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 (up to Wed), just add it to the list and the organizer (Shira) 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.

# This Week

## May 29, Evita Verheijden

### Comments on magnetic black holes

In his recent "corona paper" (2004.06084), Maldacena discusses some aspects of magnetically charged black holes, most notably a restoration of the electroweak symmetry in a region around the black hole. This region is itself surrounded by a "corona" where the Higgs field changes from zero to its usual non-zero value. I will discuss this and other aspects of these black holes, focusing mostly on the case where we ignore the dynamics of the gauge fields. If time allows I will include the U(1) gauge field dynamics.

## Papers for discussion:

The paper discussion will run as a separate event during the summer (starting June) on Tuesdays at 11:30

- Pure Gravity and Conical Defects (Benjamin, Collier, Maloney)
- Soft photon theorems from CFT Ward identites in the flat limit of AdS/CFT (Hijano, Neuenfeld)

# JCs of the Recent Past

## May 15, John Stout

### Weak Gravity without Instantons

Quantum gravity abhors exact global symmetries and tends to thrash about whenever we try to restore them. This is supposed to be true for continuous shift symmetries as well. If we take a compact scalar (an axion) and try to restore a shift symmetry by decompactifying its field space, quantum gravitational lore instructs us to expect a multitude of instanton corrections which restrict the field space to be sub-Planckian. This is normally phrased as bounds on instanton actions. However, such bounds are physically ambiguous. Instantons are not elements in the Hilbert space — I can't hold or eat one, for instance — and these bounds merely signal that some approximation has failed. I'll talk about some work in progress towards understanding what happens physically in this limit. I'll discuss how this limit may naturally be interpreted in the language of phase transitions: "instanton actions" become unsuppressed whenever a gap closes somewhere along the axion's field space. I'll illustrate this in some simple examples. I'll also discuss how this can be encoded geometrically.

and we discussed:

- The Cosmological Bootstrap: Spinning Correlators from Symmetries and Factorization (Baumann, Duaso Pueyo, Joyce, Lee, Pimentel)
- On the Virasoro six-point identity block and chaos (Anous, Haehl)

## May 8, Peng Cheng

### Maxwell edge modes of black hole

I will explain the behavior of edge modes of an U(1) gauge theory living on a black hole background with boundaries. The gauge symmetry is broken by the boundaries. We get an effective description of the would-be gauge modes and calculate the entropy of them by Euclidean path integral. The situation here is very similar to the JT gravity/Schwarzian case. If time permits, I would also discuss the possible relation with black hole information paradox.

and we discussed:

- Realize Emergent Gravity to Generic Situations (An, Cheng)
- The 5d Superconformal Index at Large $N$ and Black Holes (Crichigno, Jain)

## May 1, Sgroi, Gabriele

### Symmetries of SCFTs and quantum error correcting codes.

I will discuss a very recent paper by Harvey and Moore drawing connections between 2d SCFT supercurrents and quantum error correcting codes. I will review in detail their construction for the GTVW model and outline the connection with Mathieu Moonshine.

Papers for discussion:

- Fractional statistics in anyon collisions (Bartolomei et al.)
- Black holes, quantum chaos, and the Riemann hypothesis (Betzios, Gaddam, Papadoulaki)
- Non-Boost Invariant Fluid Dynamics (de Boer, Hartong, Have, Obers, Sybesma)

## Apr 24, Ricardo Espindola

### Effective field theories from thermal holographic baths

Recently, the study of AdS black holes coupled to external systems has been of active interest. I will describe how effective actions or influence functionals for open quantum systems can be derived using gravitational Schwinger-Keldysh backgrounds in the Feynman and Vernon paradigm. This talk is mainly based on: 2004.02888, 1906.07762 and 1812.08785.

and we discussed:

- ER = EPR revisited: On the Entropy of an Einstein-Rosen Bridge (Verlinde)
- Vortices and waves in light dark matter (Hui, Joyce, Landry, Li)

## Apr 17, Greg Mathys

### Superfluids as higher-form anomalies

I will explain how we can recast superfluid hydrodynamics as the hydrodynamic theory of a system with an emergent anomalous higher-form symmetry. The higher-form charge counts the winding planes of the superfluid. This formulation has the advantage to put all hydrodynamic equations on equal footing. In addition, the anomalous Ward identity can be used as an alternative starting point to prove the existence of a Goldstone boson, without reference to spontaneous symmetry breaking. This provides an alternative characterization of Landau phase transitions in terms of higher-form symmetries and their anomalies instead of how the symmetries are realized. I will then discuss how we think we can generalize this construction to include non-Abelian symmetries.

