UvA String Theory Journal Club Wiki

Here we will keep a list of papers for discussion at the journal club. If you see a paper that seems interesting during the week, just add it to the list and the organizer will bring it up during the journal club on Friday. To add a paper, just click on the "edit" button at the bottom of the page.

Coming weeks

12:30 Friday 12 May, Jasper van Wezel

Black hole radiation without evaporation

Abstract: Hawking’s discovery that black holes emit thermal radiation in the presence of quantum fields is a cornerstone of the modern theory of gravity, explaining why these objects appear to act as thermodynamic heat baths. However, it also introduced a paradox, suggesting that black holes evaporate into thermal radiation despite such complete annihilation not being consistent with the laws of quantum dynamics. We provide a new perspective on this old problem by introducing a model for a particular black hole that does not evaporate, despite having a well-defined temperature. This challenges the existing maxim that black hole singularities with nonzero surface gravity and entropy must produce Hawking radiation. Making use of the global definition of quantum fields allowed by the model and comparing to standard derivations of Hawking radiation for a Schwarzschild black hole, we furthermore suggest that the existence of remnant radiation at infinity has not been conclusively shown for any black hole.

Authors: Lotte Mertens and Jasper van Wezel

12:30 Friday 31 March, Igal Arav

Supersymmetric Spindles in Holography

Abstract: Spindles are two-dimensional orbifolds which are topologically a sphere, but with quantized conical deficits at its two poles. Recently interesting new solutions of supergravity have been found in the context of holography with geometries based on spindles. These solutions feature some interesting properties: Their uplifts to 10d or 11d supergravity is smooth; and supersymmetry is not realized on them via the standard topological twist. I will review some general features of spindles, and discuss some of the recently constructed holographic spindle solutions and their dual SCFTs.

Papers to be discussed:

  1. Glue-on AdS holography for TTbar-deformed CFTs (Apolo, Hao, Lai, Song)

12:30 Friday 10 March, Rishi Mouland

Black Holes in the Dual of Quantum Mechanics

Abstract: I will motivate and study a holographic duality between on one hand a theory of superconformal quantum mechanics, and on the other, M-theory on a particular background. I will exhibit a broad family of black hole solutions in the bulk theory, which fall into a wider class of “ultra-spinning” black holes in various backgrounds. Through an asymptotic study of the superconformal index in the supergravity regime, I will provide a precise microstate counting for those ultra-spinning black holes that are supersymmetric. Finally, I will briefly discuss the relation of conformal quantum mechanics and ultra-spinning black holes to notions of holomorphic factorisation and gravitation blocks, for holographic CFTs in four and six dimensions.

Papers to be discussed:

  1. A proof of a conjecture of Gukov-Pei-Putrov-Vafa (Murakami)
  2. Resurgence analysis of the Adler function at order 1/N_f^2 (Laenen, Marinissen, Vonk)
  3. Is Action Complexity better for de Sitter space in Jackiw-Teitelboim gravity? (Anegawa, Iizuka, Sake, Zenoni)

12:30 Friday 03 March, Sergio Aguilar

Euclidean axion wormholes in de Sitter space

Abstract: We study Euclidean wormholes supported by axion flux in de Sitter space. The corresponding geometry describes a handle attached to the Euclidean sphere. We compute the on-shell action and verify perturbative stability. Unlike Euclidean axion wormholes in flat or AdS space, the de Sitter embedding allows for a clear understanding thanks to the quantum cosmological setup where wormholes describe no-boundary probabilities to nucleate de Sitter universes with axion flux threading the spacelike slice. The wormhole size and axion flux is bounded and the largest wormhole (``Nariai" wormholes) corresponds to the nucleation of a de Sitter universe with critical axion density.

