Tuesday 1:00-2:00pm plus informal discussion time until 3:00pm, Online via Teams

Note: dates are subject to change, depending on whether topics run over into a second session.

Semester 1

15/09/20 - Online coffee break

22/09/20 - Ben Withers

29/09/20 - Sami Rawash

06/10/20 - Davide Bufalini - Reading group part 1

13/10/20 - Adam Chalabi - Reading group part 2

20/10/20 - Kostas Skenderis

27/10/20 - Daniel Hasenbichler, part 1

03/11/20 - Daniel Hasenbichler, part 2

10/11/20 - Charlie Woodward

17/11/20 - Gerlando Alfarano

24/11/20 - Paul Rodgers

01/12/20 - Alex Davey

08/12/20 - Raffaella Mulas

[Winter break: 14 December 2020 - 15 January 2021]

Semester 2

19/01/21 - Adam Chalabi, part 1

26/01/21 - Adam Chalabi, part 2

02/02/21 - Carsten Gundlach

09/02/21 - Enrico Parisini

17/02/21 - Ben Kitching-Morley - *note unusual day*

23/02/21 - String seminar this week: Alok Laddha (Chennai)

02/03/21 - No talk this week (H0 tension workshop)

09/03/21 - Marika Taylor

16/03/21 - String seminar this week: Gary Shiu (Wisconsin) - *unusual time* - 3:00 (student session), 3:20 (main seminar)

[Spring break: 22 March - 16 April 2021]

20/04/21 - Zezhuang Hao

27/04/21 - Zezhuang Hao (Part 2 - discussion) + Online coffee break

04/05/21 - Sami Rawash

11/05/21 - Linus Too

18/05/21 - No talk this week

25/05/21 - String seminar this week: Renann Jusinskas (Prague)

01/06/21 - Marika Taylor

[June break: 07-30 June 2021]

Summer

06/07/21 - Adam Chalabi

13/07/21 - Adam Chalabi (part 2)

20/07/21 - Davide Bufalini

27/07/21 - Federico Capone

Topics: (reverse chronological order)

Speaker:

Federico Capone: Puzzles with islands & dogmas

Two recent papers address two different aspects of the entropy problem and its resolution via the island proposal. I will present the arguments of the recent paper https://arxiv.org/abs/2107.03390 dealing with apparent inconsistencies of the island rule in theories of long-range gravity. If time permits I will also present the gedanken experiment put forward in https://arxiv.org/abs/2107.05662 with the aim of showing that the so-called central dogma is to be revisited.

Davide Bufalini: Surfing the Worldsheet in AdS3

In the last few years, our community has seen an explosion of new and interesting results concerning the most “stringy” regime of AdS3 physics. We will understand how we can capture some extremely important (and counterintuitive) features of the full String Theory with a worldsheet approach, by means of an appropriate Wess-Zumino-Witten model. After discussing some well known facts about AdS3 String Theory, we quickly review key properties of the “tensionless regime” [1] and understand how to perform the string partition function [2] and how the so-called “Factorization Problem” can be addressed and solved in this context [3].

[1]: Gaberdiel, Gopakumar [arXiv:1803.04423]

[2]: Eberhardt [arXiv:2008.07533]

[3]: Eberhardt [arXiv:2102.12355]

Adam Chalabi: Wormholes and Ensemble Averages

Recent years have seen a renewed interest in Euclidean wormholes. In this talk, I will review the role of wormholes in a simple theory of 2d gravity: Jackiw-Teitelboim gravity. In 1903.11115, the full path integral of this theory was determined exactly to all orders in a genus expansion. Unlike familiar AdS/CFT, the JT gravity path integral does not compute the partition function of an ordinary quantum system. Instead, it can be recast as a matrix integral computing an ensemble average. Wormholes play a key role in this new understanding of JT gravity. Finally, I will comment on some of the lessons that one may draw from this computation.

Marika Taylor: Islands and AdS3/CFT2

There has recently been considerable interest in using islands/replica wormholes to understand the recovery of information from black holes. Many of the works on this topic have been in the context of two dimensional JT gravity, for which the holographic dictionary is poorly understood. This talk will use the relation between JT gravity and AdS3 to discuss the interpretation of islands in the much better understood AdS3/CFT2 correspondence. The latter is one of the main playgrounds for the black hole microstate proposal and ultimately one would like to connect islands with the microstate programme. This is based on work in progress, so we won't get to this final goal, but we will sketch the steps needed to link islands and microstates.

