Seminars

Abstract: 

Abstract:  To an outside observer, a black hole appears to be an ordinary quantum mechanical system with finite entropy and highly chaotic internal dynamics. Nevertheless, the low-temperature thermodynamics of the Kerr black hole presents several puzzles. For instance, the leading order semiclassical approximation to the black hole density of states predicts a surprisingly large ground state degeneracy, while poorly understood quantum corrections are known to become increasingly important at low temperatures. I will review the modern picture of black holes as quantum systems and then discuss a recent result on the leading correction to the low-temperature thermodynamics of the Kerr black hole that resolves many of the old puzzles. 

Abstract:  The study of Black-Hole perturbation theory is a classical problem in General Relativity and crucial to study gravitational waves. Due to the high order of symmetry of the BH gravitational field and the consequent separation of variables, the first order problem reduces to the study of linear ordinary second order differential equations. The resulting ODEs are of Fuchsian type and therefore, as already observed long ago by A.M.Polyakov, can be solved exactly in terms of classical - regular or irregular - Virasoro conformal blocks. By making use of the specific explicit expressions of the latter implied by the AGT dual perspective (susy gauge theory) on the conformal field theory, it is possible to explicitly solve the connection problem of the resulting (confluent)Heun equation and give novel exact and explicit formulas for the grey body factor, quasi-normal modes and Love numbers of diverse black holes. This will be explicitly applied to 4D Kerr and Schwarzschild-(A)de Sitter BHs and BH Compton scattering amplitudes

Abstract:  Algebraically special solutions constitute a broad class of generally time dependent but analytically known solutions of Einstein's equations. The physics they describe, however, remains relatively less understood, mainly due to the boundary conditions they satisfy at asymptotic infinity. In this talk I will review algebraically special solutions with a negative cosmological constant in four dimensions and will present accelerating black holes as a particular example. I will then discuss aspects the variational problem for accelerating AdS black holes and its relation to thermodynamics.

Flyer

Abstract:  In this note, we study 1/4- and 1/2-BPS co-dimension two superconformal defects in the 6d N = (2,0) A_{N−1} SCFT at large N using their holographic descriptions as solutions of 11d supergravity. In this regime, we are able to compute the defect contribution to the sphere entanglement entropy and the change in the stress-energy tensor one-point function due to the presence of the defect using holography. From these quantities, we are then able to unambiguously compute the values for two of the twenty-nine total Weyl anomaly coefficients that characterize 4d conformal defects in six and higher dimensions.We are able to demonstrate the consistency of the supergravity description of the defect theories with the average null energy condition on the field theory side. For each class of defects that we consider, we also show that the A-type Weyl anomaly coefficient is non-negative. Lastly, we uncover and resolve a discrepancy between the on-shell action of the 7d 1/4-BPS domain wall solutions and that of their 11d uplift.

Abstract:  We investigate chaotic dynamics in tree-level S-matrices describing the scattering of tachyons, photons and gravitons on highly excited open and closed bosonic strings, motivated by the string/black hole complementarity. The eigenphase spacing distribution and other indicators of quantum chaotic scattering suggest that the dynamics is only weakly chaotic, consisting of both regular/Poisson and chaotic/Wigner-Dyson processes. Only for special values of momenta and (for photon scattering) scattering angles do we find strong chaos of random matrix type. These special values correspond to a crossover between two regimes of scattering, dominated by short versus long partitions of the total occupation number of the highly excited string; they also maximize the information entropy of the S-matrix. The lack of strong chaos suggests that perturbative dynamics of highly excited strings can never describe the universal properties and maximal chaos of black hole horizons.

Abstract:  I will discuss the problem of electrically charged, massless fermions scattering off magnetic monopoles. The interpretation of the outgoing states has long been a puzzle, as they can carry fractional quantum numbers. I will argue that such outgoing particles live in the twisted sector of a 3-dimensional topological surface, which ends on the monopole. This leads to new predictions for the outgoing state in QED as well as the Standard Model. Moreover, the surface is often non-invertible, and as such the outgoing radiation not only carries unconventional flavor quantum numbers, but is often trailed by a topological field theory.

