The 6d N = (2,0) theories are superconformal field theories believed to describe the low-energy dynamics of N coincident M5-branes. These theories don't have a known lagrangian description and remain largely mysterious, so it is an interesting question how one might calculate observables there. An exciting prospect is to use the analytical conformal bootstrap, which offers a way to systematically calculate 1/N corrections at large N. In this talk I will present the bootstrap approach to a case study, that of calculating the 2-point function of stress tensors in the presence of a surface defect. This setup turns out to be remarkably simple and helps us address some technical issues faced in similar calculations, notably we can derive a supersymmetric inversion formula and check crossing symmetry explicitly. I will also comment on the interpretation of our result in the context of holography, of the chiral algebra construction of Beem et al. and on what it can reveal about the interactions between M2 and M5-branes.
Since the 80s, radio sources have been observed to undergo extreme scattering events (ESEs): large, frequency dependent flux modulations due to scattering off the ISM. Recently, the study of these events has undergone a revived interest due to the increase in pulsar timing data, as well as the realization that FRBs will be scattered by the same structures in the ISM. Models of the structures responsible for ESEs range from spherical-cow approximations (e.g. simple Gaussian profiles) to more exotic models (e.g. plasma shells around compact dark matter). Here we present a new model in which ESEs are produced by corrugated sheets in the ISM, which, when projected onto the plane of the sky, generically form A3 cusp catastrophes. We will argue that this model naturally explains several features in scattering data, including observations of PSR 0834+06 and the original Fiedler et al. 0954+658 ESE. Moreover, this model is consistent with models of pulsar scintillation and does not require exotic physics to explain. We will discuss potential applications to FRB cosmology that arise from the universality of this model.