Linus Too: Reviewing replica wormholes and the black hole interior

To continue our discussion on the black hole information paradox and the island proposal I will be reviewing the paper Replica wormholes and the black hole interior 1911.11977. The island proposal claims the Page curve for the radiation can be recovered from the generalised entropy that takes into account the entropy of some island region in the bulk. Analogous to the derivation of Ryu-Takayanagi formula, the authors of this paper showed how to derive the entropy of the radiation by replica computation of the gravitational path integral. This talk will be focusing on seeing how the replica wormhole arises from the gravitational path integral in JT gravity.

Sami Rawash: Black holes, pieces of papers and their evaporation

The black hole information paradox is a longstanding problem in high energy physics. In this talk (based on 1101.4899) we will discuss the evaporation process of a black hole and compare it with a model of a burning piece of paper. This will enable us to discuss the differences between the two systems and why the information paradox is difficult to solve. In the second part of the tal I will introduce the fuzzball proposal and discuss how this can potentially solve the information paradox.

Zezhuang Hao: Global symmetries in Quantum Field Theory and Quantum Gravity

In this talk I will discuss the construction of symmetries from the algebraic point of view following the paper arXiv: 1810.05338 by Daniel Harlow, Hirosi Oogrui and introduce two examples to illustrate why we need to take care of the factorization of the quantum field theories. Then I will use the splittable property to argue that there are no global symmetries in quantum gravity.

Marika Taylor: Cellulations and quantum error correcting codes

In the last few years there has been considerable interest in relating quantum error codes to holography, particularly following the HaPPY approach for the hyperbolic plane. This talk will discuss the general relationship between cellulations of manifolds and certain classes of quantum error correcting codes, which has been explored within the quantum information literature. Using this background, we will then revisit HaPPY and discuss how discrete cellulations of general dimension hyperbolic manifolds may be related to dual discrete quantum theories. This is based on work in progress with Charlie Woodward.

Ben Kitching-Morley: Nonperturbative infrared finiteness in super-renormalisable scalar quantum field theory

In this talk I will present an overview of the recent paper by the LatCos Collaboration, which shares its name with this talk and can be found at arXiv:2009.14768. In this paper we demonstrate through lattice simulations and an effective theory that a class of super-renormalisable theories is non-perturbatively IR-finite. This class of theories has applications in holography, where they have been proposed as the dual theories to early universe Inflation. As well as overviewing the methodology behind the lattice evidence used, I will provide a short pedagogical review of lattice methods, for those who aren't familiar with this discipline.

Enrico Parisini: ANEC, BMS and OPEs

I will review some recent papers showing how to realize the BMS algebra on the Minkowski lightcone with appropriate insertions of the stress tensor, via the so-called light-ray operators. After a brief introduction to BMS, I will focus on the assumptions one has to make and explain what information we can extract from OPEs in the lightcone limit.

Carsten Gundlach: Notes on hyperbolicity of single-null gauges

I review formulations of the Einstein equations on null slices, and test them for a certain definition of strong hyperbolicity. I conjecture that the obstacle to strong hyperbolicity raised recently by Giannakopoulos, Hilditch and Zilhao (reviewed by Alex Davey in his seminar on 1/12/20) does not apply if the null slices start from a regular centre, rather than a non-trivial boundary.

Adam Chalabi: Coincident M2 branes, Chern-Simons-matter theories and all that

It has been known for a long time that M-theory contains extended objects called M-branes. However, it has been difficult to construct field theories that describe the low-energy dynamics on the worldvolume of multiple coincident branes. While the M5 brane is still poorly understood, significant progress was made for the M2 brane just over a decade ago. In this two-part talk, I will review certain Chern-Simons-matter theories that are conjectured to describe the superconformal fixed point of multiple M2 branes. The focus will be on the ABJM theory (0806.1218). However, I will also briefly comment on other theories and their relation to ABJM. In part one, I will describe these Chern-Simons-matter theories from a field theory perspective. In part two, I will discuss Hanany-Witten-like brane constructions of ABJM in type IIB string theory, and the gravity duals of these M2 brane theories.

Raffaella Mulas: Spectra of Hypergraphs

We define hypergraphs, we discuss some of their applications and we give an overview of their spectral theory.

Alex Davey: Hyperbolicity of GR in single-null gauges

Single-null formulations of GR have attractive properties for numerical relativity. However, their well-posedness is not well understood. I will review a recent paper (arXiv/2007.06419), which looks at the weak hyperbolicity of Bondi-Sachs and affine-null gauge, and argues that it may still be possible to recover convergence using norms other than L^2. Implications for Cauchy-Characteristic Matching are also discussed.

