Nonuniversal lower bound for the shear viscosity to entropy density ratio

The lower bound of the shear viscosity to entropy density ratio is examined using an exact representation of the ratio through the density of states. Under certain assumptions it can be shown that the lower bound of the ratio is not universal; its value is determined by the entropy density. Some examples of physical systems are discussed in the paper where one can expect violation of the conformal $1/4\pi$ value.

Figure 1

The shear viscosity to entropy ratio as a function of the imaginary self-energy.