String Theory Seminar

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: Matthew Headrick (Brandeis/IHES Paris): "Mysterious properties of holographic entropies"

15:00: Break

15:30: 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.