2,240 research outputs found
Generalized Second Law of Thermodynamics in gravity
An equilibrium picture of thermodynamics is discussed at the apparent horizon
of FRW universe in gravity, where represents the torsion
invariant and is the teleparallel equivalent of the Gauss-Bonnet term. It
is found that one can translate the Friedmann equations to the standard form of
first law of thermodynamics. We discuss GSLT in the locality of assumption that
temperature of matter inside the horizon is similar to that of horizon.
Finally, we consider particular models in this theory and generate constraints
on the coupling parameter for the validity of GSLT in terms of recent cosmic
parameters and power law solutions.Comment: 19 pages, 5 figure
Energy Conditions in Gravity
We discuss the validity of the energy conditions in a newly modified theory
named as gravity, where and represent the
scalar curvature and trace of the energy-momentum tensor. The corresponding
energy conditions are derived which appear to be more general and can reduce to
the familiar forms of these conditions in general relativity, and
theories. The general inequalities are presented in terms of recent
values of Hubble, deceleration, jerk and snap parameters. In particular, we use
two specific models recently developed in literature to study concrete
application of these conditions as well as Dolgov-Kawasaki instability.
Finally, we explore gravity as a specific case to this modified theory
for exponential and power law models.Comment: 24 pages, no figure. typos corrected, three references adde
Cosmological Evolution of Pilgrim Dark Energy
We study pilgrim dark energy model by taking IR cut-offs as particle and
event horizons as well as conformal age of the universe. We derive evolution
equations for fractional energy density and equation of state parameters for
pilgrim dark energy. The phantom cosmic evolution is established in these
scenarios which is well supported by the cosmological parameters such as
deceleration parameter, statefinder parameters and phase space of
and . We conclude that the consistent
value of parameter is in accordance with the current Planck and
WMAP results.Comment: 20 pages, 11 figure
Thermodynamics in f(R,T) Theory of Gravity
A non-equilibrium picture of thermodynamics is discussed at the apparent
horizon of FRW universe in gravity, where is the Ricci scalar and
is the trace of the energy-momentum tensor. We take two forms of the
energy-momentum tensor of dark components and demonstrate that equilibrium
description of thermodynamics is not achievable in both cases. We check the
validity of the first and second law of thermodynamics in this scenario. It is
shown that the Friedmann equations can be expressed in the form of first law of
thermodynamics , where is
the entropy production term. Finally, we conclude that the second law of
thermodynamics holds both in phantom and non-phantom phases
Thermodynamic Behavior of particular Gravity Models
We investigate the thermodynamics at the apparent horizon of the FRW universe
in theory under non-equilibrium description. The laws of
thermodynamics have been discussed for two particular models of
theory. The first law of thermodynamics is expressed in the form of Clausius
relation , where
is the energy flux
across the horizon and is the entropy production term.
Furthermore, the conditions to preserve the generalized second law of
thermodynamics are established with the constraints of positive temperature and
attractive gravity. We have illustrated our results for some concrete models in
this theory.Comment: 17 pages, 4 figure
Cosmology of Holographic and New Agegraphic Models
We consider the theory, where is the scalar curvature and is
the trace of energy-momentum tensor, as an effective description for the
holographic and new agegraphic dark energy and reconstruct the corresponding
functions. In this study, we concentrate on two particular models of
gravity namely, and . We conclude that the
derived models can represent phantom or quintessence regimes of the
universe which are compatible with the current observational data. In addition,
the conditions to preserve the generalized second law of thermodynamics are
established.Comment: 27 pages, 12 figure
Cosmic evolution in the background of non-minimal coupling in Gravity
An accelerated expansion phase is being experienced by the universe due to
the presence of an unknown energy component known as dark energy (DE). To find
out the cosmic evolution scientists ever tried to modify Einstein's
gravitational theory and its unexplored parts. We also look forward to address
the same problem with a different approach based on interaction between matter
and geometry. For this purpose we consider modified theory (where
is the Ricci Scalar, is the trace of energy-momentum tensor (EMT)
and is interaction of EMT and Ricci
Tensor ). We formulate modified field equations in the background of
Friedmann-Lematre-Robertson-Walker (FLRW) model which is defined as
, where represents the scale factor.
In this formalism energy density is found using covariant divergence of
modified field equations. involves a contribution from non-minimal
matter geometry coupling which helps to study different cosmic eras based on
equation of state (EOS). Furthermore, we apply the energy bounds to constrain
the model parameters establishing a pathway to discuss the cosmic evolution for
best suitable parameters in accordance with recent observations.Comment: 11 pages, 6 figure
Reconstructing QCD ghost models
We reconstruct theory (where is the scalar curvature and is
the trace of energy-momentum tensor) in the framework of QCD ghost dark energy
models. In this study, we concentrate on particular models of gravity
which permits the standard continuity equation in this theory. It is found that
reconstructed function can represent phantom and quintessence regimes of the
universe in the background of flat FRW universe. In addition, we explore the
stability of ghost models.Comment: 23 pages, 12 figure
Study of Anisotropic Compact Stars in Starobinsky Model
The aim of this paper is to study the formation of anisotropic compact stars
in modified theory of gravity, which is the generalization of the
Einstein's gravity. To this end, we have used the solution of Krori and Barua
to the anisotropic distribution of matter in gravity. Further, we have
matched the interior solution with the exterior solution to determine the
constants of Krori and Barua solution. Finally the constant have been
determined by using the data of compact compact stars like 4. Using the evaluated form of the solutions, we have discussed
the regularity of matter components at the center as well as on the boundary,
energy conditions, anisotropy, stability analysis and mass-radius relation of
the compact stars 4, , Comment: 20 Pages, 14 Figure
Axially Symmetric Shear-free Fluids in Gravity
In this work we have discussed the implications of shear-free condition on
axially symmetric anisotropic gravitating objects in theory.
Restricted axial symmetry ignoring rotation and reflection enteries is taken
into account for establishment of instability range. Implementation of linear
perturbation on constitutive modified dynamical equations yield evolution
equation. This equation associates adiabatic index with material and
dark source components defining stable and unstable regions in Newtonian (N)
and post-Newtonian (pN) approximations.Comment: 22 page
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