arXiv subjects

Robert C. Myers

Publications and source records attributed to Robert C. Myers.

At least 109 records · Page 6Linked to original sources

arXiv2009

Causality of Holographic Hydrodynamics

We study causality violation in holographic hydrodynamics in the gauge theory/string theory correspondence, focussing on Gauss-Bonnet gravity. The value of the Gauss-Bonnet coupling is related to the difference between the central charges of the dual conformal gauge theory. We show that, when this difference is sufficiently large, causality is violated both in the second-order truncated theory of hydrodynamics, as well as in the exact theory. We find that the latter provides more stringent constraints, which match precisely those appearing in the CFT analysis of Hofman and Maldacena.

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arXiv2009

Holographic Hydrodynamics with a Chemical Potential

We consider five-dimensional gravity coupled to a negative cosmological constant and a single U(1) gauge field, including a general set of four-derivative interactions. In this framework, we construct charged planar AdS black hole solutions perturbatively and consider the thermal and hydrodynamic properties of the plasma in the dual CFT. In particular, we calculate the ratio of shear viscosity to entropy density and argue that the violation of the KSS bound is enhanced in the presence of a chemical potential. We also compute the electrical conductivity and comment on various conjectured bounds related to this coefficient.

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arXiv2009

Beyond eta/s = 1/4pi

We use low-energy effective description of gauge theory/string theory duality to argue that the Kovtun-Son-Starinets viscosity bound is generically violated in superconformal gauge theories with non-equal central charges $c\ne a$. We present new examples (of string theory constructions and of gauge theories) where the bound is violated in a controllable setting. We consider the comparison of results from AdS/CFT calculations to the QCD plasma in the context of this discussion.

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arXiv2009

Transport Properties of Holographic Defects

We study the charge transport properties of fields confined to a (2+1)-dimensional defect coupled to (3+1)-dimensional super-Yang-Mills at large-$\nc$ and strong coupling, using AdS/CFT techniques applied to linear response theory. The dual system is described by $\nf$ probe D5- or D7-branes in the gravitational background of $\nc$ black D3-branes. Surprisingly, the transport properties of both defect CFT's are essentially identical -- even though the D7-brane construction breaks all supersymmetries. We find that the system possesses a conduction threshold given by the wave-number of the perturbation and that the charge transport arises from a quasiparticle spectrum which is consistent with an intuitive picture where the defect acquires a finite width. We also examine finite-$λ$ modifications arising from higher derivative interactions in the probe brane action.

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arXiv2008

Universal holographic hydrodynamics at finite coupling

We consider thermal plasmas in a large class of superconformal gauge theories described by a holographic dual geometry of the form $AdS_5\times M_5$. In particular, we demonstrate that all of the thermodynamic properties and hydrodynamic transport parameters for a large class of superconformal gauge theories exhibit a certain universality to leading order in the inverse 't Hooft coupling and $1/N_c$. In particular, we show that independent of the compactification geometry, the leading corrections are derived from the same five-dimensional effective supergravity action supplemented by a term quartic in the five-dimensional Weyl tensor.

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arXiv2008

On quark masses in holographic QCD

Recently certain nonlocal operators were proposed to provide quark masses for the holographic model of QCD developed by Sakai and Sugimoto. The properties of these operators at strong coupling are examined in detail using holographic techniques. We find the renormalization procedure for these operators is modified by the running of the five-dimensional gauge coupling. We explicitly evaluate the chiral condensate characterized by these operators.

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arXiv2008

Quantum corrections to eta/s

We consider corrections to the ratio of the shear viscosity to the entropy density in strongly coupled nonabelian plasmas using the AdS/CFT correspondence. In particular, higher derivative terms with the five-form RR flux, which have been ignored in all previous calculations, are included. This provides the first reliable calculation of the leading order correction in the inverse 't Hooft coupling to the celebrated result eta/s=1/4pi. The leading correction in inverse powers of the number of colours is computed. Our results hold very generally for quiver gauge theories with an internal manifold L_pqr in the holographic dual. Our analysis implies that the thermal properties of these theories will not be affected by the five-form flux terms at this order.

