Triangle Seminars

Abstract:
We will discuss how to bootstrap string theory in AdS, using N=4 SYM as a concrete example. The first lecture will review the basic ingredients of AdS/CFT: the definitions of AdS and CFT, the correspondence between Kaluza–Klein modes and single-trace operators, and the relation between string scattering in AdS and correlation functions of Kaluza–Klein modes in the CFT. The second lecture will focus on the stress-tensor multiplet four-point function, including its conformal block expansion, the constraints imposed by superconformal Ward identities, and its formulation in Mellin space. The third lecture will introduce the flat-space limit, explain how it acts on the conformal block expansion, and discuss the spectrum of single-trace operators that survives at strong coupling. In the final lecture, we will go beyond the flat-space limit and study the first AdS correction. We will introduce a worldsheet ansatz for this correction and show how matching it to the conformal block expansion fixes the correction completely.

Abstract:
In cosmological models the matter content of the universe is assumed to be a compressible fluid. Such fluids are prone to the formation of so-called shock singularities, meaning that, at a finite time, the fluid gradient blows up, while the fluid quantities themselves stay bounded. By contrast, rapidly expanding spacetimes facilitate stabilization of any fluid contained inside. This dichotomy raises the question: What amount of expansion is sufficient to prevent shocks forming for sufficiently small data? At the beginning of this talk, we will discuss the preliminaries of relativistic fluids as an initial value problem. Afterwards, I will present new results in the regime of decelerated expansion, which seek to answer this central question. This is joint work with Todd Oliynyk.

Abstract:
The talk explores the relation between gravitational saddles computing supersymmetric indices in asymptotically flat and AdS spacetimes, focusing on the D1-D5 system. Starting from five-dimensional saddles of the gravitational supersymmetric index, I will describe how a type IIB uplift and a decoupling limit yield asymptotically AdS_3 × S^3 configurations. These include horizonless geometries associated with fractional spectral flow in the dual CFT_2, and families of black hole saddles related by modular transformations. The construction extends the familiar modular family of BTZ black holes to backgrounds with nontrivial orbifold identifications and establishes their asymptotically flat origin. Their on-shell actions, including nontrivial modular phases, precisely match the corresponding contributions to the Farey-tail expansion of the D1-D5 elliptic genus, providing a sharp connection between gravitational saddles and microscopic counting.

Abstract:
I explain how to take a well-defined flat-space limit of brane models of AdS coupled to a non-gravitating bath. In the dual BCFT this amounts to a triple-scaling limit where both the number of boundary degrees of freedom and the boundary coupling are taken to infinity while the BCFT boundary piecewise approaches a lightcone. This procedure acts on the conformal generators as a Wigner-İnönü contraction, reducing the global BCFT symmetry algebra to the global symmetry algebra of flat space.
I will discuss two natural notions of entanglement entropy of the flat-space dual. These are distinguished by whether modes that have left through future null infinity are included or not and give rise to a vanishing and non-trivial Page curve, respectively. Taking the flat-space limit of topological black holes, the Page time remains finite in two-dimensions. In d > 2 the Page time diverges in the flat limit, since AdS topological black holes become flat-space Rindler horizons.

Abstract:
TBD.