Papers to be discussed:

12:30 Friday 17 February, Edward Morvan-Benhaim

Actions and Constraint Instantons in de Sitter space-times

Abstract: In this talk, I will present and summarize the results of two papers (arXiv:2203.06155v3 and arXiv:2212.12713v1). I will start with a general introduction of the techniques that we used, by first solely focusing on empty de Sitter space-time, before moving on and considering configurations with a black hole (so-called Schwarzschild-de Sitter) and finish by describing how introducing electromagnetic charges changes the picture yet again (Reissner-Nordström-de Sitter). The aim of the two papers is to provide a well-defined technique to compute actions, even when the space-time manifold is not entirely smooth , i.e. in the presence of conical singularities. This is most easily achieved with the help of the so-called Smarr relation, an integrated version of the 2nd law of thermodynamics. Armed with the general expressions for the actions, we proceeded by computing the tunneling rates to transition from one space-time configuration to another in the context of constraint instantons.

Papers to be discussed:

  1. On the universal behavior of TTbar-deformed CFTs: single and double-trace partition functions at large c (Apolo, Song, Yu)
  2. Isometric evolution in de Sitter quantum gravity (Cotler, Jensen)

12:30 Friday 3 February, Soner Albayrak

Conformal QED in 3d: The Numerical Bootstrap Approach

Abstract: The IR fixed point of quantum electrodynamics in three dimensions (QED3) is expected to be gapped for a small global symmetry group and strongly interacting for a large one. Concretely, QED3 with the flavor group SU(N) flows to a free theory for small N and to a conformal one for large N, whereas the critical flavor number is suggested to be anywhere between 1 and 10 by various theoretical and computational methods. This phase space of QED3 is very appealing, as it shows analogues of chiral symmetry breaking and confinement and hence can be used as a toy model for the quantum chromodynamics in four dimensions. Furthermore, QED3 is expected to be a part of 3d fermion-boson duality web, thus a better grasp of this theory is immediately useful for other important 3d models. Finally, QED3 by itself has direct applications in condensed matter systems, particularly in the description of Dirac spin liquids. Despite all these, the available methods to analyze QED3 have been rather insufficient as the theory is strongly coupled. In this talk, I'll detail how the non-perturbative approach of numerical conformal bootstrap differs from the other methods and what we have learned about QED3 through bootstrap studies.

Papers to be discussed:

  1. Yangian Symmetry in Holographic Correlators (Rigatos & Zhou)
  2. A note on the admissibility of complex BTZ metrics (Basile, Campoleoni, Raeymaekers)
  3. CFTs dual to quantum gravity with strongly coupled matter (Apolo, Belin, Bintanja, Castro, Keller)

12:30 Friday 20 January, Giovanni Tomaselli

Compact stars in Quantum Field Theory

Abstract: Very compact stars seem to be forbidden in General Relativity. While Buchdahl's theorem sets an upper bound on compactness, further no-go results rely on the existence of two light rings, the inner of which has been associated to gravitational instabilities. However, little is known about the role of quantum fields in these strong gravity regimes. Working in the probe approximation where the backreaction is ignored, we show that the trapping of modes around the inner light ring leads the renormalized stress tensor of Conformal Field Theories to diverge faster than the classical source in the Buchdahl limit. This leads to the violation of the Null Energy Condition, as well as the isotropy assumption used in Buchdahl's theorem. The backreaction of quantum fields in this regime therefore cannot be ignored. This happens as the star's surface approaches the Buchdahl radius 9GM/4 rather than the Schwarzschild radius, with the quantum fields having support in a small region around the center, becoming negligible at the surface. These are generic quantum features and do not depend on the details of the interactions. Our findings open a way for further investigation into the role of QFT in astrophysics.

