University of Crete HEP Seminars


Towards solving CFTs with artificial intelligence

Speaker: Costis Papageorgakis
Institution: Queen Mary University London
Time: Tuesday 28 September 2021, 13:00
Venue: Webinar
Abstract: I will introduce a novel numerical approach for solving the conformal-bootstrap equations with Reinforcement Learning. I will apply this to the case of two-dimensional CFTs, successfully identifying well-known theories like the 2D Ising model and the 2D CFT of a compactified scalar, but the method can be used to study arbitrary (unitary or non-unitary) CFTs in any spacetime dimension.
Recording: The recorded talk can be found here.

Revisiting Coleman-de Luccia transisitions in the AdS regime using holography

Speaker: Elias Kiritsis
Institution: University of Crete and APC, Paris
Time: Tuesday 05 October 2021, 13:00
Venue: Webinar
Abstract: Coleman-de Luccia processes for AdS to AdS decays in Einstein-scalar theories are studied. Such tunnelling processes are interpreted as vev-driven holographic RG flows of a quantum field theory on de Sitter space-time. These flows do not exist for generic scalar potentials, which is the holographic formulation of the fact that gravity can act to stabilise false AdS vacua. The existence of Coleman-de Luccia tunnelling solutions in a potential with a false AdS vacuum is found to be tied to the existence of exotic RG flows in the same potential. Such flows are solutions where the flow skips possible fixed points or reverses direction in the coupling. This connection is employed to construct explicit potentials that admit Coleman-de Luccia instantons in AdS and to study the associated tunnelling solutions. Thin-walled instantons are observed to correspond to dual field theories with a parametrically large value of the dimension ∆ for the operator dual to the scalar field, casting doubt on the attainability of this regime in holography. From the boundary perspective, maximally symmetric instantons describe the probability of symmetry breaking of the dual QFT in de Sitter. It is argued that, even when such instantons exist, they do not imply an instability of the same theory on flat space or on R × S3^{3}3.
Recording: The recorded talk can be found here.

Bulk geometry in gauge/gravity duality and color degrees of freedom

Speaker: Masanori Hanada
Institution: University of Surrey
Time: Tuesday 12 October 2021, 13:00
Venue: Webinar
Abstract: U(N) supersymmetric Yang-Mills theory naturally appears as the low-energy effective theory of a system of N D-branes and open strings between them. Transverse spatial directions emerge from scalar fields, which are N x N matrices with color indices; roughly speaking, the eigenvalues are the locations of D-branes. In the past, it was argued that this simple 'emergent space' picture cannot be used in the context of gauge/gravity duality, because the ground-state wave function delocalizes at large N, leading to a conflict with the locality in the bulk geometry. In this paper we show that this conventional wisdom is not correct: the ground-state wave function does not delocalize, and there is no conflict with the locality of the bulk geometry. This conclusion is obtained by clarifying the meaning of the 'diagonalization of a matrix' in Yang-Mills theory, which is not as obvious as one might think. This observation opens up the prospect of characterizing the bulk geometry via the color degrees of freedom in Yang-Mills theory, all the way down to the center of the bulk.
Recording: The recorded talk can be found here.

AdS bulk locality from sharp CFT bounds

Speaker: Dalimil Mazac
Institution: IAS Princeton
Time: Tuesday 19 October 2021, 16:00
Venue: Webinar
Abstract: It has been a long-standing conjecture that any CFT with a large central charge and a large gap M in the spectrum of single-trace operators must be dual to a local effective field theory in AdS. In my talk, I will discuss a proof of a sharp form of this conjecture. In particular, I will explain how to derive numerical bounds on bulk Wilson coefficients in terms of M using the conformal bootstrap. The bounds exhibit scaling in M expected from dimensional analysis in the bulk. The main technical tools are dispersive CFT sum rules. These provide a dictionary between CFT dispersion relations and S-matrix dispersion relations in appropriate limits. This dictionary allows one to apply recently-developed flat-space methods to construct positive CFT functionals. My talk will be based on 2106.10274, which is joint work with S. Caron-Huot, L. Rastelli, and D. Simmons-Duffin.
Recording: The recorded talk can be found here.

