Martin Lesourd (BHI, Harvard)
Title: How Do Black Holes Form ?
Abstract: Mathematical General Relativity is held together by a couple of deep conjectures. Many mathematicians like to think that by proving these (or finding counterexamples), they will finally be able to underpin (or challenge) the beliefs that Physicists and Astrophysicists have held for decades. Arguably the grandest such example is the ''Final State Conjecture'': generic vacuum spacetimes undergoing gravitational collapse will eventually resemble the Kerr solution in their exterior. At present, there is not a single example of this conjecture, nor a single vacuum spacetime with a complete future asymptotic region describing black hole formation. I will review what is known, and describe some work (still in progress) with Athanasiou in this direction.
Frederic Vincent (Paris Meudon Observatory)
Title: What can we infer from black hole images?
Abstract: The recent Event Horizon Telescope (EHT) millimeter image of the vicinity of the supermassive compact object M87* at the center of the galaxy M87 is a breakthrough. It is important to investigate what can and what cannot be inferred from such images. In this talk I will discuss a simple geometric model of the region of the accretion flow immediately surrounding M87*. I will use this model to discuss the highly-lensed features of EHT-like images, often loosely referred to as the "photon ring". I will discuss whether such features can be used as clean probes of the nature of the underlying spacetime. I will also discuss various EHT-like images of M87*, assuming this object is various kinds of compact alternatives to the standard black hole. This will allow discussing to what extent can EHT-like images help constrain the nature of the compact objects at the center of M87 or our Milky Way.