Pure Lovelock Kasner metrics

We study pure Lovelock vacuum and perfect fluid equations for Kasner-type metrics. These equations correspond to a single Nth order Lovelock term in the action in $d=2N+1,2N+2$ dimensions, and they capture the relevant gravitational dynamics when aproaching the big-bang singularity within the Lovelock family of theories. Pure Lovelock gravity also bears out the general feature that vacuum in the critical odd dimension, $d=2N+1$, is kinematic, i.e. we may define an analogue Lovelock-Riemann tensor that vanishes in vacuum for $d=2N+1$, yet the Riemann curvature is non-zero. We completely classify isotropic and vacuum Kasner metrics for this class of theories in several isotropy types. The different families can be characterized by means of certain higher order 4th rank tensors. We also analyze in detail the space of vacuum solutions for five- and six dimensional pure Gauss-Bonnet theory. It possesses an interesting and illuminating geometric structure and symmetries that carry over to the general case. We also comment on a closely related family of exponential solutions and on the possibility of solutions with complex Kasner exponents. We show that the latter imply the existence of closed timelike curves in the geometry

Global Portfolio Rebalancing and Exchange Rates

We examine international equity allocations at the fund level and show how excess foreign returns influence portfolio rebalancing, capital flows, and currencies. Our equilibrium model of incomplete foreign exchange (FX) risk trading where exchange rate risk partially segments international equity markets is consistent with the observed dynamics of equity returns, exchange rates, and fund-level capital flows. We document that rebalancing is more intense under higher FX volatility and find heterogeneous rebalancing behavior across different fund characteristics. A granular instrumental variable approach identifies a positive currency supply elasticity

Membrane Paradigm and Horizon Thermodynamics in Lanczos-Lovelock gravity

We study the membrane paradigm for horizons in Lanczos-Lovelock models of gravity in arbitrary D dimensions and find compact expressions for the pressure p and viscosity coefficients \eta and \zeta of the membrane fluid. We show that the membrane pressure is intimately connected with the Noether charge entropy S_Wald of the horizon when we consider a specific m-th order Lanczos-Lovelock model, through the relation pA/T=(D-2m)/(D-2)S_Wald, where T is the temperature and A is the area of the horizon. Similarly, the viscosity coefficients are expressible in terms of entropy and quasi-local energy associated with the horizons. The bulk and shear viscosity coefficients are found to obey the relation \zeta=-2(D-3)/(D-2)\eta.Comment: v1: 13 pages, no figure. (v2): refs added, typos corrected, new subsection added on the ratio \eta/s. (v3): some clarification added, typos corrected, to appear in JHE

The Shear Viscosity to Entropy Ratio: A Status Report

This review highlights some of the lessons that the holographic gauge/gravity duality has taught us regarding the behavior of the shear viscosity to entropy density in strongly coupled field theories. The viscosity to entropy ratio has been shown to take on a very simple universal value in all gauge theories with an Einstein gravity dual. Here we describe the origin of this universal ratio, and focus on how it is modified by generic higher derivative corrections corresponding to curvature corrections on the gravity side of the duality. In particular, certain curvature corrections are known to push the viscosity to entropy ratio below its universal value. This disproves a longstanding conjecture that such a universal value represents a strict lower bound for any fluid in nature. We discuss the main developments that have led to insight into the violation of this bound, and consider whether the consistency of the theory is responsible for setting a fundamental lower bound on the viscosity to entropy ratio.Comment: 29 pages. Invited review for Modern Physics Letters B. References and minor comments adde

The Weak Gravity Conjecture and the Viscosity Bound with Six-Derivative Corrections

The weak gravity conjecture and the shear viscosity to entropy density bound place constraints on low energy effective field theories that may help to distinguish which theories can be UV completed. Recently, there have been suggestions of a possible correlation between the two constraints. In some interesting cases, the behavior was precisely such that the conjectures were mutually exclusive. Motivated by these works, we study the mass to charge and shear viscosity to entropy density ratios for charged AdS5 black branes, which are holographically dual to four-dimensional CFTs at finite temperature. We study a family of four-derivative and six-derivative perturbative corrections to these backgrounds. We identify the region in parameter space where the two constraints are satisfied and in particular find that the inclusion of the next-to-leading perturbative correction introduces wider possibilities for the satisfaction of both constraints.Comment: 24 pages, 6 figures, v2: published version, refs added, minor clarificatio

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 studies of quasi-topological gravity

Quasi-topological gravity is a new gravitational theory including curvature-cubed interactions and for which exact black hole solutions were constructed. In a holographic framework, classical quasi-topological gravity can be thought to be dual to the large $N_c$ limit of some non-supersymmetric but conformal gauge theory. We establish various elements of the AdS/CFT dictionary for this duality. This allows us to infer physical constraints on the couplings in the gravitational theory. Further we use holography to investigate hydrodynamic aspects of the dual gauge theory. In particular, we find that the minimum value of the shear-viscosity-to-entropy-density ratio for this model is $\eta/s \simeq 0.4140/(4\pi)$.Comment: 45 pages, 6 figures. v2: References adde