Mass hierarchies from complex fixed points

Speaker: Anton Faedo
Institution: University of Oviedo
Time: Thursday 21 October 2021, 13:00
Venue: Webinar
Abstract: Our fundamental description of the Universe is filled with hierarchies and it is an important theoretical question to understand their origin. An elegant mechanism to generate large hierarchies is that of Fixed Point Annihilation (FPA), in which a pair of fixed points of the Renormalization Group merge and migrate to the complex plane. This is believed to be responsible for the walking behavior of some gauge theories or the appearance of weak first-order transitions in condensed-matter models. In this talk we will discuss the physics of these complex fixed points, the possibility of defining complex Conformal Field Theories (CFTs) associated to them and how the presence of several complex conjugate pairs can have a substantial impact on the generated hierarchies. We will present weakly coupled field-theory examples and propose a gravitational dual to FPA, the resulting complex fixed points and their related complex CFTs at strong coupling.
Recording: The recorded talk can be found here.

Bootstrapping quantum mechanical systems in 1D

Speaker: David Berenstein
Institution: UC Santa Barbara
Time: Tuesday 26 October 2021, 17:00
Venue: Webinar
Abstract: I will show how one can solve numerically for the energy levels of certain quantum mechanical systems using a recursive plus unitarity method. This method, called "bootstrap", uses the equations of motion of the system to find recursions between certain expectation values in an energy eigenstate. Combined with positivity of the expectation value of any positive operator (what one calls unitarity), one finds relations to classic problems in mathematics: the moment problem. One can check this positivity numerically and theorems guarantee the existence of a probability measure that solves the moment problem if certain conditions are satisfied.
Recording: The recorded talk can be found here.

SYK wormhole formation in real time

Speaker: Alexey Milekhin
Institution: UC Santa Barbara
Time: Tuesday 02 November 2021, 19:00
Venue: Webinar
Abstract: We study the real time formation of the ground state of two coupled SYK models. This is a highly entangled state which is close to the thermofield double state and can be viewed as a wormhole. We start from a high temperature state, we let it cool by coupling it to a cold bath. We numerically solve for the large N dynamics. Our main result is that the system forms a wormhole by going through a region with negative specific heat, taking time that is independent of N. The dynamics is smooth everywhere and it seems to follow the equilibrium thermodynamic configurations of the microcanonical ensemble. Also we comment on the relation between this coupled SYK model and Jackiw-Teitelboim gravity theory with bulk fields.
Recording: The recorded talk can be found here.

Random matrix theeory for complexity growth and black hole interiors

Speaker: Moshe Rozali
Institution: University of British Columbia
Time: Tuesday 09 November 2021, 18:30
Venue: Webinar
Abstract: We study a precise and computationally tractable notion of operator complexity in holographic quantum theories, including the ensemble dual of Jackiw-Teitelboim gravity and two-dimensional holographic conformal field theories. This is a refined, "microcanonical" version of K-complexity that applies to theories with infinite or continuous spectra (including quantum field theories), and in the holographic theories we study exhibits exponential growth for a scrambling time, followed by linear growth until saturation at a time exponential in the entropy —a behavior that is characteristic of chaos. We show that the linear growth regime implies a universal random matrix description of the operator dynamics after scrambling. Our main tool for establishing this connection is a "complexity renormalization group" framework we develop that allows us to study the effective operator dynamics for different timescales by "integrating out" large K-complexities. In the dual gravity setting, we comment on the empirical match between our version of K-complexity and the maximal volume proposal, and speculate on a connection between the universal random matrix theory dynamics of operator growth after scrambling and the spatial translation symmetry of smooth black hole interiors.
Recording: The recorded talk can be found here.

Deep learning and holographic QCD

Speaker: Koji Hashimoto
Institution: Kyoto University
Time: Tuesday 16 November 2021, 10:00
Venue: Webinar
Abstract: Bulk reconstruction in the AdS/CFT correspondence is a key idea revealing the mechanism of it, and various methods were proposed to solve the inverse problem. We use deep learning and identify the neural network as the emergent geometry, to reconstruct the bulk. The lattice QCD data such as chiral condensate, hadron spectra or Wilson loop is used as input data to reconstruct the emergent geometry of the bulk. The requirement that the bulk geometry is a consistent solution of an Einstein-dilaton system determines the bulk dilaton potential backwards, to complete the reconstruction program. We demonstrate the determination of the bulk system from QCD lattice/experiment data.
Recording: The recorded talk can be found here.

Carroll symmetry in field theory, gravity, and cosmology

Speaker: Niels Obers
Institution: NORDITA
Time: Thursday 25 November 2021, 14:00
Venue: Webinar
Abstract: Carroll symmetry arises from Poincare symmetry upon taking the limit of vanishing speed of light. I will start with a pedagogical introduction to Carroll symmetry, Carroll particles and Carroll field theories. Then I will discuss some applications of Carroll symmetry at the level of fluids and cosmology, showing in particular that Carroll symmetry might be relevant for dark energy and inflation. Finally, I will comment on Carroll gravity by considering the small speed of light expansion of general relativity.
Recording: The recorded talk can be found here.

