String Seminar Archive: 2025-2026

I will explain how the perturbative expansion of amplitudes for some quantum field theories reorganizes into a non-critical string theory amplitude giving rise to a holographic dual for the QFT. Each Feynman diagram can be mapped to a worldsheet and a CFT is constructed that reproduces the amplitude of each graph. A holographic direction naturally appears from the Liouville mode. Obtaining a fully self-consistent background requires computing the gravitational backreaction which is shown to be equivalent to renormalization of the QFT.

I will discuss the interesting and sometimes subtle effects of twisted compactifications of discrete gauge theories coupled to gravity. In the context of six-dimensional F-theory compactifications I will connect the physical expectations to the geometry of torus fibered Calabi-Yau threefolds. The condition that the discrete gauge symmetry can be unhiggsed to a continuous one lets us derive conditions for discrete anomaly cancellation via a topological Green-Schwarz mechanism. Assuming the absence of further constraints, this allows us to estimate the size of this corner of the string landscape and of the corresponding Calabi-Yau geometries. Even assuming the absence of any continuous gauge symmetry it turns out to be surprisingly large.

First-order phase transitions proceed via the nucleation of critical bubbles of the stable phase within a metastable background. Using holography, I present a fully microscopic description of these bubbles in a strongly coupled four-dimensional gauge theory at finite temperature. In the gravitational dual, the bubbles appear as static, inhomogeneous, and unstable black-brane solutions featuring localized horizon deformations. I will outline the construction of these solutions and show how they allow the nucleation rate to be extracted across the entire metastable branch. Time permitting, I will also compare this rate—and other microscopic properties—with those obtained from an effective field-theory description.

In this talk, I will propose and provide evidence for a duality between a matrix integral and type IIB supergravity. After briefly reviewing the IKKT matrix model and its supersymmetry-preserving mass deformation—the so-called polarised IKKT model—I will explain how the partition function of the deformed model can be computed using supersymmetric localisation techniques. I will then construct the corresponding dual geometry and demonstrate the explicit agreement between the polarised IKKT matrix model and its gravitational counterpart.

In this talk, I will discuss several aspects of the role of coarse-graining in both the black hole information paradox and its resolution. First, I will discuss, based on [2311.07655], how a non-unitary Hawking entropy curve can arise in a simple toy model of black hole evaporation through coarse-graining, and how replica wormholes appear in this setup to recover unitarity and the Page curve. Then I will discuss our more recent paper [2601.2207], where we study black holes in AdS in the microcanonical ensemble. Since small black holes in AdS can evaporate, this setting allows us to explore whether semiclassical AdS gravity, and the information paradox, can arise from a coarse-graining of a single boundary theory by coarse-graining over all theories that are semiclassically indistinguishable. We also investigate boundary probes that might be able to distinguish whether the bulk dual is a black hole or a thermal gas within the microcanonical ensemble.

I will discuss the recently introduced holographic product formula in the context of near-extremal hydrodynamics, which corresponds to the regime of frequency ω, momentum k and temperature T much smaller than a hard scale μ. I will explain how this formula determines the general form taken by holographic spectral functions in this regime. This form simplifies in the extremal limit T ≪ ω, k ≪ μ, for which the low-temperature gapless modes and the IR conformal behavior factorize. I will then present some examples with different types of gapless modes and IR CFTs, including new numerical results for low temperature quasi-normal modes.

13:30-14:00: Arrival and coffee

14:00 - 15:00:  Richard Myers (UvA): "The functional approach to holography in flat space"

15:00 - 15:30: Break

15:30 - 16:30: Nava Gaddam (UU): "The black hole IR triangle"

16:30: Borrel

In this talk we will construct a family complete bases for the non perturbative Hilbert space of quantum gravity, and use this to address two puzzles regarding the Hilbert space of quantum gravity. Firstly, Gibbons and Hawking proposed that the Euclidean gravity path integral with periodic boundary conditions in time computes the thermal partition sum of gravity. As a corollary, the derivative of the associated free energy with respect to the Euclidean time period results in the celebrated black hole entropy formula  S=A/4G. Why is this interpretation is correct? That is, why does this path integral compute a trace over the gravity Hilbert space? We show that the quantity computed by the Gibbons-Hawking path integral is equal to an  a priori different object -- an explicit thermal trace over the Hilbert space spanned by states produced by the Euclidean gravity path integral. Secondly, Holography suggests that the Hilbert space of quantum gravity with two asymptotic boundaries should factorise into two copies of the single-boundary gravity Hilbert space.  However, the existence of spacetimes with Einstein-Rosen bridges connecting the two boundaries seems to contradict this. We will show that the non-perturbative, two-boundary  Hilbert space factorises nonetheless. One implication of our results is that universes containing a horizon can sometimes be understood as superpositions of horizonless geometries entangled with a closed universe.

