Mohammed Lemou (IRMAR Rennes, France)
I will start by giving a short overview of the history around stability and instability issues in gravitational systems driven by kinetic equations. Conservations properties and families of non-homogeneous steady states will be first presented. A well-know conjecture in both astrophysics and mathematics communities was that "all steady states of the gravitational Vlasov-Poisson system which are decreasing functions of the energy, are stable up to space translations". We explain why the traditional variational approaches for the nonlinear stability analysis of these steady states are not sufficient to answer this conjecture. An alternative approach, inspired by astrophysics literature, will be then presented and quantitative stability inequalities will be shown, therefore solving the above conjecture for Vlasov-Poisson. This have been achieved by using refined rearrangement of functions and Poincare-like functional inequalities. For other systems like the so-called Hamiltonian Mean Field (HMF), the decreasing property of the steady states is no more sufficient to guarantee their stability. An additional explicit criteria is needed, under which the non-linear stability is proved. This criteria is sharp as instabilities can be constructed if it is not satisfied.