it allows for the extension of concepts like modular invariance to a non-relativistic setup. Furthermore, we can obtain exact results like entanglement entropies using these tools. These models might also illuminate the path to a fully covariant formalism for CFTs with an extended W_N algebra.

 

Number theory meets Higgs physics

Claude Duhr (UCL & CERN)
Wednesday, October 21st, 2015 13:00
UMons, Pentagone Building, room 0A.11

Multi-loop computations in QCD are notoriously difficult due to the complexity of the integrals involved. Inspired by results from modern number theory and algebraic geometry, a lot of progress has recently been made regarding the computation of multi-loop integrals and scattering amplitudes. I will discuss various new approaches to the computation of multi-loop integrals and scattering amplitudes, and illustrate them on the first computation of a quantity at N3LO in perturbative QCD, the inclusive Higgs-boson cross section in gluon fusion at N3LO.

 

Supergravity and the cosmological constant

Eric Bergshoeff (University of Groningen)
Wednesday, October 21st, 2015 10:30
UMons, Pentagone Building, room 0A.11

I will discuss under which conditions a cosmological constant can be added to a given supergravity theory. In the case of a positive cosmological constant I will show how the results are related to brane supersymmetry breaking using an anti-D3-brane. The talk will be arranged to try to have as little overlap as possible with the recent seminar by Renata Kallosh.

 

Topological defects in open string field theory

Martin Schnabl (AS CR, Prague)
Wednesday, October 14th, 2015 13:00
VUB, D0.05

We review some key aspects of topological defects in conformal field theory and show how they act on boundary fields generalizing results of Graham and Watts. This action can then be used in open string field theory to relate its classical solutions for different choices of boundary conditions.

 

Causality in Lovelock theories of gravity

Harvey Reall (University of Cambridge)
Wednesday, October 14th, 2015 10:30
VUB, D0.05

Lovelock theories of gravity (e.g. Einstein-Gauss-Bonnet) are an interesting alternative to General Relativity in higher dimensions. They have the unusual property that gravity can travel faster or slower than light so causality is not determined by the light cone of the metric. I will explain how causality is defined by characteristic hypersurfaces and discuss some examples in which these hypersurfaces can be determined. I will discuss problems with arguments asserting that faster than light propagation implies causality violation. In particular, I will show that small black holes in Einstein-Gauss-Bonnet theory cannot be boosted arbitrarily close to the speed of light, which prevents one from building a time machine in this theory.

 

Disorder in AdS/CFT

Leopoldo Pando-Zayas (University of Michigan)
Wednesday, October 7th, 2015 13:00
KULeuven 200N, room 00.44

We consider a direct implementation of disorder in the context of the AdS/CFT correspondence. In particular we discuss its implementation in the context of  holographic superconductors.  We study the behavior of the superfluid density and of the conductivity as a function of the strength of disorder. We observe that the behavior of the order parameter close to the transition is not mean-field type as in the clean case, rather we find robust agreement with BKT for this disorder-driven smeared transition

 

Our universe and de Sitter Supergravity

Renata Kallosh (Stanford University)
Wednesday, October 7th, 2015 10:30
KULeuven 200N, room 00.44

I will describe the standard cosmological model and the future experiments designed to test it. The early universe appears to be well described by the near de Sitter inflationary cosmology, up to slow roll parameters. Our current acceleration is likely to be described by a positive cosmological constant. I will present the recent work on  supersymmetric KKLT construction of de Sitter vacua due to anti-D3 brane with fermions on the world-volume and the relation to Volkov-Akulov goldstino. A locally supersymmetric coupling of the relevant nilpotent chiral multiplet to supergravity will be presented, which even in absence of scalars has de Sitter vacua, avoiding the long term no-go theorems on dS supergravity.