Paul Rodgers: A soliton menagerie in a Minkowski box

I will discuss soliton solutions in Einstein-Maxwell theory coupled to a charged scalar field, with a reflecting box placed at finite radial distance. Solitons can be viewed as a non-linear version of a bose condensate and at small energies is just a small scalar perturbation of Minkowski spacetime. The fundamental qualities of the solution depend sensitively on the scalar field charge. In some cases the soliton appears as the zero-horizon radius limit of hairy black holes in the same theory, which was predictable via perturbation theory. However we also find solutions which are unrelated to the hairy black holes and even ones which are not small perturbations of Minkowski. I will give a complete overview of the phase diagram of solitons in this theory.

Gerlando Alfarano: Consistent truncations and generalised geometry

Theories in more than 4 dimensions have played a crucial role over the last half a century, string theory being the most known example. Ultimately one is interested in extracting lower dimensional physics out of those theories in order to understand if and how the Standard Model can be embedded. This is usually done via a mechanism called Kaluza-Klein reduction, which produces an infinite towers of modes, followed by a truncation of the field content to a finite lower dimensional set. One may then ask whether the truncated fields are also solution of the higher dimensional equations of motion of the theory one started with, or in other words, whether the truncation is consistent or not. In this talk, I will discuss the different types of consistent truncations one may perform, their limitations and how generalised geometry has recently being brought into the subject to resolve some of the issues.

Charlie Woodward: Gravitational Lensing in the Kerr Exterior

Occultation and microlensing are different limits of the same phenomenon of one body passing in front of another body. These results have been well studied for gravitational lenses of Schwarzschild spacetime. However, in order to extend these ideas to a general astrophysical system, one must develop a general understanding of the null geodesics in Kerr spacetime. In this talk, I will begin by reviewing the basic ideas of microlensing and occultation before introducing the Kerr gravitational lens geometry and geodesic equations required to extend this to more cases of astrophysical interest.

Daniel Hasenbichler: Introduction to resurgence techniques

I present a pedagogical introduction to resurgence techniques, which can be used to describe non-perturbative physics, e.g. instantons. I discuss the relationship between the perturbative expansion of a function and its full non-perturbative content for a large class of functions.

Kostas Skenderis: Momentum space CFT (talk + informal discussion)

Adam Chalabi: Black Holes in String Theory: Part 2. Bekenstein-Hawking entropy from a microscopic counting

In this talk, we consider certain types of 5d black holes that arise from the D1-D5(-P) system. This brane configuration was introduced in Davide’s talk last week. Focussing on the open string perspective, we demonstrate how the Bekenstein-Hawking entropy of these black holes can be reproduced from a microscopic counting of degrees of freedom, which was first shown by Strominger-Vafa [hep-th/9601029].
This is the second of two talks building upon last year's PGR reading group on black holes in string theory for which we mostly followed Peet’s lectures [hep-th/0008241].

Davide Bufalini: Black Holes in String Theory: Part 1. From classical Black Holes to the D1D5(P) system

This presentation has the aim to explain how to realize Black Holes (BH) in String Theory and thus how the theory deals with well-known problems related to the semiclassical description of Black Holes. Focusing mostly on the Entropy Puzzle, in this talk we will furnish the basic knowledge needed in order to understand the microstate counting performed by Strominger and Vafa [1], which will be the focus of Adam’s talk next week.
After recalling what these fundamental problems of the semiclassical description are, we will consider the (supersymmetric) D1D5(P) system in both the closed string and open string perspective. In the latter, we will conclude by deriving the field content of a suitable (supersymmetric) gauge theory by looking at the open strings stretched between D1 and D5 branes.
This talk (the first of a series of two) is the result of what has been discussed during the “Black Holes in String Theory” Reading Group among PhD students in our STAG group.
[1]: arXiv:hep-th/9601029v2

Sami Rawash: Holographic description of black hole emission process

In this talk I will review the work in 0906.2015. In order to holographically study the emission process of a black hole, I will discuss how to couple the CFT to flat infinity and relate the emission of quanta of the gravitational solution to amplitudes of the CFT. This general formalism will then be applied to the D1D5 systems: we will first consider global AdS_3\times S^3 plus a single excitation and holographically compute the rate at which this state de-excites. A generalization of this simple process will enable to relate this result to the emission rate of a non-extremal black hole microstate.

Ben Withers: Does the hydrodynamic expansion converge?

This is one of the foundational questions in relativistic fluid mechanics, and concerns the properties of a gradient expansion at large orders. Black holes in holography provide the ideal testing ground for this question, since hundreds of orders in the expansion can be easily obtained. Here, the answer was found to be “No" for expanding plasmas and “Yes" for quasinormal mode dispersion relations. I will present recent work in which we unify these two lines of research, and provide the first definitive answer to this question for generic flows. Based on 1803.08058 and 2007.05524.