A modulated shear to entropy ratio
One of the characteristics of a relativistic fluid is its shear viscosity which describes the response of the fluid to shear flow. Over the last ten years the shear viscosity of relativistic fluids whose underlying degrees of freedom are described by gauge-theories has been extensively studied. In particular, the shear viscosity of a certain class of gauge theory fluids which are described by a holographic dual has been shown to be proportional to the entropy density of such fluids with a universal proportionality constant. The proof of this universality relies on the system being isotropic, homogenous and thermally equilibrated. In a recent paper, the shear viscosity to entropy density ratio has been studied for thermally equilibrated and spatially modulated phases. It was shown that in such phases the shear viscosity is modulated while the entropy density is not, leading to a non universal ratio.