Papers for discussion:

- Bootstraps to Strings: Solving Random Matrix Models with Positivity (Lin)
- From quantum curves to topological string partition functions II (Coman, Longhi, Teschner)
- Interfaces and the extended Hilbert space of Chern-Simons theory (Fliss, Leigh)

## Apr 08 (Wednesday), Michael Walter

### Teleportation by Size and Traversable Wormholes

We propose holographic teleportation protocols that exhibit similar behavior to that seen in recent traversable wormhole constructions: information that is scrambled into one half of an entangled system will, following a weak coupling between the two halves, unscramble into the other half. We introduce the concept of "teleportation by size" to capture how the physics of operator-size growth naturally leads to information transmission. The transmission of a signal through a semi-classical holographic wormhole corresponds to a rather special property of the operator-size distribution we call "size winding". For more general setups (which may not have a clean emergent geometry), we argue that imperfect size winding is a generalization of the traversable wormhole phenomenon. For example, a form of signaling continues to function at high temperature and at large times for generic chaotic systems, even though it does not correspond to a signal going through a geometrical wormhole, but rather to an interference effect involving macroscopically different emergent geometries.

Papers for discussion:

- Holographic duals of refined partition functions (Toldo)
- Bulk Entanglement Entropy and Matrices (Das, Kaushal, Mandal, Trivedi)

## Apr 03, Jeremy van der Heijden

### JT Gravity and Matrix Integrals

I will review a paper by Saad, Shenker and Stanford [1903.11115] which relates JT gravity to a matrix integral. In particular, I will present the main arguments underlying this JT/matrix integral correspondence. After a short introduction to random matrix theory and the genus expansion of Euclidean JT gravity I will highlight the recursive structure in the moduli space of Riemann surfaces and its relation to topological recursion. If time allows I will discuss some more recent ideas and interpretations in terms of ensemble averaging and Euclidean wormhole geometries.

Papers for discussion:

- Notes on Matrix Models (Dio, Beatrix)
- The Python's Lunch: geometric obstructions to decoding Hawking radiation (Brown, Gharibyan, Penington, Susskind)

## Mar 27, Gui Pimentel

### Spin in de Sitter

I will show how to compute and constrain cosmological correlators of primordial gravitational waves in de Sitter space, by borrowing tools from conformal field theory and intuition from the flat space S-matrix.

and we discussed

- The statistical mechanics of near-extremal black holes (Iliesiu, Turiaci)
- A Generalized Nachtmann Theorem in CFT (Kundu)

## Mar 20, Sasha Gamayun

### Transport through (non)-conformal defect

I will discuss simple microscopic realizations of the conformal and non-conformal defects in 1+1 dimensional systems. First, I review the transport through the link defect in 1D tight-binding fermionic chains. I show how to present full-counting statistics (FCS) in terms of Fredholm determinants and how to use microscopic bosonization to get its large time asymptotic. Knowing FCS allows us to address various physical quantities such as current through the link, the shot noise, and the entanglement entropy. Further, I will do small adjustments and turn the link defect into the conformal defect, which is characterized by a constant transmission coefficient at all energies. I discuss how physics is changed in this case and address the driven conformal defect.

And we discussed:

- The Coupled SYK model at Finite Temperature (Xiao-Liang Qi, Pengfei Zhang)
- N=2 Minimal Models: A Holographic Needle in a Symmetric Orbifold Haystack (Belin, Benjamin, Castro, Harrison, Keller)
- The ghost in the radiation: Robust encodings of the black hole interior(Isaac H. Kim, Eugene Tang, John Preskill)
- Geometric secret sharing in a model of Hawking radiation (Balasubramanian, Kar, Parrikar, Sárosi, Ugajin)

## Mar 13 - Cancelled due to National high energy theory seminar

## Mar 06 - Dora Nikolakopoulou

### A dynamical mechanism for the Page curve from quantum chaos.

I will briefly review some background material concerning the information paradox and its resolution through the existence of islands. I will then discuss a recent paper by Hong Liu and Shreya Vardhan (https://arxiv.org/abs/2002.05734) that provides a possible microscopic explanation of how islands are formed.

and we discussed

- Recovering Infalling Information via String Spreading (Mousatov, Silverstein)
- Horizons protect Church-Turing (Susskind)

## Mar 03 (Tuesday, seminar slot) - Austin Joyce

### (Cosmological) Bootstrap Bootcamp

How is the physics of Universe's earliest moments encoded in correlations between structures observed at cosmic distances today? How can we re-construct these correlations from general principles like locality, causality, and unitarity? What is the structure of cosmological correlation functions and what consistency conditions must they obey? Come find out.

