Joint Theory Seminar

Date	Title	Speaker	Abstract
06.10.2020	Looking for islands	Edgar Shaghoulian	I will review the role of replica wormholes in deriving a unitary Page curve for Hawking radiation and discuss extensions to flat spacetime and cosmology. I will also discuss general consistency conditions that guide the search for such nontrivial saddles in the gravitational path integral.
13.10.2020	Towards Super Teichmüller Spin TQFT	Nezhla Aghaei	The quantization of the Teichmüller theory has led to the formulation of the so-called Teichmüller TQFT for 3-manifolds. In this talk I will explain about the quantization of the super Teichmüller theory. I explain how we obtained concrete expressions for the partition functions of the super Teichmüller spin TQFT for a class of spin 3- manifold geometries, by taking advantage of the recent results on the quantization of the super Teichmüller theory. This talk is based on arXiv:2008.09829
20.10.2020
27.10.2020
03.11.2020	M-brane Realisation in Matrix Model	Yuhma Asano	An idea to formulate string theory or M-theory by a gauge theory attracts theorists and has been extensively studied. The gauge theory should be lower dimensional so that a geometry in string or M-theory, which has higher dimensions, must emerge from it. Especially, M5-brane realisation in matrix models has been a long-standing mystery. In this talk, we consider a one-dimensional matrix model, called the BMN matrix model, which is considered as a non-perturbative formulation of M-theory. In the BMN matrix model, there are some evidences that spherical M2- and M5-branes correspond to vacua of the matrix model. While it is easy to understand realisation of M2-brane geometries because the vacua form fuzzy 2-spheres, it requires non-perturbative computation in a strongly coupled regime to obtain realisation of M5-brane geometries. We solve this notorious problem by using the supersymmetric localisation method and see how the M-brane geometries are successfully obtained. Intriguingly, this result gives us a consistent picture of how the gravity-side spacetime in the gauge/gravity correspondence emerges from the matrix model.
10.11.2020	The Λ-BMS Charge Algebra in n dimensions	Adrien Fiorucci	The aim of my talk is to discuss brand-new results about the phase space of Einstein’s gravity in arbitrary dimensions with non-vanishing cosmological constant Λ. I will start by reviewing the solution space of asymptotically locally (A)dS spacetimes in the Starobinsky/Fefferman-Graham gauge and the renormalization of the presymplectic current needed when the boundary structure is allowed to fluctuate. Next, I will discuss the gravitational charge algebra for generic Al(A)dSnspacetimes, without assuming any further boundary condition than the minimal falloffs allowing for conformal compactification. Then I will introduce the Λ-BMSnalgebroid arising from some boundary gauge fixing and reducing to the Generalized BMSnalgebra in the flat limit Λ→0. The associated boundary conditions, equivalent to fix a foliation and a measure as universal structure on the boundary, do not freeze completely the transverse metric, leading to some symplectic flux through the conformal boundary. Finally, I will particularize the framework to the n=4 case and show that one can give sense to the flat limit of the phase space thanks to the incorporation of kinetic corner terms in the variational principle. The result of this process is exactly the phase space for Generalized BMS4 symmetries in asymptotically locally flat spacetimes.
17.11.2020	Do classical de Sitter string backgrounds exist?	David Andriot	A classical solution of string theory with a 4d de Sitter space-time could provide a simple and well-controlled setting to establish a connection to cosmological models. So far however, it has proven very difficult to find any example of such a solution. As will be recalled, attempts in this direction are plagued by many no-go theorems, or by the difficulty for concrete 10d supergravity solutions to fit in the classical stringy regime. This situation agrees with the recent claims of de Sitter swampland conjectures. These conjectures can actually be verified quantitatively, with a surprisingly good agreement, as will be shown. This may hint at a deep reason against classical de Sitter background, possibly to be found within the web of swampland conjectures, and the relation to the distance conjecture will be commented on.
24.11.2020	Misaligned Supersymmetry and Anti-D-branes	Flavio Tonioni	In this talk, I will review the concept of misaligned supersymmetry and discuss its relationship with anti-D-branes in type II string theories. Misaligned supersymmetry characterises a variety of non-supersymmetric string models: even if the particle spectrum lacks a one-by-one matching between particles with a different spin, there exist theories in which an infinite oscillation between fermionic and bosonic surpluses at each mass level results in a net cancellation that allows for a finite one-loop cosmological constant. Such a scenario is often discussed with regard to closed-string theories, but I will show that the spectrum of an anti-D-brane on top of an orientifold plane presents essentially the same properties. Anti-D-branes are crucial tools in many phenomenological string-theory realisations of features such as supersymmetry breaking and de Sitter uplifting, and I will comment on how an understanding of the effects of their infinite tower of states is related to their low-energy descriptions.
01.12.2020	Observables vs. gauge symmetries: lessons from the quasi-local approach	Wolfgang Wieland	An introduction to the loop representation to non-perturbative quantum gravity will be given. The talk will focus on the quasi-local approach and explain the distinction between observables, symmetries and gauge symmetries in the context of classical and quantum general relativity.
