Collections of parabolic orbits in homogeneous spaces, homogeneous dynamics and hyperkahler geometry

Let $M$ be a hyperk"ahler manifold with $b_2(M)\ge 5$. We improve our earlier results on the Morrison-Kawamata cone conjecture by showing that the Beauville-Bogomolov square of the primitive MBM classes (i.e. the classes whose orthogonal hyperplanes bound the K"ahler cone in the positive cone, or, in other words, the classes of negative extremal rational curves on deformations of $M$) is bounded in absolute value by a number depending only on the deformation class of $M$. The proof uses ergodic theory on homogeneous spaces.