## Feb 21 - Jay Armas

### A certain improvement of the Blandford-Znajek mechanism

I will review the basics of Force Free Electrodynamics and its role in certain astrophysical contexts. I will then discuss the “standard model” of astrophysical jets (the Blandford-Znajek mechanism) for which the energy is extracted from a rotating black hole via plasma currents flowing on magnetic field lines threading the horizon. I will stress that the original formulation of this mechanism is both inconsistent and incomplete and then provide a new consistent formulation based on matched asymptotic expansions. Based on https://arxiv.org/abs/2002.01972.

and we discussed:

- A dynamical mechanism for the Page curve from quantum chaos (Liu, Vardhan)
- Transcending the ensemble: baby universes, spacetime wormholes, and the order and disorder of black hole information (Marolf, Maxfield)
- Hydrodynamics for charge density waves and their holographic duals (Armas, Jain)

## Feb 14 - Antonio Rotundo

### Islands and averaging in the 3-qutrit code

I review recent progress towards a solution of the information paradox, highlighting some aspects that I find confusing. Then I briefly recall how, in the past, quantum error correcting codes have proven useful to make sense of certain confusing properties of holography. Finally, I conclude with some preliminary ideas Ben, Dora and I have been recently discussing on how to reproduce some of the confusions we face in evaporating black holes in a simple quantum error correcting code.TBA

and we discussed:

- Eigenstate Thermalization and Disorder Averaging in Gravity (Pollack, Rozali, Sully, Wakeham)
- Chaos and complementarity in de Sitter space (Aalsma, Shiu)

## Feb 7 - Beatrix Muhlmann

### Gravitational Anomalies in nAdS_2/nCFT_1

I will discuss some recent work with Alejandra in the framework of nAdS_2/nCFT_1. We revisit the holographic description of the near horizon geometry of the BTZ black hole in AdS_3 gravity with a gravitational Chern Simons term included. A dimensional reduction of this three dimensional theory allows us to examine how the UV data of the black hole are captured by the IR data and in particular where inside the 2D CFT the nCFT_1 sits.

and we discussed

- Holographic Order from Modular Chaos (de Boer, Lamprou)
- Matter Couplings and Equivalence Principles for Soft Scalars (Bonifacio, Hinterbichler, Johnson, Joyce, Rosen)
- Modular Hamiltonians for Euclidean Path Integral States (Balakrishnan, Parrikar)
- Distributions in CFT I. Cross-Ratio Space (Kravchuk, Qiao, Rychkov)

## Jan 31 - Claire Zukowski

### Wilson Lines in 3D de Sitter

I will discuss some recent work on constructing local probes in de Sitter, using the Chern-Simons formulation of 3D gravity. We construct Wilson lines in the static patch of Euclidean dS_3 gravity, built with non-unitary representations of so(4) and evaluated using certain states that satisfy a singlet condition. This allows us to reproduce the Green’s functions of massive scalar fields in de Sitter, the construction of bulk fields, and the quasinormal mode spectrum. I will also discuss the interpretation of our results in Lorentzian signature via an analytic continuation to SL(2,C) Chern-Simons theory.

and we discussed

- Double-Scaling Limit in Principal Chiral Model: a New Non-Critical String? (Kazakov, Sobko, Zarembo)
- Gravitational Wilson Lines in 3D de Sitter (Castro, Sabella-Garnier, Zukowski)

## Jan 17 - Vassilis Anagiannis

### Studying convolutional neural networks (CNNs) by using tensor networks.

I will discuss work in progress about using tools and ideas from the language of tensor networks and quantum mechanics to study a certain class of convolutional neural networks (CNNs) which are widely used in machine learning classification tasks.

and we discussed

- On the exact entropy of N=2 black holes (Gomes, het Lam, Mathys)
- Gravitational Collider Physics (Baumann, Chia, Porto, Stout)
- A Conjecture on the Minimal Size of Bound States (Freivogel, Gasenzer, Hebecker, Leonhardt)
- Real-time Dynamics of Plasma Balls (Bantilan, Figueras, Mateos)

# 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