Stability, Convergence to Self-Similarity and Elastic Limit for the Boltzmann Equation for Inelastic Hard Spheres

We consider the spatially homogeneous Boltzmann equation for inelastic hard spheres, in the framework of so-called constant normal restitution coefficients$${\alpha \in [0,1]}$$ . In the physical regime of a small inelasticity (that is $${\alpha \in [\alpha_*,1)}$$ for some constructive $${\alpha_* \in [0,1)}$$) we prove uniqueness of the self-similar profile for given values of the restitution coefficient $${\alpha \in [\alpha_*,1)}$$ , the mass and the momentum; therefore we deduce the uniqueness of the self-similar solution (up to a time translation).Moreover, if the initial datum lies in $${L^1_3}$$ , and under some smallness condition on $${(1-\alpha_*)}$$ depending on the mass, energy and $${L^1 _3}$$ norm of this initial datum, we prove time asymptotic convergence (with polynomial rate) of the solution towards the self-similar solution (the so-called homogeneous cooling state).These uniqueness, stability and convergence results are expressed in the self-similar variables and then translate into corresponding results for the original Boltzmann equation. The proofs are based on the identification of a suitable elastic limit rescaling, and the construction of a smooth path of self-similar profiles connecting to a particular Maxwellian equilibrium in the elastic limit, together with tools from perturbative theory of linear operators. Some universal quantities, such as the “quasi-elastic self-similar temperature” and the rate of convergence towards self-similarity at first order in terms of (1−α), are obtained from our study.These results provide a positive answer and a mathematical proof of the Ernst-Brito conjecture [16] in the case of inelastic hard spheres with small inelasticity.