Papers to be discussed:

  1. Action, entropy and pair creation rate of charged black holes in de Sitter space (Morvan, van der Schaar, Visser)
  2. Quantum groups, non-commutative AdS2, and chords in the double-scaled SYK model (Berkooz, Isachenkov, Narayan, Narovlansky)

12:30 Friday 13 January, Dominik Neuenfeld

Seeing Particle Production

Abstract: The physics of gapped quantum field theories is encoded in their S-matrix. Unfortunately, beyond perturbation theory and integrability we have limited tools to study its properties. I will discuss work in progress on using the time-dependent variational principle for matrix product states to study the S-matrix of Ising Field Theory (IFT). IFT is a parametrized family of strongly coupled non-integrable quantum field theories in 1+1 dimensions, interpolating between massive free fermion theory and Zamolodchikov's integrable massive $E_8$ theory. Numerical simulations allow us to extract various quantites such as scattering probabilities, the phase shift, but also resonances. I will also discuss attempts to find evidence for a conjecture by Zamolodchikov about the high energy behavior of the scattering amplitude of the lightest particle.

Papers to be discussed:

  1. General relativistic fluid spheres (Buchdahl)

12:30 Friday 16 December, Alexander van Spaendonck

From Painlevé to CFT and beyond

Abstract: In this talk I’ll review the connection between three distinct topics: the study of Fuchsian systems, which give rise to the Painlevé equations, two-dimensional conformal field theories and N=2 supersymmetric gauge theories. In the first part of the talk we’ll review what Fuchsian systems are and how their isomonodromic deformation gives rise to tau-functions that we can identify with N-point correlators in a 2D CFT. In the second part, we’ll discuss how these Fuchsian systems can also be related to the Hitchin systems that encode the BPS spectrum of N=2 gauge theories, and formulate a dictionary between both systems. This talk is mainly based on the papers [1207.0787] and [1612.06235].

Papers to be discussed:

  1. Focusing bounds for CFT correlators and the S-matrix (Hartman, Jiang, Sgarlata, Tajdini)
  2. QFT in AdS instead of LSZ (van Rees, Zhao)

12:30 Friday 09 December, Boris Post

Quantum gravity meets statistical physics

Abstract: In this JC talk I will explore to what extent bulk semiclassical gravity provides a statistical description of the microscopic boundary CFT. I will discuss various notions of coarse-graining, state averaging, ensemble averaging and operator averaging, and propose a unifying framework for these notions built around a principle of maximum entropy. The focus of the talk will be to define a probability distribution on the space of density matrices which is the 'most agnostic' about the UV complete microscopic theory, while still reproducing expectation values of low energy observables up to exponential precision in the thermodynamic entropy. This leads to a state averaging ansatz, similar in spirit to and compatible with the eigenstate thermalization hypothesis (ETH). I will end with a discussion of how variances in this ensemble are related to wormholes in gravity. The ideas in this talk are based on forthcoming work together with Jan de Boer, Diego Liška and Martin Sasieta.

Papers to be discussed:

  1. Fake Z (Dymarsky, Kalloor)
  2. Microscopic origin of the entropy of black holes in general relativity (Balasubramanian, Lawrence, Magan, Sasieta)

12:30 Friday 02 December, Ankit Aggarwal

Near-Extremal Limits of Warped CFTs

Abstract: Warped conformal field theories (WCFTs) are two-dimensional non-relativistic systems, with a chiral scaling and shift symmetry. We present a detailed derivation of the near-extremal limit for their torus partition function. This limit requires large values of the central charge, and is only consistent for non-unitary WCFT. We compare our analysis with previous studies of WCFT and its relation to a one-dimensional warped-Schwarzian theory. We discuss different ensembles of warped CFTs and contrast our results with analogous limits in two-dimensional CFTs. Based on 2211.03770.

Papers to be discussed:

  1. Wormholes from heavy operator statistics in AdS/CFT (Sasieta)
  2. A Goldstone theorem for continuous non-invertible symmetries (Etxebarria, Iqbal)

12:30 Friday 11 November, Andrew Rolph

Cost of holographic path integrals

Abstract: How complex is the time evolution of one spatial geometry to another? How complex is a path integral? In this talk, I will introduce proposals for the gravitational dual of computational cost in holographic field theories. To define terms, in Nielsen's geometric approach, complexity is the length of the shortest path between a reference and target state, while cost is the length of a general, not-necessarily-shortest path. To compare to holographic state complexity proposals, our proposals are different in that: (1) the boundary dual is cost, a quantity that can be optimised to state complexity, (2) our set of proposals is large: all functions on all bulk subregions of any co-dimension which satisfy the physical properties of cost, and (3) the proposals are by construction UV-finite. Lastly, I will explain how the path integrals, which we are proposing the cost of, fit in the framework of holographic $T\bar T$. The talk is based on arXiv:2203.08842.

