Universal power law tails of time correlation functions

The universal power law tails of single particle and multi-particle time correlation functions are derived from a unifying point of view, solely using the hydrodynamic modes of the system. The theory applies to general correlation functions, and to systems more general than classical fluids. Moreover it is argued that the collisional transfer part of the stress-stress correlation function in dense classical fluids has the same long time tail $\sim t^{-1-d/2}$ as the velocity autocorrelation function in Lorentz gases.