In this series of lectures, we will focus on simple Lie groups, their dense subgroups and the convolution powers of their measures. In particular, we will dicuss the following two questions.
Let G be a Lie group. Is every Borel measurable subgroup of G with maximal Hausdorff dimension equal to the group G?
Is the convolution of sufficiently many compactly supported continuous functions on G always continuously differentiable?
Even though the answer to these questions is no when G is abelian, the answer is yes when G is simple. This is a joint work with N. de Saxce. First, I will explain the history of these two questions and their interaction. Then, I will relate these questions to spectral gap properties. Finally, I will discuss these spectral gap properties.[-]

The weak-$A_\infty$ condition is a variant of the usual $A_\infty$ condition which does not require any doubling assumption on the weights. A few years ago Hofmann and Le showed that, for an open set $\Omega\subset \mathbb{R}^{n+1}$ with $n$-AD-regular boundary, the BMO-solvability of the Dirichlet problem for the Laplace equation is equivalent to the fact that the harmonic measure satisfies the weak-$A_\infty$ condition. Aiming for a geometric description of the open sets whose associated harmonic measure satisfies the weak-$A_\infty$ condition, Hofmann and Martell showed in 2017 that if $\partial\Omega$ is uniformly $n$-rectifiable and a suitable connectivity condition holds (the so-called weak local John condition), then the harmonic measure satisfies the weak-$A_\infty$ condition, and they conjectured that the converse implication also holds.
In this talk I will discuss a recent work by Azzam, Mourgoglou and myself which completes the proof of the Hofman-Martell conjecture, by showing that the weak-$A_\infty$ condition for harmonic measure implies the weak local John condition.[-]