# spring2017

June 9: Eva Llabres

Spinning Geodesic Witten Diagrams

In this talk, based on 1702.06128, we give a systematic procedure to evaluate conformal partial waves involving CFT$_d$ symmetric tensors using geodesic Witten diagrams in AdS$_{d+1}$. Using this procedure we discuss how to draw a line between the tensor structures in the CFT and cubic interactions in AdS. We contrast this map to known results using three-point Witten diagrams: the maps obtained via volume versus geodesic integrals differ. Despite these differences, we show how to decompose four-point exchange Witten diagrams in terms of geodesic diagrams.

And we will informally discuss:

1. Bulk RG flows and boundary states in CFTs (John Cardy)
2. Universal entanglement spectra of gapped 1-D QFTs (Cho, Ludwig and Ryu)
3. Black holes as bubbles of AdS (U. H. Danielsson, G. Dibitetto, S. Giri)
4. Nonlinear Gravity from Entanglement in Conformal Field Theories (Faulkner, Haehl, Hijano, Parrikar, Rabideau, Van Raamsdonk)
5. Einstein's Equations from Varying Complexity (Czech)

Title:
Effective matter fields in CG inspired from Cartan Geometry
Abstract:
I will present some models of massive gravity that arise from the consideration of the Chern-Simons form in 2+1 dimensions for a spin connection with a nontrivial contorsion. When splited the metric degres of freedom from the torsional ones, we regard the latter as effective matter fields, to know a scalar and a vector field wich correspond to the competelly antysymmetric part and the trace of torsion, respectively.
I will first discuss the simplest case (1504.06083), where the vector field is neglected, obtaining Chiral Gravity (CG) nonminimally coupled to a scalar field, and I will show how CG is included as the special sector of constant scalar field. This case presents interesting features, including wormhole solutions with non constant curvature that are not included in the spectrum of CG.
Then I will discuss a more general case (1603.01332), when both the scalar and vector fields are present, and I will argue that the vector field can be interpreted as a Gauge field for the Weyl symmetry, in fact I will show that this theory is classically equivalent to CG+TME (Topologycally Massive Electrodynamics).
Finally, and if time allows, I will briefly comment on the work in progress that I am doing lastly in this field.

And we will informally discuss:

1. Genus two Partition Functions: 4 recent papers (1704.08250, 1705.05855, 1705.05865, 1705.05862)
2. A proof of the weak gravity conjecture (Shahar Hod)
3. Nonlinear Gravity from Entanglement in Conformal Field Theories (Faulkner, Haehl, Hijano, Parrikar, Rabideau, Van Raamsdonk)
4. On distinguishability of black hole microstates (Bao, Ooguri)

May 10: Gerben Oling will tell us about

Diving into traversable wormholes

Gao, Jafferis and Wall (1608.05687) showed that briefly turning on a simple coupling between the two boundary theories of an eternal BTZ can violate the average null energy conditon so as to create a traversable wormhole in the bulk. I will discuss the recent work of Maldacena, Stanford and Yang (1704.05333), who study this protocol for a nearly-AdS2 eternal black hole. They show that although traversing the resulting wormhole can be an entirely smooth process, the number of qubits one can send through it until backreaction closes the bridge is limited. In fact, it is bounded by the information exhange it took to set up the boundary interaction that caused the wormhole. They also provide a simple description of this procedure in terms of the dynamics of the boundary of nearly-AdS2 spaces and give an interesting perspective on the Hayden-Preskill cloning paradox.

And we will informally discuss:

1. Line Operators in the Standard Model
2. Modular Hamiltonians of excited states, OPE blocks and emergent gauge fields (Gabor Sarosi and Tomonori Ugajin)

Apr 26: Jorrit Kruthoff will tell us about

Title: Eigenstate thermalisation hypothesis

Abstract: In this talk I will discuss the recent revival of ideas by Mark Srednicki in the 90s on how isolated quantum systems can reach thermal equilibrium, known as the Eigenstate Thermalisation Hypothesis (ETH). First I will review the mechanism and show what systems obey and do not obey ETH. Then I will go on and discuss some recent developments in understanding ETH in CFTs.

