Past seminars

Academic year : September 2006 - September 2007

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, June 6, 2007 at 13:00

In the quest for a formulation of M-theory, the existence of E11 and E10 hidden symmetries could possibly play an important role. Here we will assume the existence of this E11 algebraic structure and explore some of its consequences. We will construct an infinite multiplet of BPS states for 11D supergravity. All these solutions are related by U-dualities realised via E9 Weyl transformations in the regular embedding E9 in E10, E10 in E11. The way the solutions are constructed is giving a hint on the meaning of this symmetry through the existence of a dual formalism. We finally comment on the significance of theses results beyond the realm of BPS solutions.

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, June 6, 2007 at 10:30

Place:  Leuven, Celestijnenlaan 200D 06.34
Time:   Wednesday, May 30, 2007 at 13:00

After briefly reviewing Dynamical Supersymmetry Breaking (DSB) and the Gauge/Gravity Duality, we discuss examples of metastable DSB states both in gauge theory and in a gravity dual. We then present a set up where the same metastable state can be argued to persist both at weak and strong coupling, i.e. on both sides of the duality.

Place:  Leuven, Celestijnenlaan 200D 06.34
Time:   Wednesday, May 30, 2007 at 10:30

Assuming the standard M-theory/string theory duality relationships, one can obtain powerful constraints on the possible structure of higher derivative terms in the type II effective action. In the ten-dimensional IIA limit, we find that a term D^{2k}R^4 does not receive any perturbative contributions beyond k string loops (for any k>0). Furthermore, the maximum genus h=k contributions are determined exactly by the one-loop eleven dimensional supergravity amplitude. A plausible interpretation of these observations is that the sum of h-loop Feynman diagrams of maximally extended supergravity is ultraviolet finite in dimensions d < 4+6/h - the same bound as for N=4 Yang-Mills.

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, May 23, 2007 at 13:00

I report on work about the construction of consistent Higher-spin theories in odd dimensions D>3, which are defined as Chern-Simons theories based on infinite-dimensional higher-spin algebras. These provide a consistent coupling of higher-spin fields to a particular form of Lovelock gravity. In contrast to the well-known three-dimensional theories they do possess propagating degrees of freedom. As an example for this I consider an AdS(4) Kaluza-Klein solution of the theory in D=5 and show in the first non-trivial case of a spin-3 field that the field equations reduce in the linearisation to the required Fronsdal equations on AdS(4).

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, May 23, 2007 at 10:30

I will discuss an embedding of a known susy breaking mechanism in four dimensional minimal super QCD into a smooth, solvable string theory with five non-compact directions. One of the directions is a semi-infinite "throat" and the susy breaking is apparently localized deep inside the throat. This furnishes a controllable example of a perturbatively stable non-supersymmetric string background. I will argue that in this type of setup, one can answer the question of whether the susy breaking in string theory is spontaneous or explicit, and then proceed to do so for the system under consideration.

Place:  Leuven, Celestijnenlaan 200D 06.34
Time:   Wednesday, May 16, 2007 at 13:00

An appearance of a non-local interaction in the String Field Theory (SFT) is briefly considered and cosmological scenarios built upon the generalized non-local SFT and p-adic tachyons are examined. A general kinetic operator involving an infinite number of derivatives is studied as well as arbitrary parameter p. A late time behavior of the tachyon and the scale factor of the FRW metric in the presence of the cosmological constant or a perfect fluid with w>-1 is constructed explicitly and a possibility of non-vanishing oscillations of the total effective state parameter around the phantom divide is proven.

Place:  Leuven, Celestijnenlaan 200D 06.34
Time:   Wednesday, May 16, 2007 at 10:30

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, May 9, 2007

I will review some aspects of hyperbolic Coxeter groups and explain how these groups arise in the study of gravity close to a spacelike singularity. In this limit (the BKL-limit), the dynamics of gravity can be described as a billiard motion in a region of hyperbolic space, and this region can be identifed with the fundamental Weyl chamber of a Lorentzian Kac-Moody algebra. I will illustrate these phenomena mainly in the context of a simple example, namely that of pure gravity in four dimensions. In the second part of the talk, I will discuss some recent work which is devoted to the study of how the billiard structure changes if the theory is compactified on a non-trivial manifold M. In particular, I will explain that information about the cohomology of M is enough to determine the dynamics in the BKL-limit. It turns out that in many cases the resulting structure is described by a (regular) subalgebra of the original Kac-Moody algebra.