08.12.2020	Quantum gravity at the corner	Laurent Freidel	In this talk, I will present a new perspective on the non-perturbative quantization of gravity based on the concept of corner symmetries associated with local subregions. After introducing the corner algebra for various formulations of general relativity, I will illustrate its physical content in 4d metric and tetrad gravity. I will also present the formalism of edge modes, whose role is to restore gauge-invariance at boundaries while preserving non-trivial charges. If time permits, I'll show our recent progress on the representation theory of these algebras. I will also describe the relationship that these results entertain with standard loop gravity, asymptotic quantization, and the concept of holography.
15.12.2020	The Tadpole Problem	Severin Lüst	I will discuss string theory compactifications where a large number of moduli is stabilized by fluxes. I will present a conjecture which rules out the stabilization of all complex-structure moduli in F-theory at a generic point in moduli space by fluxes that satisfy the tadpole cancellation condition. Evidence for this conjecture comes from K3xK3 compactifications. Using evolutionary algorithms we found that moduli stabilization needs fluxes whose charge is slightly smaller than 1/2 of the number of moduli and larger than what is allowed by tadpole cancellation. I will furthermore comment on possible implications on de Sitter vacua obtained by antibrane uplift in long warped throats.
22.12.2020		Christmas Break
29.12.2020		Christmas Break
05.01.2021		Christmas Break
12.01.2021	Temperature of Black Holes in Horndeski Gravity Theories	Shahin Sheikh Jabbari	Horndeski theories are the most general family of scalar-tensor gravity with second order equations of motion. In these theories speeds of propagation of gravitons and photons are in general different and gravitons move on an effective metric different than the one seen by photons. These theories admit black hole solutions with the peculiar feature that they do not satisfy the first law with the entropy given by the Wald formula. We argue that the issue with the first law stems from the fact that black hole temperature is not given by the surface gravity of the black hole metric, but with the surface gravity of the effective metric seen by gravitons. The modified temperature, with the entropy unambiguously computed by the solution phase space method, yields the correct first law. We discuss far-reaching implications of our results for black hole thermodynamics beyond the Horndeski theories.
19.01.2021	Deformations of JT Gravity and KdV Equations	Joshua Kames-King	Recently, it was shown that the duality of double-scaled matrix models and JT gravity can be extended to include surfaces with conical defects. We show that the thermal partition function of this deformed theory may be written as the free energy of topological gravity around a shifted expansion point. By use of the KdV equation we furnish an off-shell description for both the single and multi-boundary cases of the partition function. We also rewrite the deformed partition function as a low temperature expansion, which allows for a convenient calculation of contributions up to a desired order. We use this approach to demonstrate the existence of a Hawking-Page phase transition between connected and disconnected surfaces for this instance of JT gravity. Furthermore, we calculate the spectral form factor and compare with the standard JT case. We conclude by commenting on the dS2 case.
26.01.2021	Topological Gravity as the Early Phase of our Universe	Prateek Agrawal	I will present a scenario motivated by string dualities and the swampland where the early universe is described by a topological phase of gravity. Many cosmological puzzles can be explained in terms of the topological nature of this phase. To obtain phenomenological predictions, a concrete realization of this scenario using Witten's four dimensional topological gravity will be presented. In this model, the CMB power spectrum is controlled by conformal anomaly coefficients. The model predicts the absence of tensor modes in the CMB fluctuations and significant non-Gaussianities in 4- and higher point functions.
02.02.2021
09.02.2021	Analytic Structure of Banana Feynman Integrals	Christoph Nega	In this talk I will introduce the so called banana Feynman integrals and explain their analytic properties in dimensional regularization. These Feynman integrals have a natural interpretation as relative periods of a complete intersection Calabi-Yau manifold, whose dimension is the loop order minus one of the banana integral. In particular, we find that the leading logarithmic structure in the high energy regime, which corresponds to the point of maximal unipotent monodromy, is determined by a novel Gamma-class evaluation in the ambient space of the mirror CY and the mirror CY itself. Moreover, we provide simple closed all loop formulas for the Frobenius kappa-constants at the physical threshold in terms of zeta values. I will also give a short outlook how the above structures evolve in the expansion in the dimensional regularization parameter.
16.02.2021	A new look at symmetries of 3d gravity	Marc Geiller	I will review the analysis of boundary symmetries in first order 3d gravity, and explain how the study of the boundary current algebra and the Sugawara construction actually leads to two dual notions of diffeomorphism charges. This provides a new understanding of the relationship between the second order and first order formulations, and of the existence of finite distance asymptotic (or symplectic) symmetries in topological theories. This analysis is performed on the most general theory of first order 3d gravity, which also enables to understand the duality between curvature and torsion, as well as the relationship with chiral and massive gravity.

Joint Theory Seminar

Tuesday 14:00-15:00

WS2020

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Daniel Grumiller

Associate Prof. Dr.techn.

Stefan Fredenhagen

Univ.-Prof. Dr.

Céline Zwikel

Postdoctoral researcher

Thorsten Schimannek

Postdoctoral researcher