In this talk, we will review the asymptotic structure of three-dimensional Carroll gravity with a negative cosmological constant. We formulate a consistent set of boundary conditions preserved by an infinite-dimensional extension of the AdS3 Carroll algebra, which turns out to be isomorphic to a precise generalised BMS3 algebra. We also introduce a new solution endowed with a Carroll extremal surface that fulfils this set of asymptotic conditions. By taking advantage of the Chern-Simons formulation of the theory, Carroll's thermal properties, obtained from regularity conditions, and the entropy of the configuration are also addressed. Some comments about the 3D Galilean gravity case are addressed at the end of the talk. 

Holography provides a powerful framework for probing the intricate structure of black hole interiors. Remarkably, the seminal BKL analysis from the early '70s has recently found a natural realization within AdS/CFT, offering a way to obtain and study Kasner-like dynamics inside AdS black holes. In this talk, I'll discuss how these behaviors manifest in the dual CFT as exotic RG flows, and how CFT observables can sometimes capture — or fail to capture — the physics near the singularity. I will highlight various probes, including the thermal c-function, two-sided correlators, and holographic complexity. Time permitting, I will also touch on the role of higher-derivative corrections and their impact on the structure of deep interiors.

The main goal of Carrollian Holography is to offer a non-perturbative formulation of gravitational scattering theory in asymptotically flat spacetimes. This formulation relies on a correspondence between massless particle states and conformal fields locally defined at null infinity (the spacetime conformal boundary). I will review the developments of this novel kind of "carrollian" CFT, and provide evidence that it controls gravitational scattering amplitudes. 

Holography has long been employed to explore the behavior of strongly interacting theories in regimes inaccessible to perturbative techniques, with confinement remaining one of the most intriguing phenomena. Indeed, while a holographic version of QCD still remains elusive, numerous theories that mimic certain aspects of QCD have been found with a known holographic dual: among them, the family of quiver field theories considered here. After recalling a few relevant concepts, I will introduce and discuss the theoretical framework and numerical implementation of two holographic observables in these theories: the Wilson loop and entanglement entropy, which have been shown, at least in some examples, to be linked to confinement. I will then present the numerical results and their interpretation, showing the emergence of distinct regimes corresponding to conformal, confining, and screened behaviors, and suggesting the presence of partial deconfinement.

13:30-14:00: Arrival and coffee

14:00-15:00: Matthew Headrick (Brandeis/IHES Paris): "Mysterious properties of holographic entropies"

15:00-15:30: Break

15:30-16:30: Nikolay Bobev (KU Leuven): "Precision holography for Dp-branes"

16:45: Drinks

TBA

Recently, there has been an increasing interest in the study of defects in quantum field theories, with holography providing a powerful framework to explore various aspects of these super-(conformal) gauge theories.

In this talk, I will discuss supergravity solutions that are dual to codimension-2 superconformal monodromy defects. These solutions are obtained using gauged supergravities in D=4,5,6 and 7 dimensions. I will present a prescription to compute the defect entanglement entropy, outlining the renormalization procedure needed to regularise its divergencies, which I will discuss in detail. In some cases, we are also able to express this quantity in terms of the free energy/Weyl anomaly of the defect and its conformal weight.

If time allows, I will also discuss some new results for non-conformal monodromy defects.

The low-energy expansion of closed-string genus-one amplitudes introduces a class of non-holomorphic modular forms for SL(2, Z) known as modular graph forms (MGFs). The expansion of the amplitude can be evaluated by integrating MGFs over the moduli space of the genus-one Riemann surface. A modern, more systematic approach to tackle this problem is by writing MGFs as equivariant iterated Eisenstein integrals, which can be viewed as genus-one analogues of single-valued multiple polylogarithms. Their integrals over the moduli space result in among others multiple zeta values, logarithmic derivatives of zeta values and the Euler-Mascheroni constant. In this talk, I will report on the recent four- and five-point amplitude progress.