|07.03.2023||TU||Three-dimensional gravity in the BV-BFV formalism||Giovanni Canepa||In this talk I will give a brief introduction to the BV-BFV formalism for quantum field theories. Subsequently, I will present the BV-BFV formulation of three-dimensional general relativity in the Einstein Cartan variables and draw a connection with BF and Chern-Simons theories. This talk is based on arXiv:1905.09333, a joint work with Michele Schiavina.|
|14.03.2023||UV||Relative entropy and dynamical black holes||Edoardo D'Angelo||Since the discovery of the Bekenstein-Hawking formula, there had been many attempts to derive the entropy of black holes from the entanglement between the degrees of freedom inside and outside the event horizon. This entanglement entropy reproduces the area-law, but it suffers from divergences in the continuum limit. In this talk, I show how to derive the Bekenstein-Hawking entropy from the relative entropy, which is well-defined also for continuum theories such as QFT, in the case of dynamical, spherically symmetric black holes. I first review the algebraic quantization of a free scalar field on curved space-times, and show how to compute its relative entropy from the initial data at infinity. Using the back-reaction of a free, scalar quantum field on the metric, I show that a variation in the relative entropy between coherent states of the field produces a variation of one-quarter of the black hole horizon area.|
|21.03.2023||TU||Galilean and Carrollian limits of non-relativistic strings||Gerben Oling||I will present and analyze two novel classes of non-relativistic string actions. Non-relativistic string theory usually refers to a particular limit of string theory that results in a non-relativistic spectrum, as I will briefly review from a modern geometrical perspective. However, the worldsheet geometry of such strings is still Lorentzian. Motivated in part by decoupling limits of N=4 SYM, I will introduce two related constructions that lead to novel classes of non-relativistic strings with Galilean and Carrollian structures on the worldsheet, respectively. Time permitting, I will briefly present their Hamiltonian constraint analysis, and I will comment on their potential relevance for accessing new solvable subsectors of AdS/CFT.|
|28.03.2023||UV||Some approaches to superstring cosmology||Robert Brandenberger||I will review the criteria which a modelof the early universe must satisfy in order to make successful contact with current observations. I will then show that effective field theory inevitably breaks down in the early universe, and that tools from quantum gravity must be used if we are to understand the early stages of the universe. I will then discuss models of early universe cosmology arising from superstring theory, in particular ``String Gas Cosmology".|
|18.04.2023||UV||A little piece of quantum de Sitter||Eleanor Harris||Dilaton gravity models in two dimensions, such as JT gravity, have provided fruitful results as simple holographic models in recent years. It is possible to deform such theories away from the usual AdS bulk to produce geometries that can be de Sitter-like in the interior. We will begin with a review of JT gravity, and then will see how by including a timelike Dirichlet boundary, for example, geometries that interpolate between de Sitter in the IR to anti-de Sitter near the boundary can be found that are stable under thermodynamic fluctuations. We will conclude by proposing a dual matrix model, whose degrees of freedom will hopefully be able to capture the physics of the de Sitter region in the bulk.|
|25.04.2023||TU||Non-perturbative strings, asymptotic safety, and the swampland||Alessia Platania||Quantum gravity is undoubtfully one of the most important missing pieces in the understanding of the mathematical structure of our universe. The impossibility of consistently quantizing gravity via perturbative quantum field theory has led to a plethora of different proposals, from asymptotically safe gravity to non-local gravity, loop quantum gravity, and string theory. Different approaches face different problems and have succeeded in different areas. Yet, on the conceptual side, it is not obvious that all these frameworks are inequivalent or unrelated: some theories may be low-energy approximations of others, or could even provide different mathematical descriptions of the same physics. On the technical side, the knowledge gained in an approach could be useful to investigate certain aspects of others. In this spirit, I will review progress in connecting and contrasting two theories: asymptotically safe gravity and string theory. Specifically, I will discuss how to test asymptotic safety using stringy swampland constraints, and how techniques developed in the context of asymptotically safe gravity can be exploited to compute cosmological higher-derivative corrections to all orders in string theory.|