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arXiv2008

Viscosity Bound Violation in Higher Derivative Gravity

Motivated by the vast string landscape, we consider the shear viscosity to entropy density ratio in conformal field theories dual to Einstein gravity with curvature square corrections. After field redefinitions these theories reduce to Gauss-Bonnet gravity, which has special properties that allow us to compute the shear viscosity nonperturbatively in the Gauss-Bonnet coupling. By tuning of the coupling, the value of the shear viscosity to entropy density ratio can be adjusted to any positive value from infinity down to zero, thus violating the conjectured viscosity bound. At linear order in the coupling, we also check consistency of four different methods to calculate the shear viscosity, and we find that all of them agree. We search for possible pathologies associated with this class of theories violating the viscosity bound.

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arXiv2008

The fast life of holographic mesons

We use holographic techniques to study meson quasiparticles moving through a thermal plasma in N=2 super-Yang-Mills theory, with gauge group SU(N_c) and coupled to N_f flavours of fundamental matter. This holographic approach reliably describes the system at large N_c, large 't Hooft coupling and N_f/N_c<<1. The meson states are destabilized by introducing a small quark density n_q. Spectral functions are used to examine the dispersion relations of these quasiparticles. In a low-momentum regime, the quasiparticles approach a limiting velocity which can be significantly less than the speed of light. In this regime, the widths of the quasiparticles also rise dramatically as their momentum approaches a critical value q_crit. While the spectral functions do not display isolated resonances for q>q_crit, the dispersion relations can be extended into this high-momentum regime by studying the dual quasinormal modes. A preliminary qualitative analysis of these modes suggests that the group velocity rises to the speed of light for q>>q_crit.

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arXiv2008

Viscosity Bound and Causality Violation

In recent work we showed that, for a class of conformal field theories (CFT) with Gauss-Bonnet gravity dual, the shear viscosity to entropy density ratio, $η/s$, could violate the conjectured Kovtun-Starinets-Son viscosity bound, $η/s\geq1/4π$. In this paper we argue, in the context of the same model, that tuning $η/s$ below $(16/25)(1/4π)$ induces microcausality violation in the CFT, rendering the theory inconsistent. This is a concrete example in which inconsistency of a theory and a lower bound on viscosity are correlated, supporting the idea of a possible universal lower bound on $η/s$ for all consistent theories.

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arXiv2007

Holographic spectral functions and diffusion constants for fundamental matter

The holographic dual of large-Nc super-Yang-Mills coupled to a small number of flavours of fundamental matter, Nf << Nc, is described by Nf probe D7-branes in the gravitational background of Nc black D3-branes. This system undergoes a first order phase transition characterised by the `melting' of the mesons. We study the high temperature phase in which the D7-branes extend through the black hole horizon. In this phase, we compute the spectral function for vector, scalar and pseudoscalar modes on the D7-brane probe. We also compute the diffusion constant for the flavour currents.

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arXiv2007

On the gravitational stability of D1-D5-P black holes

We examine the stability of the nonextremal D1-D5-P black hole solutions. In particular, we look for the appearance of a superradiant instability for the spinning black holes but we find no evidence of such an instability. We compare this situation with that for the smooth soliton geometries, which were recently observed to suffer from an ergoregion instability, and consider the implications for the fuzzball proposal.

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arXiv2007

Holographic phase transitions at finite chemical potential

Recently holographic techniques have been used to study the thermal properties of N=2 super-Yang-Mills theory, with gauge group SU(Nc) and coupled to Nf << Nc flavours of fundamental matter, at large Nc and large 't Hooft coupling. Here we consider the phase diagram as a function of temperature and baryon chemical potential mu. For fixed mu < Nc Mq there is a line of first order thermal phase transitions separating a region with vanishing baryon density and one with nonzero density. For fixed mu > Nc Mq there is no phase transition as a function of the temperature and the baryon density is always nonzero. We also compare the present results for the grand canonical ensemble with those for canonical ensemble in which the baryon density is held fixed [1].

