Generalized Second Law of Thermodynamics in f(T,TG)f(T,T_{G}) gravity

An equilibrium picture of thermodynamics is discussed at the apparent horizon of FRW universe in $f(T,T_G)$ gravity, where $T$ represents the torsion invariant and $T_G$ 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 f(R,T,RμνTμν)f(R,T,R_{\mu\nu}T^{\mu\nu}) Gravity

We discuss the validity of the energy conditions in a newly modified theory named as $f(R,T,R_{\mu\nu}T^{\mu\nu})$ gravity, where $R$ and $T$ 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, $f(R)$ and $f(R,T)$ 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 $f(R,T)$ 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 $\omega_\vartheta$ and $\omega'_\vartheta$. We conclude that the consistent value of parameter $\mu$ is $\mu<0$ in accordance with the current Planck and WMAP$9$ 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 $f(R,T)$ gravity, where $R$ is the Ricci scalar and $T$ 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 $T_hdS'_h+T_hd_{\jmath}S'=-dE'+W'dV$, where $d_{\jmath}S'$ is the entropy production term. Finally, we conclude that the second law of thermodynamics holds both in phantom and non-phantom phases

Cosmology of Holographic and New Agegraphic f(R,T)f(R,T) Models

We consider the $f(R,T)$ theory, where $R$ is the scalar curvature and $T$ is the trace of energy-momentum tensor, as an effective description for the holographic and new agegraphic dark energy and reconstruct the corresponding $f(R,T)$ functions. In this study, we concentrate on two particular models of $f(R,T)$ gravity namely, $R+2A(T)$ and $B(R)+\lambda{T}$. We conclude that the derived $f(R,T)$ 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 f(R,T,RμνTμν)f(R,T,R_{\mu\nu}T^{\mu\nu}) 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 $f(R,T,Q)$ modified theory (where $R$ is the Ricci Scalar, $T$ is the trace of energy-momentum tensor (EMT) $T_{uv}$ and $Q=R_{uv}T^{uv}$ is interaction of EMT $T_{\mu\nu}$ and Ricci Tensor $R_{uv}$). We formulate modified field equations in the background of Friedmann-Lema$\hat{i}$tre-Robertson-Walker (FLRW) model which is defined as $ds^2=dt^2-a(t)^2(dx^2+dy^2+dz^2 )$, where $a(t)$ represents the scale factor. In this formalism energy density is found using covariant divergence of modified field equations. $\rho$ 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

Thermodynamic Behavior of particular f(R,T)f(R,T) Gravity Models

We investigate the thermodynamics at the apparent horizon of the FRW universe in $f(R,T)$ theory under non-equilibrium description. The laws of thermodynamics have been discussed for two particular models of $f(R,T)$ theory. The first law of thermodynamics is expressed in the form of Clausius relation $T_hd\hat{S}_h=\delta{Q}$, where $\delta{Q}=-d\hat{E}+Wd\mathbb{V}+T_hd_{\jmath}\hat{S}$ is the energy flux across the horizon and $d_{\jmath}\hat{S}$ 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

Reconstructing QCD ghost f(R,T)f(R,T) models

We reconstruct $f(R,T)$ theory (where $R$ is the scalar curvature and $T$ 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 $f(R,T)$ 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 $f(R,T)$ 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 $f(R)$ 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 $f(R)$ 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$U1820-30, Her X-1, SAX J 1808-3658$. 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$U1820-30$, $Her X-1$, $SAX J 1808-3658.$Comment: 20 Pages, 14 Figure

Axially Symmetric Shear-free Fluids in f(R,T)f(R,T) Gravity

In this work we have discussed the implications of shear-free condition on axially symmetric anisotropic gravitating objects in $f(R,T)$ 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 $\Gamma$ with material and dark source components defining stable and unstable regions in Newtonian (N) and post-Newtonian (pN) approximations.Comment: 22 page