306 research outputs found
Revisiting Agegraphic Dark Energy in Brans-Dicke Cosmology
We explore a spatially homogeneous and isotropic Friedmann-Robertson-Walker
(FRW) universe which is filled with agegraphic dark energy (ADE) with mutual
interaction with pressureless dark matter in the background of Brans-Dicke (BD)
theory. We consider both original and new type of agegraphic dark energy (NADE)
and further assume the sign of the interaction term can change during the
history of the Universe. We obtain the equation of state parameter, the
deceleration parameter and the evolutionary equation for the sign-changeable
interacting ADE and NADE in BD theory. We find that, in both models, the
equation of state parameter, , cannot cross the phantom line, although
they can predict the Universe evolution from the early deceleration phase to
the late time acceleration, compatible with observations. We also investigate
the sound stability of these models and find out that both models cannot show a
signal of stability for different model parameters.Comment: 10 pages, 14 figure
Stability of HDE model with sign-changeable interaction in Brans-Dicke theory
We consider the Brans-Dicke (BD) theory of gravity and explore the
cosmological implications of the sign-changeable interacting holographic dark
energy (HDE) model in the background of Friedmann-Robertson-Walker (FRW)
universe. As the system's infrared (IR) cutoff, we choose the future event
horizon, the Granda-Oliveros (GO) and the Ricci cutoffs. For each cutoff, we
obtain the density parameter, the equation of state (EoS) and the deceleration
parameter of the system. In case of future event horizon, we find out that the
EoS parameter, , can cross the phantom line, as a result the transition
from deceleration to acceleration expansion of the universe can be achieved
provided the model parameters are chosen suitably. Then, we investigate the
instability of the sign-changeable interacting HDE model against perturbations
in BD theory. For this purpose, we study the squared sound speed whose
sign determines the stability of the model. When the model is
unstable against perturbation. For future event horizon cutoff, our universe
can be stable () depending on the model parameters. Then, we focus
on GO and Ricci cutoffs and find out that although other features of these two
cutoffs seem to be consistent with observations, they cannot leads to stable
dominated universe, except in special case with GO cutoff. Our studies confirm
that for the sign-changeable HDE model in the setup of BD cosmology, the event
horizon is the most suitable horizon which can passes all conditions and leads
to a stable DE dominated universe.Comment: 19pages, 22figure
Thermodynamics of charged rotating dilaton black branes with power-law Maxwell field
In this paper, we construct a new class of charged rotating dilaton black
brane solutions, with complete set of rotation parameters, which is coupled to
a nonlinear Maxwell field. The Lagrangian of the matter field has the form of
the power-law Maxwell field. We study the causal structure of the spacetime and
its physical properties in ample details. We also compute thermodynamic and
conserved quantities of the spacetime such as the temperature, entropy, mass,
charge, and angular momentum. We find a Smarr-formula for the mass and verify
the validity of the first law of thermodynamics on the black brane horizon.
Finally, we investigate the thermal stability of solutions in both canonical
and grand-canonical ensembles and disclose the effects of dilaton field and
nonlinearity of Maxwell field on the thermal stability of the solutions. We
find that for , charged rotating black brane solutions are
thermally stable independent of the values of the other parameters. For
, the solutions can encounter an unstable phase depending on the
metric parameters.Comment: 15 pages, 14 figures. We have revised the text to remove the overlap
Holographic Conductivity for Logarithmic Charged Dilaton-Lifshitz Solutions
We disclose the effects of the logarithmic nonlinear electrodynamics on the
holographic conductivity of Lifshitz dilaton black holes/branes. We analyze
thermodynamics of these solutions as a necessary requirement for applying
gauge/gravity duality, by calculating conserved and thermodynamic quantities
such as the temperature, entropy, electric potential and mass of the black
holes/branes. We calculate the holographic conductivity for a
-dimensional brane boundary and study its behavior in terms of the
frequency per temperature. Interestingly enough, we find out that, in contrast
to the Lifshitz-Maxwell-dilaton black branes which has conductivity for all
, here in the presence of nonlinear gauge field, the holographic
conductivity do exist provided and vanishes for . It is shown
that independent of the nonlinear parameter , the real part of the
conductivity is the same for a specific value of frequency per temperature in
both AdS and Lifshitz cases. Besides, the behavior of real part of conductivity
for large frequencies has a positive slope with respect to large frequencies
for a system with Lifshitz symmetry whereas it tends to a constant for a system
with AdS symmetry. This behavior may be interpreted as existence of an
additional charge carrier rather than the AdS case, and is due to the presence
of the scalar dilaton field in model. Similar behavior for optical conductivity
of single-layer graphene induced by mild oxygen plasma exposure has been
reported.Comment: V1: 12 pages, 5 figures (each one includes 2 subfigres) V2: 13 pages,
Some references added, Conductivity calculations improved, Accepted for
publication in PL
Holographic conductivity in the massive gravity with power-law Maxwell field
We obtain a new class of topological black hole solutions in
-dimensional massive gravity in the presence of the power-Maxwell
electrodynamics. We calculate the conserved and thermodynamic quantities of the
system and show that the first law of thermodynamics is satisfied on the
horizon. Then, we investigate the holographic conductivity for the four and
five dimensional black brane solutions. For completeness, we study the
holographic conductivity for both massless () and massive ()
gravities with power-Maxwell field. The massless gravity enjoys translational
symmetry whereas the massive gravity violates it. For massless gravity, we
observe that the real part of conductivity, , decreases as
charge increases when frequency tends to zero, while the
imaginary part of conductivity, , diverges as . For the massive gravity, we find that is
zero at and becomes larger as \ increases (temperature
decreases), which is in contrast to the massless gravity. Interestingly, we
observe that in contrast to the massless case, has a
maximum value at (known as the Drude peak) for (conformally invariant electrodynamics) where is the power parameter of
the power-law Maxwell field and this maximum increases with increasing .
Finally, we show that for high frequencies, the real part of the holographic
conductivity have the power law behavior in terms of frequency,
where . Some similar behaviors for high frequencies in
possible dual CFT systems have been reported in experimental observations.Comment: V2: 15 pages, 5 figures (each one includes \geq 3 subfigures), Some
Refs added, Some discussions regarding i) the power-law Maxwell
electrodynamics and ii) the relation between our results and experimental
observations presented, A suggestion for future extensions give
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