Title: Superluminal Liouville walls in 2d String Theory and space-like singularities

Abstract: An interesting class of time dependent backgrounds in 1+1 dimensional string theory involves worldsheet Liouville walls which move in (target space) time. When a parameter in such a background exceeds a certain critical value, the speed of the Liouville wall exceeds the speed of light, and there is no usual S-Matrix. We examine such backgrounds in the dual c=1 matrix model from the point of view of fluctuations of the collective field, and determine the nature of the emergent space-time perceived by these fluctuations. We show that so long as the corresponding Liouville wall remains time-like, the emergent space time is conformal to full Minkowski space with a time-like wall. However, for the cases where the Liouville wall is superluminal, the emergent space-time has a space-like boundary where the collective field couplings diverge. This appears as a space-like singularity in perturbative collective field theory. We comment on the necessity of incorporating finite $N$, as well as finite (double-scaled) coupling, effects to understand the behavior of the exact theory near this boundary.

NB: non-standard time!

Title: Symmetry-weighted ensemble averaging from TQFT gravity

Abstract: In a recently proposed framework of TQFT gravity (2310.13044, 2405.20366) -- a toy model of AdS3 gravity -- a bulk 3d TQFT summed over all topologies is shown to be dual to a unitary ensemble of boundary 2d CFTs. I will show that the CFTs in this ensemble are weighted by the inverse of the order of their symmetry group (relative to the categorical symmetry provided by the bulk TQFT as a SymTFT). Mathematically, this is the natural measure over the groupoid of the TQFT Lagrangian algebras that construct the CFTs, and the holographic duality then provides a generalization of the Siegel-Weil formula beyond averaging over bosonic lattice-CFTs. I will also discuss some examples for rational CFTs as well as implications to noncompact TQFTs and pure gravity.

Most objects around us are in thermal equilibrium. But are some
systems "more thermal" than others?
The emerging paradigm of deep thermalization and projected ensembles
has indeed demonstrated that there can be
different degrees of thermality, depending on how much information can
be extracted from the system and, relatedly,
the behavior of multiple copies of the system. In my talk I will discuss a
related concept of observable projected ensembles and a
field-theoretic approach to this problem based on the replica trick.
Bases on https://arxiv.org/abs/2410.21397 and on-going work.

I will describe the dynamical evolution of a universe containing a single black hole. If the black hole has sufficiently large initial charge, it will be driven very close to extremality by the emission of neutral Hawking radiation, while charged particle emission is exponentially suppressed. At low enough temperatures, quantum gravity becomes important and Hawking-style quantum field theory in curved spacetime calculations give completely incorrect answers, even for simply questions like the energy spectrum of emitted radiation. This leads to interesting new physics, e.g. in certain regimes the dominant radiation channel becomes entangled pairs of photons, as in the “forbidden’’ 2s->1s hydrogen atom transition. By careful analysis of the relevant metric fluctuations, we can calculate the quantum gravity effects in a controlled manner and tell the complete story of the black hole evaporation in both a universe with a matter content similar to ours as well as in a supersymmetric universe.