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, May 2, 2007 at 13:00

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, May 2, 2007 at 10:30

After a brief review of noncommutive scalar and gauge field theory, we present new solutions of the latter, in which coincident vortices expand into circular shells. As the theories are noncommutative, the naive definition of the locations of the vortices and shells is gauge-dependent, and so we define and calculate the profiles of these solutions using the gauge-invariant noncommutative Wilson lines introduced by Gross and Nekrasov. We find that charge 2 vortex solutions are characterized by two positions and a single nonnegative real number, which we demonstrate is the radius of the shell. We find that the radius is identically zero in all 2-dimensional solutions. If one considers solutions that depend on an additional commutative direction, then there are time-dependent solutions in which the radius oscillates, resembling a braneworld description of a cyclic universe. There are also smooth BIon-like space-dependent solutions in which the shell expands to infinity, describing a vortex ending on a domain wall.

Reference: hep-th/0702042

Place:  Leuven, Celestijnenlaan 200D 06.34
Time:   Wednesday, April 25, 2007 at 13:00

I shall review what is known about supersymmetric black holes in anti-de Sitter space and their interpretation in N=4 super Yang-Mills theory.

Place:  Leuven, Celestijnenlaan 200D 06.34
Time:   Wednesday, April 25, 2007 at 10:30

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, April 18, 2007 at 13:0

In my talk, I would like to show the dynamical equivalence between the bosonic part of pure type I supergravity in D=10 and a D=1 non-linear sigma-model on the Kac-Moody coset space DE(10)/K(DE(10)), if both theories are suitably truncated. Furthermore, I will discuss the supersymmetry transformations in the framework of this correspondence as well as representations of the generalized Lorentz group K(DE(10)).

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, April 18, 2007 at 10:30

In the context of the gauge/string correspondence, we study the dynamics of flavors by placing a set of (coincident) branes-antibranes on a background dual to a confining color theory. We show how condensation of the resulting open string tachyon is dual to chiral symmetry breaking, and discuss how this framework provides a natural description of various aspects of QCD: global flavor anomalies, the existence of a quark condensate/quark mass relation, the expected Nf^2 Goldstone bosons (for mq=0), the Gell-Mann-Oakes-Renner relation (for small mq), the \eta' mass, and linear confinement for highly excited spin-1 mesons.

Place:  Leuven, Celestijnenlaan 200D 06.34
Time:   Wednesday, March 28, 2007 at 13:00

We study Einstein equations in the degenerate situation where the metric approaches zero. Under some assumptions to define the limit, we show that Einstein equations are not satisfied at g=0. A new term is added to restore this state as a solution. We show that the new term modifies gravity as needed by dark matter.

Place:  Leuven, Celestijnenlaan 200D 06.34
Time:   Wednesday, March 28, 2007 at 10:30

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, March 21, 2007 at 13:00

Four-dimensional Einstein's General Relativity is shown to arise from a gauge theory for the conformal group, SO(4,2). The theory is constructed from the dimensional reduction of the six-dimensional Euler density integrated over a manifold with a four-dimensional topological defect. The resulting action is a four-dimensional theory defined by a gauged Wess-Zumino-Witten term. On shell there is a pure gravitational sector in which the equations of motion are Einstein's field equations and the unique coupling constant in the action is shown to be restricted to take integer values.

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, March 21, 2007 at 10:30

Place:  Leuven, Celestijnenlaan 200D 06.34
Time:   Wednesday, March 14, 2007 at 13:00
Place:  Leuven, Celestijnenlaan 200D 06.34
Time:   Wednesday, March 14, 2007 at 10:30
Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, March 7, 2007 at 13:00

Super-Liouville theory is important for its relation to Non-Critical Superstring theory. The 4-point correlation function of exponential fields is constructed in the N=1 Super-Liouville field theory . This construction involves the so-called Conformal Blocks, for which we suggest a recursion representation. Together with the explicit expression for Structure constants of Operator Product Expension(OPE) it allows to construct the 4-point function and to verify that the OPE algebra of LFT is associative.

