Usual time: Wednesdays at 1:00pm (student session), 1:20pm (main seminar).
Alternate time: Wednesdays at 3:00pm (student session), 3:20pm (main seminar).
Online via Zoom; streamed in a seminar room.

Schedule:

06/10/21 - Yolanda Lozano (1 pm)

13/10/21 - Omer Gurdogan (1 pm)

19/10/21 (Tuesday) - Avia Raviv-Moshe (3 pm)

20/10/21 - No seminar possible

27/10/21 - Horatiu Nastase (3 pm)

02/11/21 (Tuesday) - Yifan Wang (12.30 pm)

03/11/21 - No seminar possible

10/11/21 - Suvrat Raju (1 pm)

17/11/21 - Eric Perlmutter (3 pm)

24/11/21 - Giacomo Sberveglieri (1pm)

01/12/21 - Elli Pomoni (1 pm)

07/12/21 (Tuesday): Debodirna Ghosh (12:30 pm)

08/12/21 - Anton Pribytoks (1 pm)

14/12/21 (Tuesday 1pm, hybrid in 54/5025 and on Teams) - Lampros Lamprou

[Winter break: 15 December 2021 through 15 January 2022]

12/01/22 - No Seminar

26/01/22 - Costas Bachas (1 PM)

02/02/22 - Lorenz Eberhardt (3 PM)

09/02/22 - Sean Hartnoll (1 PM)

16/02/22 - Andrea Legramandi (1 PM)

23/02/22 - Joao Penedones (1 PM)

02/03/22 - Yiming Chen (3 PM)

09/03/22 - Jordan Cotler (3 PM)

16/03/22 - David Simmons-Duffin (3 PM)

23/03/22 - Charlotte Kristjansen (1 PM)

[Spring break: 26 March 2022 through 23 April 2022]

27/04/22 -No talk due to Euro Strings 2022

04/05/22 - Yoav Zigdon (1 PM)

11/05/22 - Samir Mathur (3 PM)

18/05/22 - Robert Wald (3 PM)

24/05/22 - Matteo Broccoli (Tuesday 12 PM in room 54/8031)

13/07/22 - Krishan Saraswat (Wednesday 1 PM, room 54/5027 (5A))

Titles and Abstracts (reverse chronological order):

Krishan Saraswat (Perimeter): Black Hole Thermalization and Microstructure from Microstate Statistics

Holographic investigations have revealed the importance of quantum chaos and random matrix theory in the unitary description of black holes. The spectral form factor associated with the $e^{S_{BH}}$ microstates that compose the black hole serve as a simple proxy for studying how perturbations to black holes thermalize. In this talk I will introduce a statistical model for the spectrum of black hole microstates which allows us to tune the spacing statistics between microstates. Using this, I will discuss how the details of the spectral statistics of the microstates manifest in the early and late time thermalization behaviour and connect it to the issue of whether there are modifications to the near horizon physics of black holes. I will show that depending on the precise nature of the spectral statistics, it is possible for the spectral form factor to exhibit ``regular oscillations.'' In certain cases, such regular oscillations might be interpreted as "echoes" being generated by microstructure near the horizon which effectively behaves as a fixed cutoff near the horizon with semi-reflective boundary conditions for perturbations. I will then conclude by posing some questions and describing possible connections to the SYK model, JT gravity, and wormhole saddles in the Euclidean path integral.

Matteo Broccoli (Albert Einstein Institute): Irrelevant Operators and their Holographic Anomalies

The holographic computation of anomalies provided one of the first tests of the AdS/CFT correspondence. In this talk, I will describe a technique developed in the early days of holography to compute the trace anomaly of a CFT by studying diffeomorphisms in the bulk. Then, I will discuss the features of a CFT in the presence of integer-dimensional irrelevant operators, whose holographic description was missing. Considering the effect of the backreation generated by massive scalar fields in the bulk, I will provide the dual description of such a CFT and compute its anomalies. These results represent an additional test of the AdS/CFT correspondence and I will conclude discussing possible outlooks.

Robert Wald (Chicago): Quantum Superpositions of Massive Bodies and Gravitationally Mediated Entanglement

In order to avoid contradictions with complementarity and causality in a gedankenexperiment involving a quantum superposition of a massive body, it was previously shown (in arXiv:1807.07015) that it is necessary for there to be both quantized gravitational radiation and local vacuum fluctuations of the spacetime metric We review this gedankenexperiment and the previously given "back of the envelope" arguments that resolve it. We then improve upon this analysis by providing a precise and rigorous description of the entanglement and decoherence effects (given in arXiv:2112.10798). As a by-product of our analysis, we show that under the protocols of the gedankenexperiment, there is no clear distinction between entanglement mediated by the Newtonian gravitational field of a body and entanglement mediated by on-shell gravitons emitted by the body. This suggests that Newtonian entanglement implies the existence of graviton entanglement and supports the view that the experimental discovery of Newtonian entanglement---as envisioned in proposed experiments---may be viewed as implying the existence of the graviton.

