A sticky business: the status of the conjectured viscosity/entropy density bound

Understand

There have been a number of forms of a conjecture that there is a universal lower bound on the ratio, eta/s, of the shear viscosity, eta, to entropy density, s, with several different domains of validity.

• We examine the various forms of the conjecture.
• We argue that a number of variants of the conjecture are not viable due to the existence of theoretically consistent counterexamples.
• We also note that much of the evidence in favor of a bound does not apply to the variants which have not yet been ruled out.