High Energy Physics

In order to derive the classical string action from the worldsheet, it is necessary to take string theory off shell. This can be done by a prescription of Tseytlin, who proposed taking the worldsheet sphere QFT partition function and *differentiating* it with the log of the UV cutoff. I will explain why this strange prescription always gives the correct answers, for both the S-matrix and equations of motion, to all orders in perturbation theory. I will also compare the Susskind-Uglum off-shell method of calculating black hole entropy, to the more popular (but also more dubious) orbifold method. Based on work with Amr Ahmadain (arXiv:2211.08607 and arXiv:2211.16448).

To define an algebra of observables in quantum gravity in a way that is universal and does not depend on a background spacetime, one can consider the observables along the worldline of an observer, rather than the observables in a region of spacetime.

We obtain all solutions of the Wheeler-DeWitt equation with positive cosmological constant for a closed universe in the large-volume limit. We define a natural norm on the solution space and thereby obtain a description of the Hilbert space of quantum gravity in an asymptotically de Sitter spacetime. This provides the finite G_N generalization of the Hilbert space constructed by Higuchi using group averaging. All the states in this Hilbert space share the symmetries of the Euclidean vacuum. We use this property to generalize the principle of holography of information to de Sitter space: data about cosmological correlators (defined as appropriately gauge-fixed observables) in an arbitrary small region suffices to specify them everywhere.

I will discuss the interplay between 2d de Sitter and supersymmetry in two concrete supergravity models coupled to superconformal field theories. Upon fixing a supersymmetric analogue of the Weyl gauge these theories can be viewed as supersymmetric extensions of timelike Liouville theory with N=1 and N=2 supersymmetry respectively. Supersymmetric timelike Liouville is well behaved in the UV and combines supersymmetry with a positive cosmological constant and a de Sitter saddle. The theory is amenable to a variety of techniques including systematic loop expansions and localization methods.

In this talk we will analyze renormalization group flows on d-dimensional planar defects, embedded in a D-dimensional conformal field theory. This general setup includes the case of quantum field theory with no defects ($D=d$), as well as defects of different dimensionality that are of interest in high energy and condensed matter physics. Using methods from quantum information theory, we will establish the irreversibility of renormalization group flows for defect dimensions $d \le 4$, and for all $D$. The main ingredients in the proof are strong subadditivity of the entanglement entropy, the Markov property of the conformal vacuum, and the quantum null energy condition.

I will explain a new 2d gravity/matrix model duality. The bulk theory can be defined as a minimal string theory whose matter sector is given by timelike Liouville theory instead of a minimal model. The theory admits a dual description in terms of a double-scaled matrix integral whose leading density of states is given by the universal Cardy density of states in a 2d CFT of central charge c, thus motivating us to call the bulk theory the Virasoro minimal string. The duality can be derived by exploiting a certain relation to 3d gravity compactified on a circle. For large central charge, it reduces to the duality between JT-gravity and the corresponding double-scaled matrix integral. Based on work in collaboration with Scott Collier, Beatrix Mühlmann and Victor Rodriguez.