Samir Mathur (Ohio) : The limitations of the holographic idea

The black hole information paradox arises because entangled pairs are created from the vacuum region around the horizon of a traditional hole. In string theory one finds that black hole states are actually horizon sized objects (fuzzballs) with no horizon, so there is no paradox. Why then so some people still think that Hawking's puzzle is not resolved? In this talk we will argue that opposition to the fuzzball resolution is based on a faulty extrapolation of the idea of holography. The picture proposed in these alternative resolutions (involving code subspaces which yield semiclassical islands) is actually not consistent: ideas along these directions either involve nonunitarity in the black hole interior or a violation of standard low energy physics at infinity. We argue that notions like ER=EPR, spacetime is built by entanglement etc. violate the linearity of quantum theory; the error here can be traced to use of the geometry of the eternal hole which cannot be a valid geometry in a consistent theory of quantum gravity.

Yoav Zigdon (Ben-Gurion University): A Puncture in the Euclidean Black Hole

We study a black hole in string theory which is described by an exact worldsheet conformal field theory (CFT). In Euclidean signature, the radial-time part of its geometry is described by a shrinking thermal cycle, resembling a cigar. It was pointed out that a certain condensate of winding string must be included in the CFT. Using the Horowitz-Polchinski effective field theory, we found that instead of ending with a smooth tip, the geometry in which a winding condensate backreacts approaches a ``puncture'' where the local radius of the thermal cycle and its derivatives vanish. The solution satisfies first-order equations, similarly to BPS configurations. We further argue that the entropy possessed by string modes that wrap the thermal cycle matches the Bekenstein-Hawking entropy. In the context of the black hole/string transition in type II superstring theory, the punctured black hole geometry has the same topology as a ``string geometry'', implying that there is no obstruction to a smooth transition from the point of view of the superconformal index, in contrast to a previous statement that was made in the literature. We comment on the Lorentzian interpretation of the solution.

Charlotte Kristjansen (Niels Bohr Institute) : Integrable domain walls in ABJM theory

We discuss a set-up within AdS/CFT where the field theory has a ½ BPS domain wall and correspondingly the string theory has an additional probe brane. We show that for ABJM theory as well as for N=4 SYM theory the domain wall/probe brane can be described as an integrable boundary state of the underlying integrable super spin chain. One point functions can be expressed as overlaps between Bethe eigenstates and boundary states which take the form of matrix product - or valence bond states, and thanks to integrability closed overlap formulas can be found. We furthermore discuss how certain ``microscopic duality relations” based on the QQ-system of the spin chains, can be used to predict novel overlap formulas.

David Simmons Duffin (Caltech): Carving out gravitational theory space

It has long been expected that UV consistency of quantum gravity places constraints on low energy observables. I will discuss new techniques for deriving such constraints -- techniques that arose through an interesting interplay between effective field theory and the conformal bootstrap.

Jordan Cotler (Harvard): Renormalization group flow as optimal transport

We show that Polchinski’s equation for exact renormalization group flow is equivalent to the optimal transport gradient flow of a field-theoretic relative entropy. This gives a surprising information-theoretic formulation of the exact renormalization group, expressed in the language of optimal transport. We will provide reviews of both the exact renormalization group, as well as the theory of optimal transportation. Our results allow us to establish a new, non-perturbative RG monotone, and also reformulate RG flow as a variational problem. The latter enables new numerical techniques and allows us to establish a systematic connection between neural network methods and RG flows of conventional field theories. Our techniques generalize to other RG flow equations beyond Polchinski's.

Yiming Chen (Princeton): Spectral form factor for free large N gauge theory and strings

I will discuss the spectral form factor for free large N gauge theory and highly excited strings, a quantity that has attracted much recent attention in the context of black holes. In these simpler systems, after a rapid early decay determined by the continuous density of state, the spectral form factor rises again due to new contributions. In both cases, the spectral form factor can be studied using quantities that have some "geometrical" flavor. In gauge theories, the "geometries" are various eigenvalue distributions of the thermal holonomy; For a gas of string, the "geometries" are various winding modes on the thermal manifold, or more literally, classical solutions like the Horowitz-Polchinski solution. The focus of the talk will be the new "geometries" responsible for the rise of the spectral form factor. Along the way, I will mention some potential connections to the black hole problem. The talk will be based on arXiv:2202.04741.

Joao Penedones (EPFL): Towards the non-perturbative cosmological bootstrap

I will discuss the non-perturbative bootstrap approach to Quantum Field Theory (QFT) on Anti-de Sitter, Minkowski and de Sitter spacetimes. I will argue that this approach is not yet fully understood in de Sitter space and present some first steps based on https://arxiv.org/abs/2107.13871 and on-going work.