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arXiv2007

Multi-black hole configurations on the cylinder

We construct the metric of new multi-black hole configurations on a d-dimensional cylinder R^{d-1} x S^1, in the limit of small total mass (or equivalently in the limit of a large cylinder). These solutions are valid to first order in the total mass and describe configurations with several small black holes located at different points along the circle direction of the cylinder. We explain that a static configuration of black holes is required to be in equilibrium such that the external force on each black hole is zero, and we examine the resulting conditions. The first-order corrected thermodynamics of the solutions is obtained and a Newtonian interpretation of it is given. We then study the consequences of the multi-black hole configurations for the phase structure of static Kaluza-Klein black holes and show that our new solutions imply continuous non-uniqueness in the phase diagram. The new multi-black hole configurations raise the question of existence of new non-uniform black strings. Finally, a further analysis of the three-black hole configuration suggests the possibility of a new class of static lumpy black holes in Kaluza-Klein space.

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arXiv2007

A New Perspective on DGP Gravity

We examine brane induced gravity on codimension-1 branes, a.k.a DGP gravity, as a theory of five-dimensional gravity containing a certain class four-dimensional branes. From this perspective, the model suffers from a number of pathologies which went unnoticed before. By generalizing the 5D geometry from Minkowski to Schwarzschild, we find that when the bulk mass is large enough, the brane hits a pressure singularity at finite radius. Further, on the self-accelerating branch, the five-dimensional energy is unbounded from below, implying that the self-accelerating backgrounds are unstable. Even in an empty Minkowski bulk, standard Euclidean techniques suggest that the spontaneous nucleation of self-accelerating branes is unsuppressed. If so, quantum effects will strongly modify any classical intuition about the theory. We also note that unless considered as Z_2-orbifold boundaries, self-accelerating branes correspond to `wormhole' configurations, which introduces the usual problematic issues associated with wormholes. Altogether these pathologies present a serious challenge that any proposed UV completion of the DGP model must overcome.

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arXiv2007

Thermodynamics of the brane

The holographic dual of a finite-temperature gauge theory with a small number of flavours typically contains D-brane probes in a black hole background. We have recently shown that these systems undergo a first order phase transition characterised by a `melting' of the mesons. Here we extend our analysis of the thermodynamics of these systems by computing their free energy, entropy and energy densities, as well as the speed of sound. We also compute the meson spectrum for brane embeddings outside the horizon and find that tachyonic modes appear where this phase is expected to be unstable from thermodynamic considerations.

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arXiv2007

Holographic phase transitions at finite baryon density

We use holographic techniques to study SU(Nc) super Yang-Mills theory coupled to Nf << Nc flavours of fundamental matter at finite temperature and baryon density. We focus on four dimensions, for which the dual description consists of Nf D7-branes in the background of Nc black D3-branes, but our results apply in other dimensions as well. A non-zero chemical potential mu or baryon number density n is introduced via a nonvanishing worldvolume gauge field on the D7-branes. Ref. [1] identified a first order phase transition at zero density associated with `melting' of the mesons. This extends to a line of phase transitions for small n, which terminates at a critical point at finite n. Investigation of the D7-branes' thermodynamics reveals that (d mu / dn)_T <0 in a small region of the phase diagram, indicating an instability. We comment on a possible new phase which may appear in this region.

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arXiv2006

Holographic Viscosity of Fundamental Matter

A holographic dual of a finite-temperature SU(N_c) gauge theory with a small number of flavours N_f << N_c typically contains D-branes in a black hole background. By considering the backreaction of the branes, we demonstrate that, to leading order in N_f/N_c, the viscosity to entropy ratio in these theories saturates the conjectured universal bound eta/s >= 1/4π. The contribution of the fundamental matter eta_fund is therefore enhanced at strong 't Hooft coupling lambda; for example, eta_fund ~ lambda N_c N_f T^3 in four dimensions. Other transport coefficients are analogously enhanced. These results hold with or without a baryon number chemical potential.

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