Isoperimetric domains in homogeneous three-manifolds and the isoperimetric constant of the Heisenberg group ${\mathsf{H}}^1$

In this paper we prove that isoperimetric sets in three-dimensional homogeneous spaces diffeomorphic to ${\mathbb{R}}^3$ are topological balls. We also prove that in three-dimensional homogeneous spheres isopermetric sets are either two-spheres or symmetric genus-one tori. We then apply our first result to the three-dimensional Heisenberg group ${\mathsf{H}}^1$, characterizing the isoperimetric sets and constants for a family of Riemannian adapted metrics. Using $\mathrm{\Gamma }$-convergence of the perimeter functionals, we also settle an isoperimetric conjecture in ${\mathsf{H}}^1$ posed by P.Pansu.