Abstract: For QFTs in AdS the boundary correlation functions remain conformal even if the bulk theory has a scale. This allows one to constrain RG flows with numerical conformal bootstrap methods. In this talk, I will discuss how to apply this idea to study RG flows between two-dimensional CFTs, focusing on deformations of the tricritical and ordinary Ising model. I will present non-perturbative constraints for the boundary correlation functions of these flows and compare them with conformal perturbation theory in the vicinity of the fixed points. I will also discuss a completely general constraint on the sign of the TTbar deformation in two dimensions, and how it emerges from the numerical conformal bootstrap.

Abstract:  Wilsonian effective theories exploit hierarchies of scale to simplify the description of low-energy behaviour and play as central a role for gravity as for the rest of physics. They are useful both when hierarchies of scale are explicit in a gravitating system and more generally for understanding precisely what controls the size of quantum corrections in gravitational systems. But effective descriptions are also relevant for open systems (e.g. fluid mechanics as a long-distance description of statistical systems) for which the `integrating out' of unobserved low-energy degrees of freedom complicate a straightforward application of Wilsonian methods. Observations performed only on one side of an apparent horizon provide examples where open system descriptions also arise in gravitational physics. This chapter describes some early adaptations of Open Effective Theories (i.e. techniques for exploiting hierarchies of scale in open systems) in gravitational settings. Besides allowing the description of new types of phenomena (such as decoherence) these techniques also have an additional benefit: they sometimes can be used to resum perturbative expansions at late times and thereby to obtain controlled predictions in a regime where perturbative predictions otherwise generically fail.

Abstract:  We compute tree level scattering amplitudes involving more than one highly excited states in bosonic string theory. We use these amplitudes to understand chaotic and thermal aspects of the excited string states lending support to the Susskind-Horowitz- Polchinski correspondence principle. The unaveraged amplitudes exhibit chaos in the resonance distribution as a function of kinematic parameters, which can be described by random matrix theory. Upon coarse-graining these amplitudes exponentiate, and give certain thermal indications.

Abstract:   If spacetime is holographically built up from the quantum entanglement of microscopic degrees of freedom, it should also be possible to split it apart by disentangling these same degrees of freedom. However, studying this phenomenon with holographic methods reveals a puzzle: the disentangled state appears to keep a large entanglement entropy. I will review this problem and then explain how to resolve it. Interestingly, the solution involves bulk quantum effects of a kind brought to bear on another long-standing enigma in black hole thermodynamics, namely, the entropy of near-extremal Reissner-Nordstrom black holes.

Abstract:  It is well known that in the semi-classical limit, the entropy of black holes is universally given by the Bekenstein-Hawking formula. There are in fact corrections to this formula arising from higher derivative terms in the gravitational path integral or quantum effects to due matter fields propagating on a fixed gravitational background. Evaluating such corrections is a challenging endeavor, but there is one term, of logarithmic form, that is more accessible. In this talk, I will discuss how logarithmic corrections in four-dimensional AdS gravity theories can be extracted via the heat kernel and the differences between the logarithmic term in asymptotically flat and AdS spacetimes. I will show that our results match the one-loop computations from holographic field theories when it is known, and explain how the logarithmic correction produces constraints on effective field theories coupled to gravity.

Abstract:  Why is the vacuum energy in our Universe exponentially small in natural units?  Motivated by this difficult problem, we ask a related but cleaner question: do there exist controlled anti-de Sitter solutions of string theory in which the internal space is small but the four-dimensional spacetime is exponentially large compared to the string length?  We give an affirmative answer, by explicit construction, in Calabi-Yau compactifications of type IIB string theory.  In this talk I will begin with a general overview of the problem of finding vacua in string theory, explain the physical mechanism at work in our solutions, and comment on the prospects for de Sitter solutions along similar lines.

Abstract: Top(ological) stars are smooth horizonless static solutions of Einstein-Maxwell theory in 5-d that reduce to spherically symmetric solutions of Einstein-Maxwell-Dilaton theory in 4-d. We study their scalar perturbations at linear order with different techniques and identify three classes of quasi-normal modes, all having negative imaginary parts. This suggests linear stability of top stars. Moreover we determine the tidal Love and dissipation numbers encoding the response to tidal deformations and, similarly to black holes, we find zero value in the static limit but, contrary to black holes, we find non-trivial dynamical Love numbers and vanishing dissipative effects at linear order

Abstract: We holographically study the strongly coupled dynamics of the field theory on I-branes (D5 branes intersecting on a line). In this regime, the field theory becomes (2+1) dimensional with 16 supercharges. The dual background has an IR singularity. We resolve this singularity by compactifying the theory on a circle, preserving 4 supercharges. We study various aspects: confinement, symmetry breaking and Entanglement Entropy

Abstract: The high-energy regime of string theory has long been of great interest. Both the interactions of highly-excited strings and string thermodynamics provide an approach to study this topic. We explore the connection between the two approaches, using interaction rates to find Boltzmann equations for gases of strings and detailed balance to conjecture the form of unknown interaction rates. We then use the Boltzmann equations to study the approach to equilibrium for gases of strings and discuss some applications.