References:Original ETH: https://journals.aps.org/pre/pdf/10.1103/PhysRevE.50.888; Deutsch, "Quantum statistical mechanics in a closed system"Subsystem ETH: 1610.00302; 1611.08764; PRL 96, 050403 (2006); 1703.08724Large c CFT / Holography: 1403.6829; 1501.05315
Apr 7: Subham Dutta Chowdhury will tell us about

Title: Spectral sum rules for conformal field theories in arbitrary dimensions

Abstract:
We derive spectral sum rules in the shear channel for conformal field theories at finite temperature in general $d\geq 3$ dimensions. The sum rules result from the OPE of the stress tensor at high frequency as well as the hydrodynamic behaviour of the theory at low frequencies. The sum rule states that a weighted integral of the spectral density over frequencies is proportional to the energy density of the theory. We show that the proportionality constant can be written in terms the Hofman-Maldacena variables $t_2, t_4$ which determine the three point function of the stress tensor. For theories which admit a two derivative gravity dual this proportionality constant is given by $\frac{d}{2(d+1)}$. We then use causality constraints and obtain bounds on the sum rule which are valid in any conformal field theory. Finally we demonstrate that the high frequency behaviour of the spectral function in the vector and the tensor channel are also determined by the Hofman-Maldacena variables.

And we will informally discuss:

1. Local bulk physics from intersecting modular hamiltonians (Kabat, Lifschytz)
2. Schrodinger's Cat and World History: the Many Worlds Interpretation of Alternative Facts (Banks)
3. CFT descriptions of bulk local states in AdS black hole (Goto, Takayanagi)
4. The a-theorem and the Markov property of the CFT vacuum (Casini, Teste, Torroba)

Mar 17: Faculty Corner - Alejandra Castro

Will tell us about her research.

And we will informally discuss:

1. Crossing symmetry in alpha space (Hogervorst, van Rees)
2. A Covariant Version of Verlinde's Emergent Gravity (Hossenfelder)
3. On the Heat Kernel and Weyl Anomaly of Schrödinger invariant theory (Pal, Grinstein)
4. Infrared Realization of dS2 in AdS2 (Anninos, Hofman)
5. Causal Density Matrices (Engelhardt, Fischetti)

Mar 10: Sam van Leuven

Toda theory from six dimensions

I will discuss the recent derivation of the AGT correspondence by Cordova and Jafferis (https://arxiv.org/abs/1605.03997), which is a correspondence between the BPS sector of certain 4d N=2 SCFTs on the one hand and Toda theory on the other. For this I will first need to review older work of theirs concerning a derivation of the so-called 3d-3d correspondence (https://arxiv.org/abs/1305.2891), that relates the BPS sector of certain 3d N=2 CFTs to SL(N,C) Chern-Simons theory. Finally, if time permits, I will discuss work in progress with Gerben concerning the derivation of an extension of AGT involving surface operators on the gauge theory side and a more general W-symmetry on the 2d CFT side.

and we will informally discuss:

1. Geodesic Diagrams, Gravitational Interactions & OPE Structures (Castro, Llabres, Rejon-Barrera)
2. Bulk reconstruction and the Hartle-Hawking wavefunction (Jafferis)
3. Crossing symmetry in alpha space (Hogervorst, van Rees)
4. A Covariant Version of Verlinde's Emergent Gravity (Hossenfelder)

Mar 3: Lars Aalsma

Title:
Constraining Axion Inflation with the Weak Gravity Conjecture

Abstract:
The Weak Gravity Conjecture (WGC) is a conjecture about the spectrum of an effective field theory coupled to quantum gravity. While being a general statement about low energy effective actions, the WGC has been predominantly applied to constrain axion inflation, a candidate mechanism for generating a period of large field inflation within the context of string theory. In contrast, not so much progress has been made in understanding the deeper reasons behind the WGC. Recently, a new argument has been put forward by Hebecker and Soler in arXiv:1702.06130 that gives rise to an independent constraint on axion inflation, in agreement with estimates made by using the WGC. I will review why one should care about large field axion inflation, show how the WGC can be applied to axions and discuss the new argument by Hebecker and Soler.

