arXiv subjects

Omid Saremi

Publications and source records attributed to Omid Saremi.

25 records · Page 2Linked to original sources

arXiv2009

Holographic stress tensor for non-relativistic theories

We discuss the calculation of the field theory stress tensor from the dual geometry for two recent proposals for gravity duals of non-relativistic conformal field theories. The first of these has a Schrodinger symmetry including Galilean boosts, while the second has just an anisotropic scale invariance (the Lifshitz case). For the Lifshitz case, we construct an appropriate action principle. We propose a definition of the non-relativistic stress tensor complex for the field theory as an appropriate variation of the action in both cases. In the Schrodinger case, we show that this gives physically reasonable results for a simple black hole solution and agrees with an earlier proposal to determine the stress tensor from the familiar AdS prescription. In the Lifshitz case, we solve the linearised equations of motion for a general perturbation around the background, showing that our stress tensor is finite on-shell.

hep-th↗

arXiv2008

Bulk viscosity and spectral functions in QCD

We examine the behavior of the spectral function for the trace of the stress tensor in QCD in the two regimes where it is possible to make analytical progress; weak coupling, and close to a second order QCD phase transition. We determine the behavior of the bulk viscosity in each regime. We discuss the problem of analytic continuation of the (lattice) Euclidean correlation function to determine the spectral function. In each case the spectral function has a narrow peak at small frequency; its shape would be challenging to extract accurately from lattice data with error bars.

hep-ph↗

arXiv2007

Cosmological Perturbations on a Bouncing Brane

The cosmological perturbations on a bouncing brane are studied. The brane is moving inside a Klebanov-Strassler throat where the infra-red region of the geometry is smoothly cut off. For an observer confined to the world-volume of the brane, this results in a non-singular bouncing mirage cosmology. We have calculated the scalar perturbations corresponding to the normal displacements of the brane. This is performed in the probe brane limit where the gravitational back-reaction of the brane on the bulk throat is absent. Our model provides a framework for studying the transfer of fluctuations from a contracting to an expanding phase. We find that the spectral index of the dominant mode of the metric fluctuation is un-changed, unlike what is obtained by gluing contracting to expanding Einstein universes with the help of the usual matching conditions. Assuming that the fluctuations start off in a vacuum state on sub-Hubble scales during the contracting phase, it is shown that the resulting spectral index n_s on super-Hubble scales in the expanding phase has a large blue tilt. When the brane is moving slowly inside the throat and its kinetic energy is negligible compared to its rest mass, one finds n_s=3. For a fast-rolling brane with a large kinetic energy, the spectral index is n_s = 2.3. This may put severe constraints on models of mirage cosmology.

hep-th↗

arXiv2007

Shear Waves, Sound Waves On A Shimmering Horizon

In the context of the so called ``membrane paradigm'' of black holes/branes, it has been known for sometime that the dynamics of small fluctuations on the stretched horizon can be viewed as corresponding to diffusion of a conserved charge in simple fluids. To study shear waves in this context properly, one must define a conserved stress tensor living on the stretched horizon. Then one is required to show that such a stress tensor satisfies the corresponding constitutive relations. These steps are missing in a previous treatment of the shear perturbations by Kovtun, Starinets and Son. In this note, we fill the gap by prescribing the stress tensor on the stretched horizon to be the Brown and York (or Balasubramanian-Kraus (BK) in the AdS/CFT context) holographic stress tensor. We are then able to show that such a conserved stress tensor satisfies the required constitutive relation on the stretched horizon using Einstein equations. We read off the shear viscosity from the constitutive relations in two different channels, shear and sound. We find an expression for the shear viscosity in both channels which are equal, as expected. Our expression is in agreement with a previous membrane paradigm formula reported by Kovtun, Starinets and Son.

hep-th↗

arXiv2006

The Viscosity Bound Conjecture and Hydrodynamics of M2-Brane Theory at Finite Chemical Potential

Kovtun, Son and Starinets have conjectured that the viscosity to entropy density ratio $η/s$ is always bounded from below by a universal multiple of $\hbar$ i.e., $\hbar/(4πk_{B})$ for all forms of matter. Mysteriously, the proposed viscosity bound appears to be saturated in all computations done whenever a supergravity dual is available. We consider the near horizon limit of a stack of M2-branes in the grand canonical ensemble at finite R-charge densities, corresponding to non-zero angular momentum in the bulk. The corresponding four-dimensional R-charged black hole in Anti-de Sitter space provides a holographic dual in which various transport coefficients can be calculated. We find that the shear viscosity increases as soon as a background R-charge density is turned on. We numerically compute the few first corrections to the shear viscosity to entropy density ratio $η/s$ and surprisingly discover that up to fourth order all corrections originating from a non-zero chemical potential vanish, leaving the bound saturated. This is a sharp signal in favor of the saturation of the viscosity bound for event horizons even in the presence of some finite background field strength. We discuss implications of this observation for the conjectured bound.

hep-th↗

arXiv2004

Folding Branes

We study classical dynamics of a probe Dp-brane moving in a background sourced by a stack of Dp-branes. In this context the physics is similar to that of the effective action for open-string tachyon condensation, but with a power-law runaway potential. We show that small inhomogeneous ripples of the probe brane embedding grow with time, leading to folding of the brane as it moves. We give a full nonlinear analytical treatment of inhomogeneous brane dynamics, suitable for the Dirac-Born-Infeld + Wess-Zumino theory with arbitrary runaway potential, in the case where the source branes are BPS. In the near-horizon geometry, the inhomogeneous brane motion has a dual description in terms of free streaming of massive relativistic test particles originating from the initial hypersurface of the probe brane. We discuss limitations of the effective action description around loci of self-crossing of the probe brane (caustics). We also discuss the effect of brane folding in application to the theory of cosmological fluctuations in string theory inflation.

hep-th↗

arXiv2004

Brane-antibrane systems and the thermal life of neutral black holes

A brane-antibrane model for the entropy of neutral black branes is developed, following on from the work of Danielsson, Guijosa and Kruczenski [1]. The model involves equal numbers of Dp-branes and anti-Dp-branes, and arbitrary angular momenta, and covers the cases p=0,1,2,3,4. The thermodynamic entropy is reproduced by the strongly coupled field theory, up to a power of two. The strong-coupling physics of the p=0 case is further developed numerically, using techniques of Kabat, Lifschytz et al. [2,3], in the context of a toy model containing the tachyon and the bosonic degrees of freedom of the D0-brane and anti-D0-brane quantum mechanics. Preliminary numerical results show that strong-coupling finite-temperature stabilization of the tachyon is possible, in this context.

hep-th↗