87,220 research outputs found
Viscosity bound for anisotropic superfluids in higher derivative gravity
In the present paper, based on the principles of gauge/gravity duality we
analytically compute the shear viscosity to entropy ratio corresponding to the
superfluid phase in Einstein Gauss-Bonnet gravity. From our analysis we note
that the ratio indeed receives a finite temperature correction below certain
critical temperature. This proves the non universality of shear viscosity to
entropy ratio in higher derivative theories of gravity. We also compute the
upper bound for the Gauss-Bonnet coupling corresponding to the symmetry broken
phase and note that the upper bound on the coupling does not seem to change as
long as we are close to the critical point of the phase diagram. However the
corresponding lower bound of the shear viscosity to entropy ratio seems to get
modified due to the finite temperature effects.Comment: 27 pages; v2: Details added, typos fixed, references updated; version
to appear in JHE
Entropy and universality of Cardy-Verlinde formula in dark energy universe
We study the entropy of a FRW universe filled with dark energy (cosmological
constant, quintessence or phantom). For general or time-dependent equation of
state the entropy is expressed in terms of energy, Casimir energy,
and . The correspondent expression reminds one about 2d CFT entropy only for
conformal matter. At the same time, the cosmological Cardy-Verlinde formula
relating three typical FRW universe entropies remains to be universal for any
type of matter. The same conclusions hold in modified gravity which represents
gravitational alternative for dark energy and which contains terms growing at
low curvature. It is interesting that BHs in modified gravity are more entropic
than in Einstein gravity. Finally, some hydrodynamical examples testing new
shear viscosity bound, which is expected to be the consequence of the
holographic entropy bound, are presented for the early universe in the plasma
era and for the Kasner metric. It seems that the Kasner metric provides a
counterexample to the new shear viscosity bound.Comment: LaTeX file, 39 pages, references are adde
Crossing of the w=-1 barrier in viscous modified gravity
We consider a modified form of gravity in which the action contains a power
alpha of the scalar curvature. It is shown how the presence of a bulk viscosity
in a spatially flat universe may drive the cosmic fluid into the phantom region
(w<-1) and thus into a Big Rip singularity, even if it lies in the quintessence
region (w>-1) in the non-viscous case. The condition for this to occur is that
the bulk viscosity contains the power (2 alpha-1) of the scalar expansion. Two
specific examples are discussed in detail. The present paper is a
generalization of the recent investigation dealing with barrier crossing in
Einstein's gravity: I. Brevik and O. Gorbunova, Gen. Relativ. Grav. 37 (2005)
2039.Comment: 12 pages, latex, no figure
Viscous Fluids and Gauss-Bonnet Modified Gravity
We study effects of cosmic fluids on finite-time future singularities in
modified -gravity, where and are the Ricci scalar and the
Gauss-Bonnet invariant, respectively. We consider the fluid equation of state
in the general form, , and we suppose the existence of a
bulk viscosity. We investigate quintessence region () and phantom
region () and the possibility to change or avoid the singularities
in -gravity. Finally, we study the inclusion of quantum effects in
large curvatures regime.Comment: 14 page
Holographic Aspects of a Higher Curvature Massive Gravity
We study the holographic dual of a massive gravity with Gauss-Bonnet and
cubic quasi-topological higher curvature terms. Firstly, we find the
energy-momentum two-point function of the 4-dimensional boundary theory where
the massive term breaks the conformal symmetry as expected. An -theorem is
introduced based on the null energy condition. Then we focus on a black brane
solution in this background and derive the ratio of shear viscosity to entropy
density for the dual theory. It is worth mentioning that the concept of
viscosity as a transport coefficient is obscure in a nontranslational invariant
theory as in our case. So although we use the Green-Kubo's formula to derive
it, we rather call it the rate of entropy production per the Planckian time due
to a strain. Results smoothly cover the massless limit.Comment: v2: 20 pages, typo corrected, references added; v3: section 2.2
revised; v4: section 2.2 modified, viscosity formula revised, 2 figs added,
references added; v5: 23 pages, a sign mistake in eq. (74) fixed (results
modified), eq. (34) modified, one fig added, refs added, to appear in EPJ
Higher Curvature Gravity and the Holographic fluid dual to flat spacetime
Recent works have demonstrated that one can construct a (d+2) dimensional
solution of the vacuum Einstein equations that is dual to a (d+1) dimensional
fluid satisfying the incompressible Navier-Stokes equations. In one important
example, the fluid lives on a fixed timelike surface in the flat Rindler
spacetime associated with an accelerated observer. In this paper, we show that
the shear viscosity to entropy density ratio of the fluid takes the universal
value 1/4\pi in a wide class of higher curvature generalizations to Einstein
gravity. Unlike the fluid dual to asymptotically anti-de Sitter spacetimes,
here the choice of gravitational dynamics only affects the second order
transport coefficients. We explicitly calculate these in five-dimensional
Einstein-Gauss-Bonnet gravity and discuss the implications of our results.Comment: 13 pages; v2: modified abstract, added references; v3: added
clarifying comments, modified discussio
The shear diffusion coefficient for generalized theories of gravity
Near the horizon of a black brane in Anti-de Sitter (AdS) space and near the
AdS boundary, the long-wavelength fluctuations of the metric exhibit
hydrodynamic behaviour. The gauge-gravity duality then relates the boundary
hydrodynamics for generalized gravity to that of gauge theories with large
finite values of 't Hooft coupling. We discuss, for this framework, the
hydrodynamics of the shear mode in generalized theories of gravity in d+1
dimensions. It is shown that the shear diffusion coefficients of the
near-horizon and boundary hydrodynamics are equal and can be expressed in a
form that is purely local to the horizon. We find that the Einstein-theory
relation between the shear diffusion coefficient and the shear viscosity to
entropy ratio is modified for generalized gravity theories: Both can be
explicitly written as the ratio of a pair of polarization-specific
gravitational couplings but implicate differently polarized gravitons. Our
analysis is restricted to the shear-mode fluctuations for simplicity and
clarity; however, our methods can be applied to the hydrodynamics of all
gravitational and matter fluctuation modes.Comment: 12 page
Crossing of the w=-1 Barrier in Two-Fluid Viscous Modified Gravity
Singularities in the dark energy late universe are discussed, under the
assumption that the Lagrangian contains the Einstein term R plus a modified
gravity term of the form R^\alpha, where \alpha is a constant. It is found,
similarly as in the case of pure Einstein gravity [I. Brevik and O. Gorbunova,
Gen. Rel. Grav. 37 (2005), 2039], that the fluid can pass from the quintessence
region (w>-1) into the phantom region (w<-1) as a consequence of a bulk
viscosity varying with time. It becomes necessary now, however, to allow for a
two-fluid model, since the viscosities for the two components vary differently
with time. No scalar fields are needed for the description of the passage
through the phantom barrier.Comment: 16 pages latex, no figure
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
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