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, March 7, 2007 at 10:30

Time:    Wednesday, February 28, 2007

It is known that the conifold's base is a trivial S^2 bundle over S^3. We find a simple, explicit trivialization of this bundle, thus providing global coordinates for both spheres. As an application, we consider various orbifolds of the conifold and we find that the topologies of the quotients are easily determined in our coordinates. These topologies are then checked against gauge theory expectations.

Place:  Leuven, Celestijnenlaan 200D 06.34
Time:   Wednesday, February 21, 2007 at 13:30

We construct a new giant graviton solution in AdS_5 x S^5 by letting expand gravitational waves into the CP^2 base of the S^5 and interpret this configuration as a Kaluza-Klein monopole wrapped around the S^5 and with the Taub-NUT direction in AdS_5. We find that this configuration has minimal energy when put in the center of AdS_5, where it behaves as a massless particle. We show that the construction is actually generic for all AdS_5 x Y_5 spacetimes, with Y_5 a Sasaki-Einstein manifold and we provide a possible field theory dual interpretation for the construction.

Place:  Leuven, Celestijnenlaan 200D 06.34
Time:   Wednesday, February 21, 2007 at 10:30

I will discuss how all the 10d 32-dimensional component spinor supergravities can be found as different real slices of two complex (pseudo)-actions. By writing these in an explicit Lorentz covariant form we discover some new unexpected properties of *-theories. Furthermore I will show how the pseudo-supersymmetry, introduced by Skenderis and Townsend, can be understood in terms of II and II* theories. This talk is based on work in progress and I invite everyone to come and discuss.

Place:  Leuven, Celestijnenlaan 200D 06.34
Time:   Wednesday, February 14, 2007 at 13:30

Place:   Leuven, Celestijnenlaan 200D 06.34
Time:    Wednesday, February 14, 2007 at 10:30

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, February 7, 2007 at 13:30

Compactification can control chaotic Mixmaster behavior in gravitational systems with p-form matter: we consider this in light of the connection between supergravity models and Kac-Moody algebras. We show that different compactifications define "mutations" of the algebras associated with the noncompact theories. We list the algebras obtained in this way, and find novel examples of wall systems determined by Lorentzian (but not hyperbolic) algebras. Cosmological models with a smooth pre-big bang phase require that chaos is absent: we show that compactification alone cannot eliminate chaos in the simplest compactifications of the heterotic string on a Calabi-Yau, or M theory on a manifold of G_2 holonomy.

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, February 7, 2007 at 10:30

Two models of a grainy structure of the planckian substructure of vacuum are reviewed, Sorkin's causets and Wheeler's foam resp. Both are shown to give a possible account of dark energy and inflation. But whereas I have not succeeded in wheedling out an inflaton collective type field in the causet scenario, it is a quite natural construction using modes and foam wherein the fluctuation of the number density of modes of O(m-planck cubed) serves the purpose.

Nevertheless I am dissatisfied by my lack of comprehension of the quantitative inflaton parameters, mass and amplitude during inflation. I shall indicate how one might be able to come to grips with this connundrum through a more thorough understanding of the transplanckian problem encountered in black hole evaporation.

More speculative one cannot get...Yet... Roma non fu fatta in una giornata.

Place:  Leuven, Celestijnenlaan 200C 01.02
Time:   Thursday, December 21, 2006 at 13:30

I will review discoveries in particle physics in the 1950's and 1960's and then describe the influences and events that led to my role in discovering the color degree of freedom in hadronic physics.

Place:  Leuven, Celestijnenlaan 200C 01.02
Time:   Thursday, December 21, 2006 at 10:30

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Thursday, December 14, 2006 at 13:30

Near-horizon extremal black hole solutions can be determined from a variational principle based on the entropy function. I will discuss the relationship of the entropy function to the black hole potential and the attractor mechanism for static extremal black holes of N=2 supergravity in 4 dimensions. As an illustration, I will present solutions to the attractor equations for a one-modulus prepotential associated with the conifold, mentioning their implications in the context of the entropic principle. I will also comment on the entropy function in 5-dimensional N=2 supergravity.