Papers to be discussed:

  1. Complexity Equals Anything II (Belin, Myers, Ruan, Sárosi, Speranza)
  2. On the resurgent structure of quantum periods (Gu, Mariño)

12:30 Friday 04 November, Suchetan Das

Driven CFTs: OTOC and holography

Abstract: Studying the notion of scrambling or chaos in non-equilibrium systems is an active area of research which may uncover some interesting physics behind thermalization or ergodicity in an open system as well as dynamics of blackhole creation or evaporation in holography. Floquet CFTs provide us with an analytically tractable class of models for studying certain out of equilibrium properties. The most interesting feature of such floquet CFTs is the emergence of a rich phase structure in the parameter space of the theory- the so-called - heating phase, non heating phase and a transition between them. I will summarize some of our recent results and new features on computations of Out of Time Ordered Correlators (OTOC) in Conformal Field Theories (CFTs) subjected to either discrete or continuous drive protocols and how they are different from the behavior of OTOCs in thermal equilibrium. I will also discuss some ongoing work on the holographic realizations of different phases in the same setup.

Papers to be discussed:

12:30 Friday 21 October, Kamran Salehi Vaziri

Character Analysis of Quantum Field Theory in de Sitter

Abstract: In this talk, I will first motivate the study of unitary irreducible representations of SO(d+1,1) in the context of Nonperturbative Cosmological Bootstrap and in particular the spectral decomposition of bulk two-point functions and boundary four-point functions in dS. Then, I will focus on the decomposition of (1.)tensor products and (2.)SO(d+1,2) (CFT in bulk) into SO(d+1,1) irreps in two ways: First, numerically diagonalizing the Casimir matrix and second the analysis of Harish-Chandra characters. I will conclude with some of the physical applications.

Papers to be discussed:

  1. Infinite Distances and Factorization (John Stout)
  2. Gravity and the Crossed Product (Witten)
  3. Rational Wavefunctions in de Sitter Spacetime (Goodhew)

12:30 Friday 14th October, Ana-Maria Raclariu

Towers of soft operators and celestial holography

Abstract: The tree-level soft theorems were recently shown to arise from the conservation of infinite towers of charges extracted from the asymptotic Einstein equations. There is evidence this tower promotes the extended BMS algebra to an infinite higher-spin symmetry algebra. In this talk I will introduce towers of canonically conjugate memory and Goldstone operators, highlighting their role in parameterizing the gravitational phase space. I will discuss the conditions under which these towers provide a complete set of scattering states and demonstrate that they are the building blocks of both soft and hard charges. I will finally show that the tower of tree level soft symmetries can be used to extend the Dirac (Faddeev-Kulish) dressings to include the infinite towers of Goldstones and comment on their implications for the gravitational S-matrix.

Papers to be discussed:

  1. The Outside View of the Cosmological Horizon (Lars Aalsma, Sergio E. Aguilar-Gutierrez, Watse Sybesma)

1pm Thursday 29th September, Ruben Lier

Relaxational Goldstone hydrodynamics and odd viscoelasticity

Abstract: Strange metals, charge density waves and Wigner crystals are some examples of electron phases that display spontaneous translational ordering. Often such systems are so complicated that a microscopic description is impossible and one relies instead on an effective theory to find all the possible transport phenomena. One way to do this is by formulating a hydrodynamic effective theory that incorporates the translational Goldstone field. There are at least two ways in which the Goldstone field can be subjected to relaxational phenomena that mitigate the translationally ordered nature of the crystal, namely due to the presence of either dislocations or impurities. The former makes the Goldstone field configuration singular, giving rise to plasticity, whereas the latter explicitly breaks the symmetry and leads to pinning. We explain how to incorporate these phenomena into a hydrodynamic effective theory and explain the distinct effects that these phenomena have on transport as well as their interplay.