Analyticity and unitarity for cosmological correlators

Speaker: Shota Komatsu
Institution: CERN
Time: Tuesday 30 November 2021, 13:00
Venue: Webinar
Abstract: I will discuss the fundamentals of quantum field theory on a rigid de Sitter space. First, I will show that the perturbative expansion of late-time correlation functions to all orders can be equivalently generated by a non-unitary Lagrangian on a Euclidean AdS geometry. This finding systematizes recent findings in the literature on the relation between dS and AdS Feynman diagrams, as well as allows us to establish basic properties of these correlators, which comprise a Euclidean CFT. Second, I use this to infer the analytic structure of the spectral density that captures the conformal partial wave expansion of a late-time four-point function, to derive an OPE expansion, and to constrain the operator spectrum. Third, I will prove that unitarity of the de Sitter theory manifests itself as the positivity of the spectral density. This statement does not rely on the use of Euclidean AdS Lagrangians and holds non-perturbatively.
Recording: The recorded talk can be found here.

Quasi-normal modes of black holes and black branes from quantum Seiberg-Witten curves

Speaker: Massimo Bianchi
Institution: University of Rome
Time: Thursday 02 December 2021, 14:30
Venue: Webinar
Abstract: After reviewing the role Quasi-Normal Modes (QNMs) play in the Gravitational Wave (GW) signals emitted in the ring-down phase of Black-Hole (BH) mergers, we present a novel efficient approach to compute QNMs of BHs, D-branes and fuzz-balls, based on quantum Seiberg-Witten (SW) curves for N=2 supersymmetric Yang-Mills (SYM) theories. We find remarkable agreement with numerical results obtained by means of Leaver's method of continuous fractions and with `semi-classical' results obtained in the eikonal approximation, based on geodetic motion. Finally we discuss the extension to D3-branes and their bound states of Couch-Torrence (CT) conformal inversions, that exchange horizon and infinity, and show that they keep the photon-sphere (or photon-halo) fixed.
Recording: The recorded talk can be found here.

Exploring continuum quantum field theories of fracton order

Speaker: Andreas Karch
Institution: University of Texas, Austin
Time: Tuesday 07 December 2021, 16:00
Venue: Webinar
Abstract: Recently discovered solvable lattice models, known as fractons, have forced us to rethink the very basic question of what a quantum field theory is. In order to describe the continuum limit of fracton lattice models, novel quantum field theories had to be written down that go beyond what textbooks taught us, exhibiting unusual features such as subsystem symmetries, UV sensitivity of IR quantities, and broken rotation invariance. This talk will describe several explorations of the interesting new properties these unusual field theories exhibit.
Recording: The recorded talk can be found here.

Towards the non-perturbative cosmological bootstrap

Speaker: Kamran Salehi Vaziri
Institution: EPFL
Time: Thursday 09 December 2021, 14:00
Venue: Webinar
Abstract: We study Quantum Field Theory (QFT) on a background de Sitter spacetime dS_d+1. Our main tool is the Hilbert space decomposition in irreducible unitarity representations of its isometry group SO(d+1,1). Throughout this work, we focus on the late-time physics of dS_d+1, in particular on the boundary operators that appear in the late-time expansion of bulk local operators. As a first application of the Hilbert space formalism, we recover the Källen-Lehmann spectral decomposition of bulk two-point functions. In the process, we exhibit a relation between poles in the corresponding spectral densities and boundary CFT data. Next, we study the conformal partial wave decomposition of four-point functions of boundary operators. These correlation functions are very similar to the ones of standard conformal field theory, but have different positivity properties that follow from unitarity in de Sitter. We conclude by proposing a non-perturbative conformal bootstrap approach to the study of these late-time four-point functions, and we illustrate our proposal with a concrete example for QFT in dS_2.
Recording: The recorded talk can be found here.

On scale separated AdS vacua of string theory

Speaker: Thomas Van Riet
Institution: KU Leuven
Time: Tuesday 14 December 2021, 14:00
Venue: Webinar
Abstract: Finding phenomenological vacua that are on a trustworthy footing within string theory is hard. Some crucial properties of such vacua are: moduli stabilisation and a small cosmological constant in units of the fundamental scale. In this talk I will review the progress on achieving this for SUSY AdS vacua.
Recording: The recorded talk can be found here.