Effects of Anisotropy on Scalar Field Ghost Dark Energy and the Non-Equilibrium Thermodynamics in Fractal Cosmology

Since the fractal cosmology has been created in early universe, therefore their models were mostly isotropic.The majority of previous studies had been based on FRW universe, while in the early universe, the best model for describing fractal cosmology is actually the anisotropic universe. Therefore in this work, by assuming the anisotropic universe, the cosmological implications of ghost and generalized ghost dark energy models with dark matter in fractal cosmology has been discussed. Moreover, the different kinds of dark energy models such as quintessence and tachyon field, with the generalized ghost dark energy in fractal universe has been investigated. In addition, we have reconstructed the Hubble parameter, H, the energy density, ρ, the deceleration parameter, q, the equations of state parameter, ωD,for both ghost and generalized ghost dark energy models. This correspondence allows us to reconstruct the potential and the dynamics of a fractal canonical scalar field according to the evolution of generalized ghost dark energy density.Eventually, thermodynamics of the cosmological apparent horizon in fractal cosmology was investigated and the validity of the Generalized second law of thermodynamics(GSLT) have been examined in an anisotropic universe. The results show the influence of the anisotropy on the GSLT of thermodynamics in a fractal cosmology.     

Viscosity Effects on Anisotropic Universe in Curvature-Matter Coupling Gravity

In this paper, we study evolution of the universe in the background of f(R, T) gravity using LRS Bianchi type-Ⅰ model. We discuss scale factors as well as deceleration parameter in dark energy dominated era for different bulk viscosity models. The occurrence of big-rip singularity is also examined. It is concluded that expansion is faster when bulk viscosity is proportional to Hubble parameter as compared to other models.     

New Agegraphic Dark Energy in Brans-Dicke Theory

In this paper, we investigate the new agegraphic dark energy model in the framework of Brans-Dicke theory, which is a natural extension of the Einstein＇s general relativity. In this framework the form of the new agegraphic dark energy density takes as pq = 3n^2Ф（t）η^-2, where η is the conformal age of the universe and Ф（t） is the Brans-Dicke scalar field representing the inverse of the time-variable Newton＇s constant. We derive the equation of state of the new agegraphic dark energy and the deceleration parameter of the universe in the Brans-Dicke theory. It is very interesting to find that in the Brans-Dicke theory the agegraphic dark energy realizes quintom-like behavior, i.e., its equation of state crosses the phantom divide ω= -1 during the evolution. We also compare the situation of the agegraphic dark energy model in the Brans-Dicke theory with that in the Einstein＇s theory. In addition, we discuss the new agegraphic dark energy model with interaction in the framework of the Brans-Dicke theory.     

Evolution of Holographic Dark Energy in Interacting Modified Chaplygin Gas Model

We investigate the modified Chaplygin gas （MCG） with interaction between holographic dark energy proposed by Li and dark matter. In this model, evolution of the universe is described in detail, which is from deceleration to acceleration. Specifically, the evolutions of related cosmological quantities such as density parameter, the equation of state of holographic dark energy, deceleration parameter and transition redshift are discussed. Moreover, we also give their present values which are consistent with the lately observations. Furthermore, the results given by us show such a model can accommodate a transition of the dark energy from a normal state wx 〉 -1 to ωx 〈 -1 phantom regimes.     

Quintessence Reconstruction of Interacting HDE in a Non-Flat Universe

In this paper we consider quintessence reconstruction of interacting holographic dark energy in a non-fiat background. As system＇s IR cutoff we choose the radius of the event horizon measured on the sphere of the horizon, defined as L = at（t）. To this end we construct a quintessence model by a real, single scalar field. Evolution of the potential, V（φ）, as well as the dynamics of the scalar field, φ, is obtained according to the respective holographic dark energy. The reconstructed potentials show a cosmological constant behavior for the present time. We constrain the model parameters in a fiat universe by using the observational data, and applying the Monte Carlo Markov chain simulation. We obtain the best fit values of the holographic dark energy model and the interacting parameters as c=1.0576-0.6632-0.6632^＋0.3010＋0.3052 and ζ =0.2433-0.2251-.2251^＋0.6373＋0.6373 , respectively. From the data fitting results we also find that the model can cross the phantom line in the present universe where the best fit value of the dark energy equation of state is WD=-1.2429.     

New Agegraphic Pilgrim Dark Energy in f(T,T_G) Gravity

In this work, we briefly discuss a novel class of modified gravity like f(T, TG) gravity. In this background,we assume the new agegraphic version of pilgrim dark energy and reconstruct f(T, TG) models for two specific values of s. We also discuss the equation of state parameter, squared speed of sound and wDE-w DEplane for these reconstructed f(T, TG) models. The equation of state parameter provides phantom-like behavior of the universe. The wDE-w DEplane also corresponds to ΛCDM limit, thawing and freezing regions for both models.     

