content_image/304033.jpg content_image/304032.jpg
ratio, η/s . By comparing the correlation function for the low energy scatter- ing cross section used in the motivation for the correspondence to the Kubo formula for the viscosity of a strongly-coupled SYM plasma [333] one finds that 

<img src='content_image/304033.jpg'>

where [334] found that the cross section for extended p -branes has a universal low energy limit of the area of the horizon. Using this with Eq. (2.67), one quite trivially arrives at the famous strong coupling limit of viscosity to entropy 

<img src='content_image/304032.jpg'>

The result led to another conjecture [335], that this is in fact a universal lower bound. As noted in Section 2.1.1, qualitative estimates of the viscosity at Rhic [121] appear to require just such values of η/s ∼ . 1 ∼ 1 / 4 π . This gives great impetus to look for additional observables calculable in AdS/CFT. 

In fact we can learn a great deal by extending the AdS/CFT conjecture further by breaking some of the field theory supersymmetry encoded in the S 5 part of the geometry. Specifically the introduction of M ≪ N D 7 branes that fill space from the asymptotic 3 + 1 D Minkowski boundary to some finite distance above r = 0 and wrapping an equatorial S 3 ⊂ S 5 corresponds to introducing M fundamental flavors in the gauge theory [336]. The M ≪ N requirement allows the D 3 branes to backreact and warp spacetime in the large N limit while the D 7 branes act only as probes. An open string ending 