and we will informally discuss:

1. Bulk Connectedness and Boundary Entanglement (Bao, Remmen)
2. Analyticity in Spin in Conformal Theories (Caron-Huot)

Feb 17: Fotios Dimitrakopoulos

2D CFT Partition Functions at Late Times

Based on 1611.04592

Abstract:
"One of the deepest questions in Quantum Gravity is the origin of the discrete spectrum of black hole microstates from the bulk perspective. In this paper, the late time behaviour of the analytically continued partition function Z(β+it)Z(β-it) in holographic 2dCFTs is discussed, both for chaotic and integrable theories, with the hope to shed some light on the bulk mechanism responsible for the restoration of the naively lost information."

And we will informally discuss:

1. Scrambling the spectral form factor: unitarity constraints and exact results (Sonner et al.)
2. Emergent gravity in galaxies and in the Solar System (Hees, Famaey, Bertone) Testing Verlinde's Emergent Gravity with the Radial Acceleration Relation (Lelli, McGaugh, Schombert)

Feb 10: Alberto Faraggi

will give the following talk:

“Phase Transitions in Higher Spin Gravity”

I will talk about the thermodynamic phase diagram of three-dimensional higher spin black holes. By analyzing the semi-classical partition function I will show the existence of Hawking-Page transitions from black hole states to the Ads_3 vacuum, first order phase transitions among black hole states and a second order critical point.

The reference paper is

https://arxiv.org/abs/1611.08025

And we will informally discuss:

1. No Simple Dual to the Causal Holographic Information? (Engelhardt & Wall)
2. Weak Gravity Conjecture and Extremal Black Holes (Cottrell, Shiu and Soler)

Feb 3: Juan Pedraza

will give the following talk:

Complexity

I will review two recent proposals for quantum complexity: the Complexity-Volume conjecture (arXiv:1402.5674, arXiv:1406.2678) and the Complexity-Action conjecture (arXiv:1509.07876, arXiv:1512.04993). If time allows, I will also discuss some selected additions to the literature of holographic complexity (arXiv:1606.08307, arXiv:1610.02038, arXiv:1612.00433, arXiv:1612.05439).

And we will informally discuss:

1. The Conformal BMS Group (Haco, Hawking, Perry, Bourjaily)
2. On the Uniqueness of Liouville and the Universality of BTZ (Yin et al)
3. BPS spectrum on AdS3xS3xS3xS1 (Eberhardt, Gaberdiel, Gopakumar, Li)

Jan 20: Victor Godet

will give the following talk:

Logarithmic corrections are universal?

In the appropriate regime, quantum corrections to the Bekenstein-Hawking entropy depend only on the low energy data and can serve as an "infrared window into the microstates". I will explain how to compute them and how they compare with the microscopic predictions. A recent computation in supergravity suggests that they possess a remarkable universality property: they seem to be independent of the black hole parameters.

And we will informally discuss:

1. From Global to Local Energy Conditions (Wall)

Jan 13: Gui Pimentel

Will give the following talk:

Title: Vacuum decay from particle production

In general curved backgrounds, the vacuum is unstable to particle production. Calculating the vacuum persistence rate in the general case is a hard problem. For free gapped matter, there is an interesting regime, in which the background is “almost strong enough” to produce particles. I will identify the threshold singularity in the vacuum persistence rate. The result is universal, in the sense that it does not depend on many features of the background. For gapless matter in 2D, I will present an exact formula for the decay rate.

Based on work in progress and some old work (1512.06721) with A. Polyakov and G. Tarnopolsky.

And we will informally discuss

1. Towards a QFT analog of the SYK model(Turiaci, Verlinde)
2. Modular invariance on S1×S3 and circle fibrations (Shaghoulian)