Title:

Hydrodynamics : from effective field theory to holography

Hydrodynamics is an effective theory that is extremely successful in describing a wide range of physical phenomena in liquids, gases and plasmas. However, our understanding of the structure of the theory, its microscopic origins and its behaviour at strong coupling is far from complete. To understand how an effective theory of dissipative hydrodynamics could emerge from a closed microscopic system, we analyse the structure of effective SchwingerKeldysh ClosedTimePath theories. We use this structure and the action principle for open systems to derive the energymomentum balance equation for a dissipative fluid from an effective CTP Goldstone action. Near hydrodynamical equilibrium, we construct the firstorder dissipative stressenergy tensor and derive the NavierStokes equations. Shear viscosity is shown to vanish, while bulk viscosity and thermodynamical quantities are determined by the form of the effective action. The exploration of strongly interacting states of matter, particularly in the hydrodynamic regime, has been a major recent application of gauge/string duality. The strongly coupled theories involved are typically deformations of large$N$ SUSY gauge theories with exotic matter that are unusual from a lowenergy point of view. In order to better interpret holographic results, an understanding of the weakcoupling behaviour of such gauge theories is essential. We study the exact and SUSYbroken N=1 and N=2 superQED with finite densities of electron number and Rcharge, respectively. Despite the fact that fermionic fields couple to the chemical potentials, the strength of scalarfermion interactions, fixed by SUSY, prevents a Fermi surface from forming. This is important for hydrodynamical excitations such as zero sound. Intriguingly, in the absence of a Fermi surface, the total charge need not be stored in the scalar condensates alone and fermions may contribute. GaussBonnet gravity is a useful laboratory for nonperturbative studies of the higher derivative curvature effects on transport coefficients of conformal fluids with holographic duals. It was previously known that shear viscosity can be tuned to zero by adjusting the GaussBonnet coupling, λ_{GB}, to its maximal critical value. To understand the behaviour of the fluid in this limit, we compute the secondorder transport coefficients nonperturbatively in λ_{GB} and show that the fluid still produces entropy, while diffusion and sound attenuation are suppressed at all order in the hydrodynamic expansion. We also show that the theory violates a previously proposed universal relation between three of the second order transport coefficients. We further compute the only secondorder coefficient thus far unknown, λ_{2}, in the N=4 super YangMills theory with the leadingorder 't Hooft coupling correction. Intriguingly, the universal relation is not violated by these leadingorder perturbative corrections. Finally, by adding higherderivative photon field terms to the action, we study charge diffusion and nonperturbative parameter regimes in which the charge diffusion constant vanishes.
