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Steven S. Gubser

Publications and source records attributed to Steven S. Gubser.

At least 73 records · Page 4Linked to original sources
arXiv2009

Low-temperature behavior of the Abelian Higgs model in anti-de Sitter space

We explore the low-temperature behavior of the Abelian Higgs model in AdS_4, away from the probe limit in which back-reaction of matter fields on the metric can be neglected. Over a significant range of charges for the complex scalar, we observe a second order phase transition at finite temperature. The symmetry-breaking states are superconducting black holes. At least when the charge of the scalar is not too small, we observe at low temperatures the emergence of a domain wall structure characterized by a definite index of refraction. We also compute the conductivity as a function of frequency.

hep-th↗
arXiv2008

Time warps

I reconsider asymmetrically warped compactifications, in which time and space have different warp factors. I call such compactifications time warps if the bulk geometry has neither entropy nor temperature. I provide an example starting from an asymptotically AdS_5 spacetime where the speed of light, measured in a fixed coordinate system, is larger near the boundary than it is deep in the interior. This example follows the general plan of earlier work on superconducting black holes. To obtain a normalizable, four-dimensional graviton, one can introduce a Planck brane whose action includes a wrong-sign Einstein-Hilbert term. The equation of state of the Planck brane has w < -1, which is a violation of the null energy condition. I show, in an almost dimension-independent fashion, that such a violation must occur in a static time warp geometry. Time warps of the type I describe provide an extra-dimensional description of boost invariance as an emergent symmetry in the infrared. High-energy violations of Lorentz symmetry, if confined to a strongly coupled unparticle sector dual to the time warp geometry, might manifest themselves through unusual kinematic constraints. As an example, I explain how modifications of unparticle phase space would affect the decay of a heavy particle into a light visible sector particle plus unparticle stuff.

hep-th↗
arXiv2008

Gluon energy loss in the gauge-string duality

We estimate the stopping length of an energetic gluon in a thermal plasma of strongly coupled N=4 super-Yang-Mills theory by representing the gluon as a doubled string rising up out of the horizon.

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arXiv2008

The gravity dual of a p-wave superconductor

We construct black hole solutions to the Yang-Mills equations in an AdS_4-Schwarzschild background which exhibit superconductivity. What makes these backgrounds p-wave superconductors is that the order parameter is a vector, and the conductivities are strongly anisotropic in a manner that is suggestive of a gap with nodes. The low-lying excitations of the normal state have a relaxation time which grows rapidly as the temperature decreases, consistent with the absence of impurity scattering. A numerical exploration of quasinormal modes close to the transition temperature suggests that p-wave backgrounds are stable against perturbations analogous to turning on a p+ip gap, whereas p+ip-wave configurations are unstable against turning into pure p-wave backgrounds.

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arXiv2008

Expanding plasmas and quasinormal modes of anti-de Sitter black holes

We compute the gravitational quasinormal modes of the global AdS_5-Schwarzschild solution. We show how to use the holographic dual of these modes to describe a thermal plasma of finite extent expanding in a slightly anisotropic fashion. We compare these flows with the behavior of quark-gluon plasmas produced in relativistic heavy ion collisions by estimating the elliptic flow coefficient and the thermalization time.

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arXiv2008

The gravity dual to a quantum critical point with spontaneous symmetry breaking

We consider zero temperature solutions to the Abelian Higgs model coupled to gravity with a negative cosmological constant. With appropriate choices of parameters, the geometry contains two copies of anti-de Sitter space, one describing conformal invariance in the ultraviolet, and one in the infrared. The effective speed of signal propagation is smaller in the infrared. Green's functions and associated transport coefficients can have unusual power law scaling in the infrared. We provide an example in which the real part of the conductivity scales approximately as omega^3.5 for small omega.

hep-th↗
arXiv2008

Bulk viscosity of strongly coupled plasmas with holographic duals

We explain a method for computing the bulk viscosity of strongly coupled thermal plasmas dual to supergravity backgrounds supported by one scalar field. Whereas earlier investigations required the computation of the leading dissipative term in the dispersion relation for sound waves, our method requires only the leading frequency dependence of an appropriate Green's function in the low-frequency limit. With a scalar potential chosen to mimic the equation of state of QCD, we observe a slight violation of the lower bound on the ratio of the bulk and shear viscosities conjectured in arXiv:0708.3459.