|02.05.2023||UV||An asymptotic framework for gravitational scattering||Geoffrey Compère||I will first review the main difficulties in defining scattering in the presence of gravity. I will then outline some concepts useful at defining scattering in asymptotic Minkowski spacetime: a puzzle-piece diagram with 5 asymptotic expansions and 4 overlapping regions that faithfully describe massless and massive fields. I will show in which sense particles, stars and black holes can be described with the same tools in a unified way. I will discuss the BMS covariance that inevitably occurs when gravitational radiation is present and contrast when is BMS symmetry important (to define memory effects) and when it can be bypassed (to define invariant Lorentz charges and intrinsic spin of bodies). I will finally discuss infrared divergences and propose how to regulate them.|
|09.05.2023||TU||Covariant Lagrangians for (twisted) self-duality equations of (non-)chiral p-forms and their Abelian interactions||Karapet Mkrtchyan||We present a covariant Lagrangian formulation for p-form fields in Minkowski spaces of arbitrary dimensions that treat electric and magnetic degrees of freedom on equal footing. This formulation allows the inclusion of arbitrary abelian self-interactions. In d=4k+2, we cover all abelian self-interactions of a chiral (self-dual) 2k-form, the most interesting example being d=6 (we also comment on d=2 and 10), where the general abelian self-interactions are parametrized by a function of one variable. For d=4k, we cover all abelian self-interactions of (2k-1)-forms, including those with SO(2) duality symmetry. For d=4, we give a simple democratic actionfor arbitrary non-linear electrodynamics involving an arbitrary function of two variables and its duality symmetric subclass manifesting SO(2) symmetry, parametrized by a function of one variable. This construction, in particular, covers all interesting examples in the literature.|
|16.05.2023||UV||Infrared Finite Scattering Theory in QFT and Quantum Gravity||Gautam Satishchandran||A long-standing problem in QFT and quantum gravity is the construction of an “IR-finite" S-matrix. In the gravitational case, the existence of these “infrared divergences” is intimately tied to the “memory effect” (i.e. the permanent displacement of test masses due to the passage of a gravitational wave). In this talk, I shall explain the origin of these connections and illustrate that the construction of an IR-finite S-matrix requires the inclusion of states with memory (which do not lie in the standard Fock space). In massive QED an elegant solution to this problem was provided by Faddeev and Kulish who constructed an incoming/outgoing Hilbert space of charged particles “dressed” with memory. We illustrate the ``preferred status'' of such states and their relationship to the superselection/decoherence structure of QED. However, we show that this construction fails in the case of massless QED, Yang-Mills theories, linearized quantum gravity with massless/massive sources, and in full quantum gravity. In the case of quantum gravity, we prove that the only "Faddeev-Kulish" state is the vacuum state. We also show that “non-Faddeev-Kulish” representations are also unsatisfactory. Thus, in general, it appears there is no preferred Hilbert space for scattering in QFT and quantum gravity. We argue that, at a fundamental level, one must formulate scattering theory without an a priori choice of Hilbert space. We outline the framework of such a manifestly IR-finite scattering theory.|
|23.05.2023||TU||Kaluza-Klein Spectrometry for String Theory Compactifications||Emanuel Malek||I will present a powerful new method that for the first time allows us to compute the Kaluza-Klein spectrum of a large class of string theory compactifications, including those arising in maximal gauged supergravities and beyond. This includes geometries with little to no remaining (super-)symmetries, completely inaccessible by previous methods. I will show how these insights can be used to holographically compute the anomalous dimensions of protected and unprotected operators in strongly-coupled CFTs, as well as to study global properties of their conformal manifolds. I will also show how the method can be used to determine the perturbative stability of non-supersymmetric AdS vacua. We will see the importance of higher Kaluza-Klein modes to the physics of string compactifications, e.g. in realising the compactness of moduli spaces, and in destabilising vacua that appear to stable in lower-dimensional supergravities. If time permits, I will comment on ongoing work extending these methods to capturing cubic interactions in supergravity, where we will encounter a surprising new structure that is inherited from the consistent truncation to maximal gauged supergravity.|
