Arnaud Brothier, University of New South Wales, Sydney

Abstract: Motivated by Vaughan Jones’ reconstruction program of conformal field theories, we have defined a class of groups constructed with tree-diagrams named “forest-skein (FS) groups”. These groups are interesting on their own, satisfying exceptional properties and having powerful extra-structures for studying them. They are isotropy subgroups of forest-skein categories: categories of planar diagrams mod out by certain skein relations (just like planar algebras are in Jones’ subfactor framework). We will present explicit examples and explain how the theory of forest-skein groups is surprisingly well-behaved. In particular, we will present new infinite families of infinite simple and finitely presented groups. Some results are joint work with Ryan Seelig.

Alex Lupsasca- Vanderbilt University

What does a black hole look like? The first images of the supermassive black hole M87* display a bright ring encircling the event horizon, which appears as a dark patch in its surrounding emission. But Einstein’s theory of general relativity predicts that within this image there also lies a thin “photon ring” consisting of multiple mirror images of the main emission. These images arise from photons that orbited around the black hole multiple times, probing the warped space-time geometry just outside its horizon. The photon ring carries an imprint of the strong gravity in this region and encodes fundamental properties of the black hole. A measurement of this predicted (but not yet observed) ring could provide a precise test of general relativity and will be one of the main targets of a NASA mission proposed to fly within the next decade: the Black Hole Explorer (BHEX).