Transport properties of cascading gauge theories

Cascading gauge theories of Klebanov et al. provide a model within a framework of gauge theory/string theory duality for a four-dimensional nonconformal gauge theory with a spontaneously generated mass scale. Using the dual supergravity description, we study sound wave propagation in strongly coupled cascading gauge theory plasma. We analytically compute the speed of sound and the bulk viscosity of cascading gauge theory plasma at a temperature much larger than the strong coupling scale of the theory. The sound wave dispersion relation is obtained from the hydrodynamic pole in the stress-energy tensor two-point correlation function. The speed of sound extracted from the pole of the correlation function agrees with its value computed in Phys. Rev. D 72, 066003 (2005), using the equation of state. We find that the bulk viscosity of the hot cascading gauge theory plasma is nonzero at the leading order in the deviation from conformality.