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Transport coefficients and universality in high T holography


The quark-gluon plasma produced at RHIC appears to behave as a strongly-coupled fluid. This has focused much recent attention on strongly-coupled fluids, which are theoretically intractable using standard field-theoretic methods.  The gauge/gravity duality is the only known technique that gives theoretical access to the properties of strongly-coupled fluids described by gauge theories that bear some resemblance to QCD.  While no dual to QCD is known, it may still be useful to look at fluids with gravity duals to obtain some otherwise unavailable intuition about what can happen to fluids at strong coupling.  To this end,  a number of transport coefficients in a broad class of strongly-coupled fluids described by theories with gravity duals are investigated.  These fluids stay strongly coupled even at high temperatures, in contrast to what happens to the QCD plasma. It turns out that transport coefficients exhibit interesting universal behaviors at high T.  Furthermore, there is a bound on the speed of sound v_s in these theories, with v_s^2 always approaching 1/3 the speed of light from below at high T.   These results are compared with what one finds in theories that are weakly coupled at high T.