Andrea Legramandi (Swansea) : Islands in The Stream of Hawking radiation

We consider the island formula for the entropy of an arbitrary subset of the Hawking radiation in the adiabatic limit, including the effect of a grey-body factor. We find a simple concrete ‘on-shell’ formula for the generalized entropy which involves the image of the island out in the stream of radiation, the ‘island in the stream’. The quantum information properties of the radiation are discussed together with the entanglement monogamy problem. The grey-body factor allows us to analyse the role of irreversibility in the evaporation. In particular, we show that irreversibility leads to multiple saddles to dominate the entropy, and we will match this result with a generalization of Page's theorem that involves a nested temporal sequence of unitary averages.

Sean Hartnoll (Cambridge): Entanglement in the quantum Hall matrix model

Quantum mechanical theories describing large N by N matrices of oscillators can lead to an emergent space as N -> infinity. In the most fully fledged version, the emergent space is dynamical and gravitating. However, there are also simpler, lower dimensional versions of this phenomenon. One of the simplest occurs in the so-called quantum Hall matrix model, in which a 2 dimensional space emerges and supports Chern-Simons dynamics. I will describe how this solvable model leads to insights about the emergence of space from matrices. In particular, I will describe how the emergent spatial locality is reflected in the entanglement structure of the ground state of theory.

Lorenz Eberhardt (Princeton): A perturbative CFT dual for pure NS-NS AdS3 strings

I will discuss perturbative string theory on AdS3 with pure NS-NS flux and propose a dual spacetime CFT. It is given by a symmetric orbifold of a linear dilaton theory deformed by a marginal operator from the twist-2 sector. I explain the computation of two- and three-point functions on the CFT side to 4th order in conformal perturbation theory at large N. They agree with the string computation at genus 0, thus providing ample evidence for a duality. I also explain how to match the full spectra of both short and long strings.

Costas Bachas (ENS Paris): Transport across Holographic Interfaces

Conformal interfaces are ubiquitous both in condensed-matter physics and in studies of holographic duality. I will present some recent results on the phase diagram, the transport of energy and the non-equilibrium steady states in the simplest model of holographic interfaces in two dimensions.

Lampros Lamprou (British Columbia): Falling inside holographic black holes

I will present a bulk reconstruction technique in AdS/CFT suitable for addressing a facet of the black hole information problem: how to unambiguously predict the results of measurements accessible to an infalling observer in the black hole interior.

Anton Pribytoks (Trinity College Dublin): Automorphic symmetries, Free fermions and AdS integrable deformations

We begin by showing implications of underlying automorphic symmetries in two-dimensional integrable systems and implementation of boost operator. We will develop the method of integrable generators for finding new models and demonstrate Hamiltonian bottom-up approach for constructing associated R-matrices, which also allows to resolve classification problem. Universality of the method admits generalisation to higher dimensional quantum spaces, as well as extends to generic spectral dependence, where its mapping to string integrable backgrounds and S-matrix arises. We shall make this exact in the structure of spin chain and string pictures. Description of the novel models and their properties will be provided based on various Bethe Ansätze, including emergence of free fermion condition for AdS string integrable models. Based on that, the new classes of deformed integrable models will be shown in AdS_3 \times S^3 \times M^4 and AdS_2 \times S^2 \times M^6 backgrounds along with their limits, and relation to double q-deformed supercoset models in the former case. We will also discuss notes on deformation flows and proposal for S-matrix for higher parametric Sigma models and their quantum sector.

Elli Pomoni (DESY): Dynamical spin chains in 4D N = 2 SCFTs

In this talk we will revisit the study of spin chains capturing the spectral problem of 4d N = 2 SCFTs in the planar limit. At one loop and in the quantum plane limit, we will discover a quasi-Hopf symmetry algebra, defined by the R-matrix read off from the superpotential. This implies that when orbifolding the N = 4 symmetry algebra down to the N = 2 one and then marginaly deforming, the broken generators are not lost, but get upgraded to quantum generators. Thereafter, we will demonstrate that these chains are dynamical, in the sense that their Hamiltonian depends on a parameter which is dynamically determined along the chain. At one loop we will show how to map the holomorphic SU(3) scalar sector to a dynamical 15-vertex model, which corresponds to an RSOS model, whose adjacency graph can be read off from the gauge theory quiver/brane tiling. One scalar SU(2) sub-sector is described by an alternating nearest-neighbour Hamiltonian, while another choice of SU(2) sub-sector leads to a dynamical dilute Temperley-Lieb model. These sectors have a common vacuum state, around which the magnon dispersion relations are naturally uniformised by elliptic functions. For the example of the SU(N)xSU(N) quiver theory we will study these dynamical chains by solving the one- and two-magnon problems with the coordinate Bethe ansatz approach.

