String Seminar Series

Infinite distance limits have been observed to enjoy a number of universal properties: they have "logarithmic" metric singularities, are always associated with weak-coupling limits, and are associated with the appearance of a tower of exponentially light fields as described by the Swampland Distance Conjecture. Using information theory, I will explain how the first two of these universal features are a consequence of unitarity, which dictates that infinite distance limits always correspond to limits in which observables factorize and enforces a logarithmic metric singularity. I will also explain why such limits necessarily have dramatic quantum gravitational behavior. Gravity couples universally to stress-energy and obstructs factorization limits. Gravity must then decouple in consistent factorization limits. I will explain how this both provides a bottom-up motivation for the Swampland Distance Conjecture and points towards ways around it. 

This talk will start with an introduction to hydrodynamisation in an FRW universe. I first briefly describe the set-up and show how a hydrodynamic plasma dilutes and falls out of equilibrium due to expansion towards empty de Sitter spacetime [1]. Next, I will show new technical advances that allowed us to dynamically evolve the boundary metric in accordance with the Friedmann equations [2]. Previously unphysical renormalisation constants now become physical parameters and depending on the boundary cosmological constant this leads to de Sitter, asymptotically flat or Big Crunch cosmologies. Time permitting I will show some new results where we have a rolling inflation on the boundary that heats up the boundary QFT.

[1] Jorge Casalderrey-Solana, Christian Ecker, David Mateos and WS, Strong-coupling dynamics and entanglement in de Sitter space, 2011.08194 (SciPost Phys)

[2] Christian Ecker, WS, David Mateos and Jorge Casalderrey-Solana, Holographic Evolution with Dynamical Boundary Gravity, 2109.10355 (JHEP)

In large-𝑁 gauge theories, evidence has emerged recently that between confined and deconfined phases a partially-deconfined phase can appear, in which only a subset of colours deconfine. The existence of such a phase has implications for the map between degrees of freedom under gauge/gravity duality and black hole phase diagrams, where a counterpart to the partially-deconfined saddle should be present. We investigate properties of partial deconfinement on the field theory side, first considering the partially-deconfined saddle of large-𝑁 pure Yang-Mills theory. Here, the colour degrees of freedom split into confined and deconfined sectors. We argue with the use of numerical simulations that a linear confinement potential is generated in the confined sector, implying the formation of flux tubes, whereas the potential is screened in the deconfined sector and behaves instead according to the perimeter law. Furthermore, we find that the onset of partial deconfinement coincides with the breaking of chiral symmetry, providing an order parameter for the partially-deconfined phase. We conjecture that global symmetries can be used to signify partial deconfinement, leading also to an associated order parameter. As another, cleaner example of this, we show that CP symmetry breaking coincides precisely with the emergence of the partially-deconfined phase in supersymmetry-broken N=1 super-Yang-Mills with a theta-angle 𝜃=𝜋, for both large finite 𝑁 and the formal large-𝑁 limit. Finally, we discuss consequences of these findings for holography and the QCD crossover.

Determining the phase structure of Quantum Chromodynamics (QCD) and its Equation of State (EoS) at densities and temperatures realised inside neutron stars and their mergers is a long-standing open problem. I will present a framework for the EoS of dense and hot QCD that describes the deconfinement phase transition between a dense baryonic and quark matter phase via the holographic V-QCD model. This model is then used in fully general relativistic hydrodynamic simulations of binary systems that are consistent with the first ever observed neutron star merger event GW170817 and the consequences on the formation of quark matter and the emitted gravitational wave signal are studied.

The experimental absence of low-energy supersymmetry, together with theoretical swampland arguments, leave the door open for high-energy supersymmetry breaking. Non-supersymmetric string theory provides such an arena, and it is important to investigate its consistency with swampland principles. I will present some of the first results in this direction, discussing brane dynamics in simple flux vacua. A number of swampland criteria hold: de Sitter vacua are obstructed, gravity is the weakest force and (suitably generalized) infinite-distance limits in the space of vacua behave according to the emergent string conjecture, possibly hinting at a mysterious novel realization of S-duality. To wit, at the edges of the space of vacua either extra dimensions decompactify or a unique (D-)string becomes tensionless, and both phenomena occur exponentially fast in terms of a properly defined measure of distance.

