Emergence of Hydrodynamics in Expanding Relativistic Plasmas

I consider a simple set of equations that govern the expansion of boost-invariant plasmas of massless particles. These equations describe the transition from a collisionless regime at early time to hydrodynamics at late time. Their mathematical structure encompasses all versions of second order hydrodynamics. We emphasize that the apparent success of Israel-Stewart hydrodynamics at early time has little to do with ``hydrodynamics'' proper, but rather with a particular feature of Israel-Stewart equations that allows them to effectively mimic the collisionless regime.