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Thursday, December 14, 2006 at 10:30

We study the effect of inhomogeneities on inflation. Because the problem is in general quite hard, we consider a specific situation that (we believe) captures some of the same physics, by playing local curvature effects against the might of the cosmological constant. We study the classical evolution of bubbles of a new vacuum in an inhomogeneous ambient Universe. For concreteness and tractability, we work with a specific radially inhomogeneous background: the LeMaitre-Peebles cosmology. Using the Israel junction conditions, we evolve the bubble numerically. We find a spectrum of evolution patterns for various choices of inhomogeneity profiles. The preliminary results suggest that inhomogeneities can radically affect the evolution of an inflating region and sometimes trigger recollapse. One motivation for this problem is its possible significance in the landscape context.

Place:  Leuven, Celestijnenlaan 200C 01.02
Time:   Thursday, December 7, 2006 at 13:30

Place:  Leuven, Celestijnenlaan 200C 01.02
Time:   Thursday, December 7, 2006 at 10:30

The dynamics of the scalar fields along the N=1 domain wall solutions of type IIB supergravity compactified on a Calabi-Yau manifold in the presence of RR and NSNS electric and magnetic fluxes is described by gradient flow equations controlled by a superpotential W related to the fluxes. A geometrical interpretation in terms of the mirror symmetric compactification of type IIA is proposed. It relies on generalized Hitchin flow equations of a manifold with SU(3)xSU(3)structure which is fibered over the direction transverse to the domain wall.

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Thursday, November 30, 2006 at 13:30

I study a class of timedependent (via lightcone coordinate) 1/4 BPS solutions in N=2, d=5 gauged sugra with vectors and hypers, containing Domain wall solution as special case. The main application is in the prospective of a timedepedent generalization of AdS/CFT proposed by various authors that is relevant for characterizing holographically lightlike singularities of the gravity solution.

Based on hep-th/0610300.

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Thursday, November 30, 2006 at 10:30

Cosmology applications of supergravity need a term that generates positive energy. This is usually provided by the so-called D-term. However, its presence in supergravity implies restrictions on the remainder of the theory. We discuss these problems and the consequences for cosmic strings and cosmology. We spent some time on the way in which D-terms are produced from D7 branes in string theory in a way that is consistent with the supergravity constraints. This is a new step in the understanding of cosmology models that follow the ideas of the KKLT construction.

Place:  Leuven, Celestijnenlaan 200C 01.02
Time:   Thursday, November 23, 2006 at 13:30

In this talk we discuss cosmological scaling solutions in gauged supergravity. We start by introducing scaling cosmologies and motivate why they are interesting. The examples I will give will be in N=8 gauged supergravity in 4 dimensions. The solutions have the interesting property that they describe geodesic curves on the scalar manifold. The scaling solutions describe eternal accelerating and decelerating power-law universes, which are unstable. Then we quickly outline how the solutions can be seen as 11-dimensional solutions. Finally we shortly comment on the fact that N=2 gauged supergravity allows accelerating stable scaling solutions (whereas N= does not have stable solutions ).

Place:  Leuven, Celestijnenlaan 200C 01.02
Time:   Thursday, November 23, 2006 at 10:30

The precision reached by the recent CMB measurements gives new insights into the shape of the primordial power spectra of the cosmological perturbations. In the context of inflationary cosmology, this implies that the CMB data are now sensitive to the form of the inflaton potential. Most of the current approaches devoted to the derivation of the inflationary primordial power spectra, or to the inflaton potential reconstruction problem, rely on approximate analytical treatments that may break down for exotic models. In this talk, I discuss numerical methods to solve the inflationary evolution of both the background and all the perturbed quantities. The usefulness of such an approach to deal with CMB data is illustrated by analysing the WMAP third year data in the context of single field models. For this purpose, we introduce a new inflationary related parameter encoding the basic properties of the reheating era. This reheating parameter has significant observable effects and provides a self-consistency test of inflationary models that has been overlooked so far.

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Thursday, November 16, 2006 at 13:30

We have studied the problem of deforming the sum of Fronsdal lagrangians for spin1, spin 2 and spin 3 fields, in the antifield formalism. Using these cohomological tools, we have found the only interacting cubic vertices deforming the gauge algebra. These deformations of the gauge algebra are not automatically consistent at second order of deformation and we have found relations on the internal coefficients of the vertices.

