String Theory Seminar

In this talk, I will present irregular representations of Kac-Moody algebras where we restrict ourselves to the sl(2,C) case. I show how such irregular representations correspond to irregular Gaiotto-Teschner representations of the Virasoro algebra. The associated KZ equations for conformal blocks with one irregular operator at infinity and their integral representations are derived. By connecting to 2d Liouville theory, I will show how the conformal blocks governed by our irregular KZ equation correspond to 4d Argyres-Douglas theories with surface operator insertions. The corresponding flat connections describe braiding between such operators on the Gaiotto curve.

In this talk I will discuss a new constructive approach for classifying and finding integrable models. By using our method we have found new integrable models in fields ranging from string theory to condensed matter systems. I will discuss some of our new models and applications to for example AdS/CFT.

I will describe how to consider the flat space limit of scattering in AdS relative to a point (where scattering occurs). The kinematics is related to the Wigner-Inonu contraction. In particular, I will discuss how to take the proper limits of wave functions in AdS (times extra dimensions) to understand a notion of in states and out states and how a scattering amplitude should be conceived.  This will make use of the embedding formalism, where the description of these wave functions is simple. I will show how these wave functions are related to other constructions in AdS/CFT and suggest how the Mellin parameters of these other setups arise from integral representations of the wave functions in terms of Schwinger parameters.

Whether flux compactifications can feature scale separation, i.e. cosmological constant can be much smaller than the KK mass scale, is an important and still debated issue. In this talk, I will provide evidence for the existence of such scale-separated, supersymmetric AdS4 vacua in M-theory of the Freund-Rubin type. The internal space has weak G2-holonomy, which is obtained from the lift of AdS vacua in massless type IIA on a specific SU(3)-structure with O6-planes. Such lifts require a local treatment of the O6-planes, therefore going beyond the usual smeared approximation. I will show how this can be achieved while preserving supersymmetry manifestly, extending on previous work.

We will review the classical double copy, which maps exact solutions of classical gauge theories like electromagnetism, to solutions of general relativity. We will cover both the Kerr-Schild and Weyl formulations of this map.  Following a survey of known exact solutions, we will review why a position-space classical solution double copy is even feasible.  We will briefly discuss some perturbative approaches, and then close by relating several gravitational objects (including horizons and Penrose limits) to their gauge theory analogues.

I will discuss the evidence for a spatially modulated instability within the deconfined quark-gluon plasma phase of QCD. This evidence is based on robust predictions from generic bottom-up holographic models, which have been accurately fitted to lattice data. The instability, known as the Nakamura-Ooguri-Park instability, is driven by the Chern-Simons term mandated by QCD flavor anomalies. Contrary to common expectations, this instability occurs universally across holographic models at surprisingly low densities and high temperatures, within the crossover region accessible to lattice and experimental studies. However, the precise extent of the instability region remains sensitive to flavor dependence which is not yet incorporated in current models.

Hydrodynamics is a cornerstone of physics. Formulated initially to describe the flow of water, it has since been successfully applied to model the collective motion of various systems at vastly different length scales, ranging from cosmological distances to the size of an atomic nucleus. In the hydrodynamic description, microscopic and quantum details of specific systems are "averaged over," producing an effective classical theory. Yet, we know that the physical world consists of interacting particles exhibiting inherent quantum properties. Thus, it is natural to ask how such collective behavior emerges from the underlying quantum theory and whether there are scenarios where quantum effects become amplified, influencing macroscopic hydrodynamic processes. In this talk, I will discuss the latest developments in the formulation of relativistic hydrodynamics with spin and quantum anomalies. I will focus on applications to the physics of the quark-gluon plasma, a relativistic quantum fluid created in heavy-ion collisions, which is also believed to have filled the Universe a few microseconds after the Big Bang.

One of the main challenges for string theory phenomenology is to reliable identify consistent four-dimensional vacua of string theory with low or broken supersymmetry. This task is difficult as one expects no such vacua in the asymptotic limits where one sends all coupling constants to zero or (equivalently) all moduli fields to infinity. On the other hand, once these fields take finite values there exist various quantum and string corrections to the low-energy effective action that are difficult to compute and often unknown.

In this talk I will propose an index formulation based on contour integration techniques that relates the existence of critical points of effective potentials to their behaviour at the asymptotic boundaries of the field space. I will demonstrate this technique and discuss possible challenges for the specific example of IIB / M-theory flux compactifications with a one-dimensional complex structure moduli space and the resulting N=1 supersymmetric F-term potential.