|06.06.2023||UV||New higher-spin topological systems in 3D||Nicolas Boulanger||Motivated by the generation of action principles from off-shell dualisation, we present a general class of free, topological theories in three dimensions that exhibit higher-spin gauge invariance. In the spin-2 case, we recover the triplet system already known before. The higher-spin systems that we obtain, on the other hand, seem to be new. We show that the flat limit of our action around (A)dS_3 background gives rise to a one-parameter family of inequivalent actions in Minkowski space, whose non-Abelian deformations are studied.|
|13.06.2023||UV||Dark energy and modified gravity: the cosmographic perspective||Rocco D'Agostino||The success and the shortcomings of the standard cosmological model are discussed in view of the most recent theoretical and observational constraints. The motivations for considering extensions and/or modifications of General Relativity are taken into account, with the possibility to describe the gravitational dynamics under different frameworks. A powerful approach to discriminate among competing cosmological scenarios is given by the cosmographic method. After reviewing the main features of standard cosmography, we highlight how to overcome the convergence issue jeopardizing current low redshift-cosmographic distances by means of rational polynomials. Based on numerical bounds over the cosmographic series, we investigate the origin of cosmic acceleration and the possibility of going beyond the standard cosmological model to explain the dark energy problem.|
|20.06.2023||TU||Modifying General Relativity using Topological Terms||Francesco Bajardi||Despite the success and the formal elegance, general relativity suffers from shortcomings at any scale of energy. For this reason, several alternatives have been proposed over the years, with the aim to address issues provided by Einstein's gravity. Within the plethora of modified theories of gravity, particular interest is gained by modifications including topological invariants, such as the Gauss-Bonnet term. Extending the gravitational action with the Gauss-Bonnet invariant leads to interesting dynamics in cosmological and spherically symmetric backgrounds. Specifically, the early and late-time Universe bahaviours can be predicted without invoking any Dark Energy, which is thus mimicked by geometric contributions. Moreover, considering the Gauss-Bonnet term into the action can settle theoretical issues, currently faced by General Relativity. After selecting the form of the action, the gravitational field equations can yield analytic solutions whose free parameters can be constrained by experiments and observations. Specifically, the starting model can be selected by the so called "Noether Symmetry Approach", a selection criterion aimed at finding easily-handled models containing symmetries. The resulting conserved quantities can be used to reduce the minisuperspace dimension and find out exact solutions. In this talk, different modifications of General Relativity including the Gauss-Bonnet term are discussed, such as scalar-tensor models, non-local models, extended and teleparallel models.|
|27.06.2023||TU||Non-relativistic geometry and higher-spins in the fractional quantum Hall effect||Patricio Salgado-Rebolled||It is well-known that the low-energy regime of the fractional quantum Hall effect (FQHE) can be described by Abelian and non-AbelianChern-Simons theories, while the corresponding chiral edge states are described by rational conformal field theories. However, other collective excitations exist in the FQHE that cannot be described by topological field theory. One example is the long-wavelengthlimit of the Girvin-MacDonald-Platzman (GMP) mode, a gapped spin-2 mode associated with an internal metric of the system, which can be described in terms of non-relativistic geometry. Moreover, in addition to the GMP mode, additional collective excitationsdescribed by non-relativistic higher-spin fields need to be introduced in the known effective field theory of the FQHE to reproduce the complete GMP algebra. In the first part of this talk, we propose a non-relativistic model based on a Chern-Simons theory invariant under a supersymmetric extensionof the Nappi-Witten algebra, capturing the physical properties of the Moore-Read state in the FQHE. The boundary dynamics of the theory are described by a supersymmetric WZW model, which reduces to a chiral boson and a chiral Majorana fermion, consistent withthe boundary states of the Moore-Read state. In the second part of the seminar, we explore the possibility of formulating effective descriptions for higher-spin fields in the FQHE by considering suitable non-relativistic limits of relativistic higher-spin fieldsin 2+1 dimensions.|
The Joint Theory Seminar is held alternately at the University of Vienna and the Technische Universität Wien, either on site or online via Zoom.
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Black holes and holography
Technische Universität Wien
University of Vienna
Black holes and holography
Technische Universität Wien
University of Vienna