The marked length spectrum rigidity question asks whether two closed negatively curved manifolds , are isometric if they are homeomorphic with a homeomorphism which maps a closed geodesic on to a curve on which is freely homotopic to a closed geodesic of the same length. The lecture discusses the work of Guillarmou and Lefeuvre who used novel tools from microlocal analysis to give an affirmative answer to a local version of this question.
NB: A youtube link is available on bourbaki.fr