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Thursday, November 16, 2006 at 10:30

Place:  Leuven, Celestijnenlaan 200C 01.02
Time:   Thursday, November 9, 2006 at 13:30

We show that there is a natural correspondence between domain walls and cosmologies foliated by slices of maximal symmetry and we obtain first order equations for all such solutions. For Minkowski and AdS-sliced domain walls (flat and closed FLRW cosmologies) we related these first order equations to the existence of (pseudo)Killing spinors and discuss their stability.

Place:  Leuven, Celestijnenlaan 200C 01.02
Time:   Thursday, November 9, 2006 at 10:30

We give evidence in favour of a string/black hole transition in the case of BPS fundamental string states of the Heterotic string. Our analysis goes beyond the counting of degrees of freedom and considers the evolution of dynamical quantities in the process. As the coupling increases, the string states decrease their size up to the string scale when a small black hole is formed. We compute the absorption cross section for several fields in both the black hole and the perturbative string phases. At zero frequency, these cross sections can be seen as order parameters for the transition. In particular, for the scalars fixed at the horizon the cross section evolves to zero when the black hole is formed.

Place:  VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Thursday, October 26, 2006 at 14:00

2006 International Solvay Chair in Physics

Place:  VUB/ULB, Campus Plaine, building A, room A2 120
Time:   Thursday, October 26, 2006 at 11:00

Place:  VUB/ULB
Time:   Thursday, October 19, 2006

Visit the website: http://wsrtn06.na.infn.it/

Time:   Thursday, October 12, 2006

Both the open string sector of Witten's topological B-model on the supertwistor space CP^3/4, and the alternative theory of open twistor strings due to Berkovits, describe perturbative tree amplitudes in N=4 super Yang-Mills. However, in computations of gauge theory loop scattering amplitudes, conformal supergravity modes are unavoidably mixed in with the gauge theory ones. This suggests that the original twistor string models of these authors are inconsistent, and limits their usefulness for the purpose of computing amplitudes as there appears to be no decoupling limit giving pure super-Yang-Mills amplitudes. The aim of my talks will be to present a modification of the Berkovits string which gives Einstein supergravity (with second order field equations for the graviton) coupled to Yang-Mills, and has a limit in which the gravity modes can be decoupled to give pure gauge theory amplitudes. I will first review a number of relevant aspects of twistor theory, including: special features associated with different space-time signatures; supertwistor space; the Penrose transform; the infinity twistor; and Penrose's non-linear graviton construction. I will then review the Witten and Berkovits strings, with emphasis on the latter. The world-sheet formulation of the Berkovits model involves so-called beta-gamma systems; I will describe the symmetries of such systems and their gauging, and explain how the analysis can be applied to the construction of a family of new gauged Berkovits twistor strings which are free from world-sheet anomalies. I will also describe the corresponding spectra in space-time, and show that they give Einstein supergravities instead of the higher derivative conformal supergravities arising in the original twistor string models. The new theories include one with the spectrum of N=8 supergravity, two theories with the spectrum of N=4 supergravity coupled to N=4 Yang-Mills, a family of N>0 models with the spectra of self-dual supergravity coupled to self-dual super-Yang-Mills, and a non-supersymmetric string with the spectrum of self-dual gravity coupled to self-dual Yang-Mills and a scalar.

Place:  Leuven, Celestijnenlaan 200C 01.10
            Leuven, Celestijnenlaan 200D 06.32
Time:   Thursday, October 5, 2006 at 10:30
            Thursday, October 5, 2006 at 13:30

We discuss an instanton solution preserving AdS boundary conditions. It arises in a compactification of M-theory to four dimensions, keeping a single scalar coupled to gravity with a phi^4 potential. We study the holography of this solution in the context of a toy model, where the effective boundary CFT is a conformally coupled scalar with a phi^6 potential in three dimensions. We match bulk and boundary instanton solutions as well as fluctuations around them. Using a form of radial quantization we show that quantum states in the bulk correspond to multiply-occupied single particle states in the boundary theory. We discuss the interpretation of the instanton in the dual CFT as a deformation by a triple-trace operator, and how the intanton signals an instability of the theory under this deformation.

Note the special time: 16:00.

Place:  VUB/ULB, Campus Plaine, building NO, 6th floor, Room 2.NO.610
Time:   Thursday, September 28, 2006 at 16:00

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