In the context of inflationary cosmology a natural question arises on what precedes inflation itself. We review aspects of the Hartle-Hawking (no boundary) and Vilenkin (tunneling) proposals, and their relation to the Wheeler-DeWitt equation. These raise some paradoxes that are complementary in nature and in clash with observations. On the other hand theories of quantum gravity are better defined in the presence of a negative cosmological constant. I will propose a new type of wavefunction of the universe with asymptotically AdS boundary conditions in the far (Euclidean) past. In the semiclassical limit, it describes a Euclidean (half)-wormhole geometry with properties that result in an expanding universe upon analytic continuation to Lorentzian signature. In this context some of the aforementioned phenomenological issues can be resolved.  

I will give an overview of a very rich worldsheet string theory obtained by coupling two Liouville theories with complex central charges. This worldsheet theory enjoys a duality with a double scaled matrix integral and can be used to probe many aspects of string theory and quantum gravity: It can be completely solved from the worldsheet by an analytic bootstrap approach, which effectively consistutes a first principle derivation of the duality with the matrix model. It can also be used to study observables with boundaries in string theory and is a simple laboratory to explore non-perturbative effects in string theory. It is classically equivalent to a theory of dilaton gravity on the worldsheet with a sine potential, which exhibits infinitely many vacua, both with positive and negative cosmological constant and thus provides a rigorous window into the study of such theories. Finally, it can be related to cosmological correlators in dS3 where the matrix model provides a microscopic realization of dS3.

Interacting quantum field theories typically thermalize, leading to the emergence of hydrodynamics at late times. I will talk about (1+1)d QFTs at high and low temperatures, where the proximity to a CFT results in parametrically slow thermalization, with much of the associated dynamics tractable. I will first explain how the UV effective theory – conformal perturbation theory – breaks down universally at late times due to the unsuppressed exchange of stress tensors, giving room for hydrodynamics to emerge. Specialising to the case of large central charge, I will then argue that the IR effective theory – hydrodynamics -- has universal transport coefficients and use this to show that it breaks down at early times due to the existence of thermal CFT excitations. The timescales at which the two effective theories break down agree.

13h30: Tea & Coffee.
14h00: Speaker 1: Andrei Parnachev (Dublin), “Operator product expansion, geodesics and black hole singularities”
15h15: Coffee break.
15h45: Speaker 2: Nabil Iqbal (Durham/UvA),  “Machine learning approach to duality in statistical physics”
17h00: Borrel (drinks and snacks): Cafe Minnaert
 

Compared to their counterparts with enhanced supersymmetry, little is known about the field spaces of theories of gravity with minimal supersymmetry in four dimensions away from asymptotic weak coupling limits. In this talk, I will focus on 4d N=1 theories obtained as compactifications of the heterotic string on Calabi-Yau threefolds and investigate the interior of its field space. In particular, I will focus on certain strong coupling singularities for the heterotic gauge group that arise at the perturbative level and naively cap off the moduli space at finite distance. In the dual M-theory compactification on a Calabi-Yau threefold times an interval the perturbative strong coupling singularity of the heterotic string manifests itself as an end-of-the-world domain wall. In this dual setup, I will discuss the fate of the strong coupling singularity at the non-perturbative level by studying the instanton corrections to the BPS equations of the domain wall which are inherited from the 5d N=1 parent theory. I will argue that these instanton corrections effectively resolve the end-of-the-world singularity of the domain solution which translates into a resolution of the original strong coupling singularity of the heterotic string. 

A notion of distance on the moduli space of low-energy effective field theories is crucial for the Swampland program, and specifically for the Distance Conjecture. In this talk, I will show how geometric flow equations, and in particular generalizations of the Ricci flow, offer a different and elegant viewpoint in the more general case of a scalar field space with potential. After a brief introduction to Ricci flow and its generalization to backgrounds involving a NSNS two-form, I will review the known realization as a gradient flow of the string-effective action and the associated beta-functions, ultimately proposing a suitable notion of distance along the flow as well as a generalized Ricci flow Conjecture. I will discuss examples highlighting the new and intriguing implications as well as the connection to previous work on the role of (diverging) potentials. 

This work is in collaboration with Saskia Demulder and Dieter Lüst.