41 research outputs found
The stress tensor of a quark moving through N=4 thermal plasma
We develop the linear equations that describe graviton perturbations of
AdS_5-Schwarzschild generated by a string trailing behind an external quark
moving with constant velocity. Solving these equations allows us to evaluate
the stress tensor in the boundary gauge theory. Components of the stress tensor
exhibit directional structures in Fourier space at both large and small
momentum. We comment on the possible relevance of our results to relativistic
heavy ion collisions.Comment: 33 pages, 5 figures. v2: improved low K discussion; other minor
improvement
String creation in cosmologies with a varying dilaton
FRW solutions of the string theory low-energy effective actions are
described, yielding a dilaton which first decreases and then increases. We
study string creation in these backgrounds and find an exponential divergence
due to an initial space-like singularity. We conjecture that this singularity
may be removed by the effects of back-reaction, leading to a solution which at
early times is de Sitter space.Comment: 15 pages, latex, one figur
Dissipation from a heavy quark moving through N=4 super-Yang-Mills plasma
Using AdS/CFT, we compute the Fourier space profile of generated by
a heavy quark moving through a thermal plasma of strongly coupled N=4
super-Yang-Mills theory. We find evidence of a wake whose description includes
gauge fields with large momenta. We comment on the possible relevance of our
results to relativistic heavy ion collisions.Comment: 17 pages, 2 figures. v2: reference added, other minor improvements.
v3: improved the phrasing describing directional structure
Shock waves in strongly coupled plasmas
Shock waves are supersonic disturbances propagating in a fluid and giving
rise to dissipation and drag. Weak shocks, i.e., those of small amplitude, can
be well described within the hydrodynamic approximation. On the other hand,
strong shocks are discontinuous within hydrodynamics and therefore probe the
microscopics of the theory. In this paper we consider the case of the strongly
coupled N=4 plasma whose microscopic description, applicable for scales smaller
than the inverse temperature, is given in terms of gravity in an asymptotically
space. In the gravity approximation, weak and strong shocks should be
described by smooth metrics with no discontinuities. For weak shocks we find
the dual metric in a derivative expansion and for strong shocks we use
linearized gravity to find the exponential tail that determines the width of
the shock. In particular we find that, when the velocity of the fluid relative
to the shock approaches the speed of light the penetration depth
scales as . We compare the results with second
order hydrodynamics and the Israel-Stewart approximation. Although they all
agree in the hydrodynamic regime of weak shocks, we show that there is not even
qualitative agreement for strong shocks. For the gravity side, the existence of
shock waves implies that there are disturbances of constant shape propagating
on the horizon of the dual black holes.Comment: 47 pages, 8 figures; v2:typos corrected, references adde
Anisotropic Drag Force from 4D Kerr-AdS Black Holes
Using AdS/CFT we investigate the effect of angular-momentum-induced
anisotropy on the instantaneous drag force of a heavy quark. The dual
description is that of a string moving in the background of a Kerr-AdS black
holes. The system exhibits the expected focussing of jets towards the impact
parameter plane. We put forward that we can use the connection between this
focussing behavior and the angular momentum induced pressure gradient to
extrapolate the pressure gradient correction to the drag force that can be used
for transverse elliptic flow in realistic RHIC. The result is recognizable as a
relativistic pressure gradient force.Comment: 22 pages and 4 figure
Quark-Gluon Plasma - New Frontiers
As implied by organizers, this talk is not a conference summary but rather an
outline of progress/challenges/``frontiers'' of the theory. Some fundamental
questions addressed are:
Why is sQGP such a good liquid? Do we understand (de)confinement and what do
we know about ``magnetic'' objects creating it? Can we understand the AdS/CFT
predictions, from the gauge theory side? Can they be tested experimentally? Can
AdS/CFT duality help us understand rapid equilibration/entropy production? Can
we work out a complete dynamical ``gravity dual'' to heavy ion collisions?Comment: final talk at Quark Matter 2008, Jaipur, India, Feb.200
On the Beaming of Gluonic Fields at Strong Coupling
We examine the conditions for beaming of the gluonic field sourced by a heavy
quark in strongly-coupled conformal field theories, using the AdS/CFT
correspondence. Previous works have found that, contrary to naive expectations,
it is possible to set up collimated beams of gluonic radiation despite the
strong coupling. We show that, on the gravity side of the correspondence, this
follows directly (for arbitrary quark motion, and independently of any
approximations) from the fact that the string dual to the quark remains
unexpectedly close to the AdS boundary whenever the quark moves
ultra-relativistically. We also work out the validity conditions for a related
approximation scheme that proposed to explain the beaming effect though the
formation of shock waves in the bulk fields emitted by the string. We find that
these conditions are fulfilled in the case of ultra-relativistic uniform
circular motion that motivated the proposal, but unfortunately do not hold for
much more general quark trajectories.Comment: 1+33 pages, 2 figure
Black Branes in a Box: Hydrodynamics, Stability, and Criticality
We study the effective hydrodynamics of neutral black branes enclosed in a
finite cylindrical cavity with Dirichlet boundary conditions. We focus on how
the Gregory-Laflamme instability changes as we vary the cavity radius R. Fixing
the metric at the cavity wall increases the rigidity of the black brane by
hindering gradients of the redshift on the wall. In the effective fluid, this
is reflected in the growth of the squared speed of sound. As a consequence,
when the cavity is smaller than a critical radius the black brane becomes
dynamically stable. The correlation with the change in thermodynamic stability
is transparent in our approach. We compute the bulk and shear viscosities of
the black brane and find that they do not run with R. We find mean-field theory
critical exponents near the critical point.Comment: 23 pages, 3 figures. v2: added comments on first-order phase
transitio