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arXiv2008

Entropy production in collisions of gravitational shock waves and of heavy ions

We calculate the area of a marginally trapped surface formed by a head-on collision of gravitational shock waves in AdS_D. We use this to obtain a lower bound on the entropy produced after the collision. A comparison to entropy production in heavy ion collisions is included. We also discuss an O(D-2) remnant of conformal symmetry which is present in a class of gravitational shock wave collisions in AdS_D and which might be approximately realized (with D=5) in central heavy-ion collisions.

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arXiv2008

Thermodynamics and bulk viscosity of approximate black hole duals to finite temperature quantum chromodynamics

We consider classes of translationally invariant black hole solutions whose equations of state closely resemble that of QCD at zero chemical potential. We use these backgrounds to compute the ratio zeta/s of bulk viscosity to entropy density. For a class of black holes that exhibits a first order transition, we observe a sharp rise in zeta/s near T_c. For constructions that exhibit a smooth cross-over, like QCD does, the rise in zeta/s is more modest. We conjecture that divergences in zeta/s for black hole horizons are related to extrema of the entropy density as a function of temperature.

hep-th↗
arXiv2008

Mimicking the QCD equation of state with a dual black hole

We present numerical and analytical studies of the equation of state of translationally invariant black hole solutions to five-dimensional gravity coupled to a single scalar. As an application, we construct a family of black holes that closely mimics the equation of state of quantum chromodynamics at zero chemical potential.

hep-th↗
arXiv2008

Colorful horizons with charge in anti-de Sitter space

An abelian gauge symmetry can be spontaneously broken near a black hole horizon in anti-de Sitter space using a condensate of non-abelian gauge fields. There is a second order phase transition between Reissner-Nordstrom-anti-de Sitter solutions, which are preferred at high temperatures, and symmetry breaking solutions, which are preferred at low temperatures.

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arXiv2008

Linearized hydrodynamics from probe-sources in the gauge-string duality

We study the response of an infinite, asymptotically static N=4 plasma to a generic localized source in the probe approximation. At large distances, the energy momentum tensor of the plasma includes a term which satisfies the constitutive relations of linearized hydrodynamics, but it can also include a non-hydrodynamical term which contributes at the same order as viscous corrections, or even at leading order in some cases. The conditions for the appearance of a laminar wake far behind the source and its relevance for phenomenological models used to explain di-hadron correlations are discussed. We also consider the energy momentum tensor near the source, where the hydrodynamical approximation can be expected to break down. Our analysis encompasses a wide range of sources which are localized in the bulk of AdS, including trailing strings, mesonic and baryonic configurations of strings, and point particles.

hep-th↗
arXiv2008

Breaking an Abelian gauge symmetry near a black hole horizon

I argue that coupling the Abelian Higgs model to gravity plus a negative cosmological constant leads to black holes which spontaneously break the gauge invariance via a charged scalar condensate slightly outside their horizon. This suggests that black holes can superconduct.

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arXiv2007

Shock waves from heavy-quark mesons in AdS/CFT

We calculate the far-field stress-energy tensor of a quark gluon plasma due to a heavy-quark meson moving through it, using a semi-classical description in string theory of the meson as a string hanging into anti-de Sitter space. We find that these mesons create a shock wave but no diffusion wake, in contrast to single heavy quarks described via a trailing string, where the diffusion wake is strong. If thermal QCD responds similarly to heavy quarks and heavy quarkonia, the presence or suppression of the diffusion wake constitutes a prediction which can in principle be checked in heavy-ion collisions.

hep-th↗
arXiv2007

Universality of the diffusion wake in the gauge-string duality

As a particle moves through a fluid, it may generate a laminar wake behind it. In the gauge-string duality, we show that such a diffusion wake is created by a heavy quark moving through a thermal plasma and that it has a universal strength when compared to the total drag force exerted on the quark by the plasma. The universality extends over all asymptotically anti-de Sitter supergravity constructions with arbitrary scalar matter. We discuss how these results relate to the linearized hydrodynamic approximation and how they bear on our understanding of di-hadron correlators in heavy ion collisions.

hep-th↗
arXiv2007

Energy disturbances due to a moving quark from gauge-string duality

Using AdS/CFT, we calculate the energy density of a quark moving through a thermal state of N=4 super-Yang-Mills theory. Relying on previous work for momentum-space representations as well as asymptotic behaviors, we Fourier transform to position space and exhibit a sonic boom at a speed larger than the speed of sound. Nontrivial structure is found at small length scales, confirming earlier analytical work by the authors.

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