Giacomo Sberveglieri (SISSA): Resurgence and 1/N Expansion in Integrable Field Theories

In theories with renormalons the perturbative series is factorially divergent even after restricting to a given order in 1/N, making the 1/N expansion a natural testing ground for the theory of resurgence. We studied in detail the interplay between resurgent properties and the 1/N expansion in various integrable field theories with renormalons, namely the non-linear sigma model, the principal chiral field and the Gross-Neveu models. We focused on the free energy in the presence of a chemical potential coupled to a conserved charge, which can be computed exactly with the thermodynamic Bethe ansatz (TBA). In this talk, after an introduction on the nature of perturbative series and having set the key ingredients and tools of our study, I’ll present and discuss the results: they turned out to be different in the three models. While in some examples, the terms in the 1/N expansion can be fully decoded in terms of a resurgent trans-series in the coupling constant, in others each term in the 1/N expansion includes non-perturbative corrections which can not be predicted by a resurgent analysis of the corresponding perturbative series. Notably, in the principal chiral field we found a new, explicit solution for the large N free energy which can be written as the median resummation of a trans-series with infinitely many, analytically computable IR renormalon corrections.

Eric Perlmutter (IPhT, Saclay): On 2D CFT Spectra

I will explain how the spectra of 2D CFTs, both rational and irrational, exhibit surprising rigidity and determinacy. These structures follow from new ways of leveraging discreteness, integrality and modularity constraints on torus observables, via technology from the world of modular forms. Our techniques suggest alternative bootstrap approaches to the space of 2D CFTs.

Suvrat Raju (ICTS, India): Holography of Information and Massive Islands

We will review recent results that suggest that, in any standard theory of quantum gravity, information available on the bulk of a Cauchy slice

must also be available near the boundary of the slice. These ideas indicate how holography should be extended to four dimensional

asymptotically flat spacetimes, shed light on the origins of AdS/CFT and provide a robust resolution of some versions of the information paradox.

We contrast this picture with the paradigm of islands and argue that islands are consistent only in theories of massive gravity.

Yifan Wang (NYU): Fusion Category Symmetries in Quantum Field Theory

Topological defects provide a modern perspective on symmetries in quantum field theory. They generalize the familiar invertible symmetries described by groups to non-invertible symmetries described by fusion categories. Such generalized symmetries are ubiquitous in quantum field theory and provide new constraints on renormalization group flows and the IR phase diagram. In this talk I'll review some recent progress in identifying and understanding fusion category symmetries in 1+1d conformal field theories. Time permitting, I'll also comment on higher dimensional generalizations.

Horatiu Nastase (IFT, Sao Paulo): A 4d duality web

In 3 dimensions, in 2016 a web of dualities was found that includes particle-vortex duality

(bose-bose) and Son's duality (fermi-fermi), and is derived from a basic bosonization duality

(fermi-bose). The duality is between field theories with Chern-Simons terms ("with flux added"),

and has been used to obtain implications for physical systems, as in the case of the usual

electric-magnetic duality in 4 dimensions. A derivation of a massive deformation of the basic

bosonization step was found, based on a combination of particle-vortex duality and the BQ

bosonization map. We use an extension of this procedure to 4 dimensions to generate

(constructively) a duality web for field theories with theta terms.

In the review part, I will review dualities, in particular T-duality, particle-vortex and bosonization

described as a duality.

Avia Raviv-Moshe (Stony Brook): Renormalization Group Flows on Line Defects

In this talk, we will consider line defects in d-dimensional CFTs. The ambient CFT places nontrivial constraints on renormalization group flows on such line defects. We will see that the flow on line defects is consequently irreversible and furthermore a canonical decreasing entropy function exists. This construction generalizes the g theorem to line defects in arbitrary dimensions. We will demonstrate this generalization in some concrete examples, including a flow between Wilson loops in 4 dimensions, and an O(3) bosonic theory coupled to impurities with large isospin.

Omer Gurdogan: Coactions of Feynman periods from integrability

Results in Quantum Field Theories, such as anomalous dimensions or scattering amplitudes are known to exhibit rich properities under the motivic coaction that acts on period integrals. I will show how these phenomena arise directly in an integrability setup describing the anomalous dimensions of an integrable four-dimensional scalar model.

Yolanda Lozano (Oviedo): New advancements in AdS_3/CFT_2 and AdS_2/CFT_1

I will describe recent constructions of AdS_3 and AdS_2 solutions to Type II supergravities with (0,4) supersymmetries and their proposed 2d and 1d dual superconformal field theories. I will briefly address the interpretation of some of these solutions as describing 2d and 1d defects within higher dimensional CFTs.