Abstract: I’ll define and explain how to compute a cosmological S matrix in de sitter. I’ll also mostly advertise some upcoming work on a new set of techniques to compute cosmological correlators using boundary differential equations.

Abstract: Back in 1973, Sidney Coleman and Nobel Prize winner David Gross proved that only non-abelian gauge theories can have asymptotic freedom. More recently, Michael Aizenman and Hugo Duminil-Copin (Field Medalist 2022) proved that scalar field theories are quantum trivial in four dimensions. Both of these important proofs have the same loophole: they assume that the coupling constant in the UV is positive definite. While a reasonable physics assumption, the results obtained in non-Hermitian (PT-symmetric) quantum mechanics indicate that certain field theories with negative coupling exist and fulfill the basic requirements of unitarity and stability. I will show an explicit calculation on how a particular field theory (the O(N) model) in 3+1 dimensions exploits this loophole, leading to a candidate for an asymptotically free interacting scalar field theory, and a potential alternative to the Standard Model Higgs sector.

Abstract:  We derive an equivalence between exact renormalization group (RG) flow equations and stochastic Langevin PDEs.  After reviewing latent diffusion models, a prominent machine learning method, we show that a certain class of latent diffusion models are based on Langevin equations taking the precise form of lattice RG flow equations.  We leverage our findings to construct ML-based models for studying field theories, in which the models learn the inverse process to an explicitly specified RG scheme. We detail how these models define a class of adaptive bridge samplers for lattice field theories and provide numerical demonstrations.

Abstract:  Scattering of matter against a conformal interface in two-dimensional CFTs exhibits a broad universality in terms of the reflection and transmission coefficients, which get completely determined by the central charges of the underlying theories (and an additional central charge-like parameter, to be explained). This is rather counterintuitive in generic quantum field theories, where these coefficients are expected to depend on the nature of incident excitation as well as the momentum it carries. It thus remains to be an interesting question to understand how is the universality modified as we depart the CFT fixed point along some RG flow. In this talk, I shall first give a brief derivation of the universality. both from the field theory as well as a (bottom-up) holographic point of view. Then I shall introduce the T\bar{T} deformation of 2d CFTs, which generates an integrable flow in the space of field theories, and outline how can one adopt the techniques in the holographic dual of the deformed ICFT to compute these coefficients.

Abstract: The accelerated expansion of the universe leads to a horizon beyond which we cannot see.   Structure arises from the quantum variance of early universe fields stretched across the horizon, according to the simplest theory fitting observations.  In concrete new models in string- (or `M-’) theory, the accelerating universe itself relies on the variance of extra dimensional fields, analogously to the quantum stability of atoms, stars and materials.  We derive these models starting from several favorable properties of hyperbolic compactifications, combined with magnetic flux and the automatically generated Casimir energy from internal fields.  

Theoretical calculations also suggest an enormous unobserved entropy associated with the de Sitter horizon. Recent and ongoing work of multiple research groups has refined and sharpened the meaning of this entropy along with other thermodynamic quantities such as energy and temperature.  Timelike features – such as a fixed-metric boundary or an observer within the system – play a key role, as do novel deformations of (2d) field theories defining a family of quantum systems with a finite Hilbert space.  These structures combine naturally into a new proposal for holography of a bounded patch of (3d) de Sitter (at large radius and with matter included),  realizing the finite entropy as a microstate count in a non-gravitational boundary theory.

Abstract: String amplitudes famously accomplish several extraordinary and interrelated mathematical feats, including an infinite spin tower, tame UV behavior, and dual resonance: the ability of the amplitude to be represented as a sum over a single scattering channel. But how unique are these properties to string amplitudes? In this talk, I will demonstrate that it is possible to construct infinite new classes of tree-level, dual resonant amplitudes with customizable, non-Regge mass spectra. Crucial ingredients are Galois theory and a particular dlog transformation of the Veneziano amplitude. The formalism generalizes naturally to n-point scattering and allows for a worldsheet-like integral representation. In the case of a Regge spectrum, I will investigate whether the structure of the Veneziano amplitude can be bootstrapped from first principles. Even there, we will find that there is extra freedom in the dynamics, allowing for a new class of dual resonant hypergeometric amplitudes with a linear spectrum.

