Optimal gradient estimates and asymptotic behaviour for the   Vlasov-Poisson system with small initial data
  The Vlasov-Poisson system describes interacting systems of collisionless particles. For solutions with small initial data in three dimensions it is known that the spatial density of particles decays like $t^{-3}$ at late times. In this paper this statement is refined to show that each derivative of the density which is taken leads to an extra power of decay so that in $N$ dimensions for $N\ge 3$ the derivative of the density of order $k$ decays like $t^{-N-k}$. An asymptotic formula for the solution at late times is also obtained. 
Hyung Ju Hwang, Alan D. Rendall and Juan J. L. Velazquez