The Shear Viscosity to Entropy Ratio:. a Status Report
Abstract
This review highlights some of the lessons that the holographic gauge/gravity duality has taught us regarding the behavior of the shear viscosity to entropy density in strongly coupled field theories. The viscosity to entropy ratio has been shown to take on a very simple universal value in all gauge theories with an Einstein gravity dual. Here we describe the origin of this universal ratio, and focus on how it is modified by generic higher derivative corrections corresponding to curvature corrections on the gravity side of the duality. In particular, certain curvature corrections are known to push the viscosity to entropy ratio below its universal value. This disproves a longstanding conjecture that such a universal value represents a strict lower bound for any fluid in nature. We discuss the main developments that have led to insight into the violation of this bound, and consider whether the consistency of the theory is responsible for setting a fundamental lower bound on the viscosity to entropy ratio.
 Publication:

Modern Physics Letters B
 Pub Date:
 2011
 DOI:
 10.1142/S0217984911027315
 arXiv:
 arXiv:1108.0677
 Bibcode:
 2011MPLB...25.1867C
 Keywords:

 Quarkgluon plasma;
 hydrodynamics;
 AdSCFT correspondence;
 black holes in string theory;
 High Energy Physics  Theory;
 General Relativity and Quantum Cosmology;
 High Energy Physics  Phenomenology;
 Nuclear Theory
 EPrint:
 29 pages. Invited review for Modern Physics Letters B. References and minor comments added