Kubo formulae for the shear and bulk viscosity relaxation times and the scalar field theory shear τ_π calculation

In this paper we provide a quantum field theoretical study on the shear and bulk relaxation times. First, we find Kubo formulas for the shear and the bulk relaxation times, respectively. They are found by examining response functions of the stress-energy tensor. We use general properties of correlation functions and the gravitational Ward identity to parametrize analytical structures of the Green functions describing both sound and diffusion mode. We find that the hydrodynamic limits of the real parts of the respective energy-momentum tensor correlation functions provide us with the method of computing both the shear and bulk viscosity relaxation times. Next, we calculate the shear viscosity relaxation time using the diagrammatic approach in the Keldysh basis for the massless $\lambda\phi^4$ theory. We derive a respective integral equation which enables us to compute $\eta\tau_\pi$ and then we extract the shear relaxation time. The relaxation time is shown to be inversely related to the thermal width as it should be.