Universal strongly coupled shear-viscosity bound

Determine whether the shear-viscosity-to-entropy-density ratio of a strongly coupled system universally reaches the value \(\eta/s=1/4\pi\).

Background

The thesis discusses the use of the ratio between the heavy-quark spatial diffusion coefficient and the shear-viscosity-to-entropy-density ratio to distinguish strongly and weakly coupled media. In the strongly coupled limit, gauge–gravity duality motivates the values $D_s\approx 1/(2\pi T)$ and $\eta/s=1/4\pi$. The universal status of the shear-viscosity value is presented as a conjecture rather than as an established result. The thesis subsequently finds that the corresponding proposed lower limit can be violated within the Quasi-Particle Model, reinforcing the unresolved nature of the conjectured bound.