Holographic bulk viscosity: GPR vs EO

Recently Eling and Oz (EO) proposed a formula for the holographic bulk viscosity, in arXiv:1103.1657, derived from the null horizon focusing equation. This formula seems different from that obtained earlier by Gubser, Pufu and Rocha (GPR) in arXiv:0806.0407 calculated from the IR limit of the two-point function of the trace of the stress tensor. The two were shown to agree only for some simple scaling cases. We point out that the two formulae agree in two non-trivial holographic theories describing RG flows. The first is the strongly coupled N=2* gauge theory plasma. The second is the semi-phenomenological model of Improved Holographic QCD.Comment: 21 pages, 2 figure

Viscosity Bound and Causality in Superfluid Plasma

It was argued by Brigante et.al that the lower bound on the ratio of the shear viscosity to the entropy density in strongly coupled plasma is translated into microcausality violation in the dual gravitational description. Since transport properties of the system characterize its infrared dynamics, while the causality of the theory is determined by its ultraviolet behavior, the viscosity bound/microcausality link should not be applicable to theories that undergo low temperature phase transitions. We present an explicit model of AdS/CFT correspondence that confirms this fact.Comment: 27 pages, 5 figures. References added, typos fixe

Holographic dual of the Eguchi-Kawai mechanism

archiveprefix: arXiv primaryclass: hep-th reportnumber: NORDITA-2014-40, UUITP-03-14, QMUL-PH-14-08 slaccitation: %%CITATION = ARXIV:1404.0225;%%archiveprefix: arXiv primaryclass: hep-th reportnumber: NORDITA-2014-40, UUITP-03-14, QMUL-PH-14-08 slaccitation: %%CITATION = ARXIV:1404.0225;%%archiveprefix: arXiv primaryclass: hep-th reportnumber: NORDITA-2014-40, UUITP-03-14, QMUL-PH-14-08 slaccitation: %%CITATION = ARXIV:1404.0225;%%The work of K.Z. was supported by the ERC advanced grant No 341222, by the Marie Curie network GATIS of the European Union’s FP7 Programme under REA Grant Agreement No 317089, and by the Swedish Research Council (VR) grant 2013-4329. DY acknowledges NORDITA where this work was begun, during his time as a NORDITA fellow

On effective action of string theory flux compactifications

We discuss four dimensional effective actions of string theory flux compactifications. These effective actions describe four dimensional gravity coupled to overall Kahler modulus of the compactification manifold. We demonstrate the agreement between ten dimensional equations of motion of supergravity with localized branes, and equations of motion derived from the effective action. The agreement is lost however if one evaluates the full effective action on the equations of motion for a subset of the supergravity modes, provided these modes depend on-shell on the Kahler modulus.Comment: 25 pages; v2: refs adde

Transport coefficients, membrane couplings and universality at extremality

We present an efficient method for computing the zero frequency limit of transport coefficients in strongly coupled field theories described holographically by higher derivative gravity theories. Hydrodynamic parameters such as shear viscosity and conductivity can be obtained by computing residues of poles of the off-shell lagrangian density. We clarify in which sense these coefficients can be thought of as effective couplings at the horizon, and present analytic, Wald-like formulae for the shear viscosity and conductivity in a large class of general higher derivative lagrangians. We show how to apply our methods to systems at zero temperature but finite chemical potential. Our results imply that such theories satisfy $\eta/s=1/4\pi$ universally in the Einstein-Maxwell sector. Likewise, the zero frequency limit of the real part of the conductivity for such systems is shown to be universally zero, and we conjecture that higher derivative corrections in this sector do not modify this result to all orders in perturbation theory.Comment: 29 pages, v2: Small text changes for clarity, typos correcte

Stability of the non-extremal enhancon solution I: perturbation equations

We consider the stability of the two branches of non-extremal enhancon solutions. We argue that one would expect a transition between the two branches at some value of the non-extremality, which should manifest itself in some instability. We study small perturbations of these solutions, constructing a sufficiently general ansatz for linearised perturbations of the non-extremal solutions, and show that the linearised equations are consistent. We show that the simplest kind of perturbation does not lead to any instability. We reduce the problem of studying the more general spherically symmetric perturbation to solving a set of three coupled second-order differential equations.Comment: 20 pages, 1 figure, references added, typos fixed, version to appear in PR

Dynamics of Fundamental Matter in N=2* Yang-Mills Theory

We study the dynamics of quenched fundamental matter in $\mathcal{N}=2^\ast$ supersymmetric large $N$ SU(N) Yang-Mills theory at zero temperature. Our tools for this study are probe D7-branes in the holographically dual $\mathcal{N}=2^\ast$ Pilch-Warner gravitational background. Previous work using D3-brane probes of this geometry has shown that it captures the physics of a special slice of the Coulomb branch moduli space of the gauge theory, where the $N$ constituent D3-branes form a dense one dimensional locus known as the enhancon, located deep in the infrared. Our present work shows how this physics is supplemented by the physics of dynamical flavours, revealed by the D7-branes embeddings we find. The Pilch-Warner background introduces new divergences into the D7-branes free energy, which we are able to remove with a single counterterm. We find a family of D7-brane embeddings in the geometry and discuss their properties. We study the physics of the quark condensate, constituent quark mass, and part of the meson spectrum. Notably, there is a special zero mass embedding that ends on the enhancon, which shows that while the geometry acts repulsively on the D7-branes, it does not do so in a way that produces spontaneous chiral symmetry breaking.Comment: 24 pages, 8 figures. Corrected typos, added comment about counterterm. To appear in JHE

The effect of higher derivative correction on η/s\eta /s and conductivities in STU model

In this paper we study the ratio of shear viscosity to entropy, electrical and thermal conductivities for the R-charged black hole in STU model. We generalize previous works to the case of a black hole with three different charges. Actually we use diffusion constant to obtain ratio of shear viscosity to entropy. By applying the thermodynamical stability we recover previous results. Also we investigate the effect of higher derivative corrections.Comment: revised versio

Hydrodynamics of fundamental matter

First and second order transport coefficients are calculated for the strongly coupled N=4 SYM plasma coupled to massless fundamental matter in the Veneziano limit. The results, including among others the value of the bulk viscosity and some relaxation times, are presented at next-to-leading order in the flavor contribution. The bulk viscosity is found to saturate Buchel's bound. This result is also captured by an effective single-scalar five-dimensional holographic dual in the Chamblin-Reall class and it is suggested to hold, in the limit of small deformations, for generic plasmas with gravity duals, whenever the leading conformality breaking effects are driven by marginally (ir)relevant operators. This proposal is then extended to other relations for hydrodynamic coefficients, which are conjectured to be universal for every non-conformal plasma with a dual Chamblin-Reall-like description. Our analysis extends to any strongly coupled gauge theory describing the low energy dynamics of Nc>>1 D3-branes at the tip of a generic Calabi-Yau cone. The fundamental fields are added by means of 1<<Nf<<Nc homogeneously smeared D7-branes.Comment: 24 pages. V2: Important improvements in the discussion of the results in section 1. References adde