Abstract: It has been recently pointed out that nonlinear effects are necessary to model the ringdown stage of the gravitational waveform produced by the merger of two black holes giving rise to a remnant Kerr black hole. We show that this nonlinear behavior is explained, both on the qualitative and quantitative level, by near-horizon symmetries of the Kerr black hole within the Kerr/CFT correspondence.

Abstract:Theories with defects and boundaries have  multiple applications in condensed matter and high energy physics. Here we will discuss general scalar CFT's with a linear defect in four and six dimensions. Using holography and the Hamilton-Jacobi formalism, we will show that the beta functions for the defect RG flow are the gradient of the entropy function. This enables the proof that the relevant C-functions decrease monotonically throughout  the RG flow. We will also obtain closed formulas for the dimension of scalar operators and discuss possible instabilities.

Abstract: In the context of theories with a first order phase transition, we propose a general covariant description of coexisting phases separated by domain walls using an additional order parameter-like degree of freedom. In the case of a holographic dual to a confining and a de-confing phases, the resulting model extends hydrodynamics and has a simple formulation in terms of an action and a corresponding energy-momentum tensor. The proposed description leads to simple analytic profiles of domain walls, including the surface tension density, which agree nicely with holographic numerical solutions. We show that for such systems, the domain wall or bubble velocity can be expressed in a simple way in terms of a perfect fluid hydrodynamic formula, and thus in terms of the equation of state. We test the predictions for various holographic domain walls.

Abstract: In my talk I will discuss recently discovered implications of relativistic causality on transport. In particular, I will explain how causality in holographic QFTs explains the presence of short-lived quasinormal modes of dual black branes and, similarly, how causality forces upon us the known ways of formulating effective equations of motion for relativistic fluids. Finally, I will discuss a new way of thinking about relativistic transport by adopting a bootstrap perspective based on relativistic causality. Based on 2007.05524, 2212.07434 and 2305.07703.

Abstract: CFTs with type IIB string theory duals have correlation functions which are tractable in a double expansion. For maximally supersymmetric Yang-Mills, one parameter is 1/N^2 representing the number of loops in supergravity. The other parameter is the 'tHooft coupling which keeps track of stringy corrections away from supergravity. I will discuss the first example of this analysis being done for a less supersymmetric CFT where the bulk dual includes not just super-gravitons but super-gluons as well. This will be a USp(2N) gauge theory with eight supercharges which has received attention in F-theory, To obtain high spin CFT data in this theory, it is enough to impose superconformal symmetry, crossing symmetry and Regge boundedness. The low spin data requires extra information which can be supplied with supersymmetric localization. I will explain this procedure and how it sheds light on the Veneziano amplitude in AdS. I will also show how some observables can be understood at finite gauge coupling by combining localization with instanton counting

Abstract:The ε expansion was invented more than 50 years ago and has been used extensively ever since to study aspects of renormalisation group flows and critical phenomena. In the first part of this talk we will discuss the structure of the ε expansion and the fixed points that can be obtained within it, focusing mostly on scalar theories in 4−ε dimensions. In the second part we will consider line defect deformations of various ε expansion fixed points and discuss aspects of the infrared defect conformal field theories that arise

Abstract: I will discuss the numerical implementation of the Lanczos method, the evaluation of zero temperature spectral functions,the extension to finite temperatures by the microcanonical Lanczos method and recent ideas about Lanczos coefficients, operator growth and complexity.

Abstract: Ultracold atom experiments have demonstrated their potential as quantum simulators of QFT models. Tunnel-coupled one-dimensional Bose gases, in particular, have been successfully used to implement strongly interacting models of QFT and explore their equilibrium and dynamical properties. I will discuss how this platform can be used to study measures of quantum information in QFT and their scaling with size, dubbed as volume and area law. Based on a version of quantum tomography suitable for continuous fields, we achieve a detailed reconstruction of the system’s quantum state, allowing measurements of the von Neumann entropy and mutual information in thermal equilibrium states.