We study the vacua of 4d heterotic toroidal orbifolds using effective theories consisting of an overall Kähler modulus, the dilaton, and non-perturbative corrections to both the superpotential and Kähler potential that respect modular invariance. We prove three de Sitter no-go theorems for several classes of vacua and thereby substantiate and extend previous conjectures. Additionally, we provide evidence that extrema of the scalar potential can occur inside the PSL(2,Z) fundamental domain of the Kähler modulus, in contradiction of a separate conjecture. We also illustrate a loophole in the no-go theorems and determine criteria that allow for metastable de Sitter vacua. Finally, we identify inherently stringy non-perturbative effects in the dilaton sector that could exploit this loophole and potentially realize de Sitter vacua.

Gauge-gravity duality plays a key role in understanding quantum gravity and strongly interacting gauge theories, however, lacks a satisfactory microscopic derivation. Fundamental questions such as, how does gravity emerge directly from quantum field theory observables and how to determine which QFTs are holographic, which are not, remain unanswered. In this talk, I propose a primitive form of gauge-string duality based on the worldline formulation of perturbative QFT. In particular we consider L loop quantum corrections to correlation functions in an holographic QFT where a Schwinger parameter is associated to each internal propagator in the corresponding Feynman diagrams. We argue that embedding of the holographic coordinate in string theory emerges from the collection of these Schwinger parameters in the continuum limit of the Feynman diagrams. As a by product, we provide a novel Kallen-Lehmann representation of two-point functions as a sum over boundary-to-boundary propagators of massive bulk scalars in AdSd+1 with masses determined by L. This novel approach can be generalized to arbitrary N. Therefore it might have two potential uses: to provide  i) a non-perturbative approach to quantum gravity in terms of perturbative QFT at finite N, ii) a true bottom-up holographic construction for confining gauge theories like QCD derived directly from QCD amplitudes.

Positive scalar potentials in string effective theories could provide an origin to Dark Energy, responsible for the accelerated expansion of our universe today or during inflation. It is thus crucial to characterise these scalar potentials, namely their slope, their critical points (de Sitter solutions) and the associated stability, as also advocated by the Swampland Program. We will present such characterisations. Going further, we will also discuss negative scalar potentials, and make related observations on anti-de Sitter solutions, in particular on a new mass bound, as well as comments on scale separation.

Within the AdS/CFT correspondence, the entanglement properties of the CFT are related to wormholes in the dual gravity theory. This gives rise to questions about the factorisation properties of the Hilbert spaces on both sides of the correspondence.  We  show how the Berry phase, a geometrical phase encoding information about topology, may be used to reveal similarities between the Hilbert space structure on both sides of the correspondence. Mathematical concepts such as coadjoint orbits play an important role. Moreover, there is a relation to the von Neumann algebras recently studied in the context of AdS/CFT. In addition to its relevance for quantum gravity, this analysis also suggests how to experimentally realise the Berry phase and its relation to entanglement in table-top experiments involving photons or electrons. This provides a new example for relations between very different branches of physics that follow from the AdS/CFT correspondence and its generalisations.

Based on 2109.06190, 2202.11717 and work in progress.

In this talk we introduce particular TQFTs in three dimensions, known as Symmetry Topological Field Theories (or SymTFTs), to identify line defects of two-dimensional CFTs arising from the compactification of 6d (2,0) SCFTs on 4-manifolds M4. The mapping class group of M4 and the automorphism group of the SymTFT switch between different absolute 2d theories or global variants. Using the combined symmetries, we realize the topological defects in these global variants. Our main example is P1 x P1. For N M5-branes the corresponding 2d theory inherits ZN 0-form symmetries from the SymTFT. Moreover, we find duality defects at fixed points of the coupling constant under elements of the mapping class group which together with the ZN symmetries form a TY[ZN] fusion category. We also comment on other Hirzebruch surfaces, del Pezzo surfaces, as well as the connected sum of P1 x P1.