Odd elasticity is the odd analogue of classical elasticity, in the sense that the elastic tensor is odd under exchange of indices. Odd elasticity is a chiral phenomenon that requires parity-violation. Odd elasticity can only occur in active systems, because when odd elasticity is present in the elastic tensor, performing closed quasistatic cycles of deformations can either destroy or generate energy. However, when one looks at odd elasticity not as an equilibrium phenomenon but from the point of view of odd viscosity or Hall viscosity, one finds a non-hydrostatic dissipative odd strain correction in the stress tensor, which allows for a transient notion of passive odd elasticity. Including these contributions as well as others, we formulate the most general class of passive odd viscoelastic models. We also look at a lift force on a bead in a two-dimensional viscoelastic material. Lift force is interesting because it is a phenomenon that is allowed by symmetry for odd viscoelastic materials due to the parity breaking nature, but occurs only when the material is compressible, as will be explained.

Papers to be discussed:

  1. Love symmetry (Charalambous, Dubovsky, Ivanov)
  2. The Stranger Things of Symmetric Product Orbifold CFTs (Benjamin, Bintanja, Castro, Hollander)

Past meetings (Fall 2022)

1pm Friday 23rd September, Bartłomiej Czech

The gauge theory of measurement-based quantum computation

Abstract: Measurement-Based Quantum Computation (MBQC) is a model of quantum computation, which uses local measurements instead of unitary gates. Here we explain that the MBQC procedure has a fundamental basis in an underlying gauge theory. This perspective provides a theoretical foundation for global aspects of MBQC. The gauge symmetry reflects the freedom of formulating the same MBQC computation in different local reference frames. The main identifications between MBQC and gauge theory concepts are: (i) the computational output of MBQC is a holonomy of the gauge field, (ii) the adaptation of measurement basis that remedies the inherent randomness of quantum measurements is effected by gauge transformations. The gauge theory of MBQC also plays a role in characterizing the entanglement structure of symmetry-protected topologically (SPT) ordered states, which are resources for MBQC. Our framework situates MBQC in a broader context of condensed matter and high energy theory.

Papers to be discussed:

  1. None.

1pm Friday 16th September, Facundo Rost

From celestial correlators to AdS, and back

Abstract: I will present a general relation between perturbative contributions to celestial correlation functions in d dimensions and Witten diagrams in (d+1)-dimensional EAdS, found in the paper 2208.01629. Celestial contact diagrams turn out to be proportional to contact Witten diagrams, and celestial particle exchanges can be recast as a continuum of particle exchanges in EAdS where the exchanged particles carry unitary principal series representations of SO(1,d+1). One can then import familiar EAdS techniques to study properties of celestial correlators like their conformal partial wave expansion, or constraints from unitarity. This extends similar relations recently uncovered between boundary correlators in dS and Witten diagrams in EAdS, suggesting that EAdS could play a central role in efforts towards holography for all Lambdas.

Papers discussed:

  1. None.

JCs of the past

This has been restored from the backup of our previous referata website.
Journal Clubs of Spring 2015
Journal Clubs of Fall 2015
Journal Clubs of Spring 2016
Journal Clubs of Fall 2016
Journal Clubs of Spring 2017
Journal Clubs of Fall 2017
Journal Clubs of Spring 2018
Journal Clubs of Fall 2018
Journal Clubs of Spring 2019
Journal Clubs of Fall 2019
Journal Clubs of Spring/Summer 2020
Journal Clubs of Fall 2020
Journal Clubs of Spring/Summer 2021
Journal Clubs of Fall 2021
Journal Clubs of Spring/Summer 2022

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