112 research outputs found
A modulated shear to entropy ratio
We study correlation functions in an equilibrated spatially modulated phase
of Einstein-Maxwell two-derivative gravity. We find that the ratio of the
appropriate low frequency limit of the stress-stress two point function to the
entropy density is modulated. The conductivity, the stress-current and
current-stress correlation functions are also modulated. At temperatures close
to the phase transition we obtain analytic expressions for some of the
correlation functions.Comment: 38 pages, 2 figures; v2: added reference, minor correctio
Using subthreshold events to characterize the functional architecture of the electrically coupled inferior olive network
The electrical connectivity in the inferior olive (IO) nucleus plays an important role in generating well-timed spiking activity. Here we combined electrophysiological and computational approaches to assess the functional organization of the IO nucleus in mice. Spontaneous fast and slow subthreshold events were commonly encountered during in vitro recordings. We show that whereas the fast events represent intrinsic regenerative activity, the slow events reflect the electrical connectivity between neurons ('spikelets'). Recordings from cell pairs revealed the synchronized occurrence of distinct groups of spikelets; their rate and distribution enabled an accurate estimation of the number of connected cells and is suggestive of a clustered organization. This study thus provides a new perspective on the functional and structural organization of the olivary nucleus and a novel experimental and theoretical approach to study electrically coupled networks
Implications of area scaling of quantum fluctuations
Quantum fluctuations of a certain class of bulk operators defined in spatial
sub-volumes of Minkowski space-time, have an unexpected area scaling property.
We wish to present evidence that such area scaling may be ascribed to a
boundary theory. We first highlight the implications of area scaling with two
examples in which the boundary area of the spatial regions is not monotonous
with their volume. Next, we prove that the covariance of two operators that are
restricted to two different regions in Minkowski space scales linearly with
their mutual boundary area. Finally, we present an example which demonstrates
why this implies an underlying boundary theory.Comment: 12 pages, 5 figure
Holographic bulk viscosity: GPR vs EO
Recently Eling and Oz (EO) proposed a formula for the holographic bulk
viscosity, in arXiv:1103.1657, derived from the null horizon focusing equation.
This formula seems different from that obtained earlier by Gubser, Pufu and
Rocha (GPR) in arXiv:0806.0407 calculated from the IR limit of the two-point
function of the trace of the stress tensor. The two were shown to agree only
for some simple scaling cases. We point out that the two formulae agree in two
non-trivial holographic theories describing RG flows. The first is the strongly
coupled N=2* gauge theory plasma. The second is the semi-phenomenological model
of Improved Holographic QCD.Comment: 21 pages, 2 figure
Fluid dynamics of R-charged black holes
We construct electrically charged AdS_5 black hole solutions whose charge,
mass and boost-parameters vary slowly with the space-time coordinates. From the
perspective of the dual theory, these are equivalent to hydrodynamic
configurations with varying chemical potential, temperature and velocity
fields. We compute the boundary theory transport coefficients associated with a
derivative expansion of the energy momentum tensor and R-charge current up to
second order. In particular, we find a first order transport coefficient
associated with the axial component of the current.Comment: 31 pages, v2: published version; added some references, discussion of
the charge-current changed, results unchanged, v3: typo in formula (15)
changed, v4: added footnote 3 in order to clarify the relation of our results
to those of arXiv:0809.259
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
Parity-Violating Hydrodynamics in 2+1 Dimensions
We study relativistic hydrodynamics of normal fluids in two spatial
dimensions. When the microscopic theory breaks parity, extra transport
coefficients appear in the hydrodynamic regime, including the Hall viscosity,
and the anomalous Hall conductivity. In this work we classify all the transport
coefficients in first order hydrodynamics. We then use properties of response
functions and the positivity of entropy production to restrict the possible
coefficients in the constitutive relations. All the parity-breaking transport
coefficients are dissipationless, and some of them are related to the
thermodynamic response to an external magnetic field and to vorticity. In
addition, we give a holographic example of a strongly interacting relativistic
fluid where the parity-violating transport coefficients are computable.Comment: 39+1 page
The effect of higher derivative correction on and conductivities in STU model
In this paper we study the ratio of shear viscosity to entropy, electrical
and thermal conductivities for the R-charged black hole in STU model. We
generalize previous works to the case of a black hole with three different
charges. Actually we use diffusion constant to obtain ratio of shear viscosity
to entropy. By applying the thermodynamical stability we recover previous
results. Also we investigate the effect of higher derivative corrections.Comment: revised versio
The Gluonic Field of a Heavy Quark in Conformal Field Theories at Strong Coupling
We determine the gluonic field configuration sourced by a heavy quark
undergoing arbitrary motion in N=4 super-Yang-Mills at strong coupling and
large number of colors. More specifically, we compute the expectation value of
the operator tr[F^2+...] in the presence of such a quark, by means of the
AdS/CFT correspondence. Our results for this observable show that signals
propagate without temporal broadening, just as was found for the expectation
value of the energy density in recent work by Hatta et al. We attempt to shed
some additional light on the origin of this feature, and propose a different
interpretation for its physical significance. As an application of our general
results, we examine when the quark undergoes oscillatory motion,
uniform circular motion, and uniform acceleration. Via the AdS/CFT
correspondence, all of our results are pertinent to any conformal field theory
in 3+1 dimensions with a dual gravity formulation.Comment: 1+38 pages, 16 eps figures; v2: completed affiliation; v3: corrected
typo, version to appear in JHE
Lectures on hydrodynamic fluctuations in relativistic theories
These are pedagogical lecture notes on hydrodynamic fluctuations in normal
relativistic fluids. The lectures discuss correlation functions of conserved
densities in thermal equilibrium, interactions of the hydrodynamic modes, an
effective action for viscous fluids, and the breakdown of the derivative
expansion in hydrodynamics.Comment: 55 pages. Based on lectures given at the Seattle INT Summer School on
Applications of String Theory in July 201
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