Modified Chaplygin gas as an interacting holographic dark energy model

The modified Chaplygin gas (MCG) as an interacting model of holographic dark energy in which dark energy and dark matter are coupled together is investigated in this paper. Concretely, by studying the evolutions of related cosmological quantities such as density parameter Ω, equation of state w, deceleration parameter q and transition redshift zT, we find the evolution of the universe is from deceleration to acceleration, their present values are consistent with the latest observations, and the equation of state of holographic dark energy can cross the phantom divide w = -1. Furthermore, we put emphasis upon the geometrical diagnostics for our model, i.e., the statefinder and Om diagnostics. By illustrating the evolutionary trajectories in r - s, r - q, w -w and Om planes, we find that the holographic constant c and the coupling constant b play very important roles in the holographic dark energy (HDE) model. In addition, we also plot the LCDM horizontal lines in Om diagrams, and show the discrimination between the HDE and LCDM models.     

A Dark Energy Model with Higher Derivative and Cosmological Evolution

We construct dark energy models which have high derivative terms. In the framework of a single scalar field model with a higher derivative, the equation of state （EOS） ω might have some special character in the evolvement, which states that the universe will evolve in some interesting ways. We study the different kinds of dark energy models, and classify them into several genera by their character in the evolvement.     

Analysis of Generalized Ghost Dark Energy in LQC and Galileon Gravity

A so-called ghost dark energy was recently proposed to explain the present acceleration of the universe.The energy density of ghost dark energy,which originates from Veneziano ghost of Quantum Chromodynamics(QCD),in a time dependent background,can be written in the form,ρD=αH + βH~2 where H is the Hubble parameter.We investigate the generalized ghost dark energy(GGDE) model in the setup of loop quantum Cosmology(LQC) and Galileon Cosmology.We study the cosmological implications of the models.We also obtain the equation of state and the deceleration parameters and differential equations governing the evolution of this dark energy model for LQC and Galileon Cosmology.     

A New Exponential Gravity

We propose a new exponential f（R） gravity model with f（R） = （R - λc） e^λ（c/R）n and n 〉 3, λ ≥ 1, c 〉 0 to explain late-time acceleration of the universe. At the high curvature region, the model behaves like the A CDM model. In the asymptotic future, it reaches a stable de-Sitter spaeetime. It is a cosmologically viable model and can evade the local gravity constraints easily. This model shares many features with other f（R） dark energy models like Hu-Sawicki model and ExponentiM gravity model. In it the dark energy equation of state is of an oscillating form and can cross phantom divide line ωde = -1. In particular, in the parameter range 3 〈 n ≤ 4, λ ～ 1, the model is most distinguishable from other models. For instance, when n = 4, λ = 1, the dark energy equation of state will cross -1 in the earlier future and has a stronger oscillating form than the other models, the dark energy density in asymptotical future is smaller than the one in the high curvature region. This new model can evade the local gravity tests easily when n 〉 3 and λ 〉 1.     

New Agegraphic Pilgrim Dark Energy in f(T,TG) Gravity

In this work, we briefly discuss a novel class of modified gravity like f(T, T_G) gravity. In this background, we assume the new agegraphic version of pilgrim dark energy and reconstruct f(T, T_G) models for two specific values of s. We also discuss the equation of state parameter, squared speed of sound and w_DE-w'_DE plane for these reconstructed f(T, T_G) models. The equation of state parameter provides phantom-like behavior of the universe. The w_DE-w'_DE plane also corresponds to CDM limit, thawing and freezing regions for both models.     

Statefinder Parameters for Different Dark Energy Models with Variable G Correction in Kaluza-Klein Cosmology

In this work, we have calculated the deceleration parameter, statefinder parameters and EoS parameters for different dark energy models with variable G correction in homogeneous, isotropic and non-flat universe for Kaluza-Klein Cosmology. The statefinder parameters have been obtained in terms of some observable parameters like dimensionless density parameter, EoS parameter and Hubble parameter for holographic dark energy, new agegraphic dark energy and generalized Chaplygin gas models.     

Cosmological analysis of reconstructed $${\mathcal {F}}(T,T_{\mathcal {G}})$$ models

In this paper, we analyze cosmological consequences of the reconstructed generalized ghost pilgrim dark energy \({\mathcal {F}}(T,T_{\mathcal {G}})\) models in terms of redshift parameter z. For this purpose, we consider power-law scale factor, scale factor for two unified phases and intermediate scale factor. We discuss graphical behavior of the reconstructed models and examine their stability analysis. Also, we explore the behavior of equation of state as well as deceleration parameters and \(\omega _{\Lambda }-\omega _{\Lambda }^{'}\) as well as \(r-s\) planes. It is found that all models are stable for pilgrim dark energy parameter 2. The equation of state parameter satisfies the necessary condition for pilgrim dark energy phenomenon for all scale factors. All other cosmological parameters show great consistency with the current behavior of the universe.     

Holographic Dark Energy (DE) Cosmological Models with Quintessence in Bianchi Type-V Space Time

In this paper, author studied homogeneous and anisotropic Bianchi type-V universe filled with matter and holographic dark energy (DE) components. The exact solutions to the corresponding Einstein’s field equations are obtained for exponential and power-law volumetric expansion. The holographic dark energy (DE) EoS parameter behaves like constant, i.e. ω _Λ =?1, which is mathematically equivalent to cosmological constant (Λ) for exponential expansion of the model, whereas the holographic dark energy (DE) EoS parameter behaves like quintessence for power-law expansion of the model. A correspondence between the holographic dark energy (DE) models with the quintessence dark energy (DE) is also established. Quintessence potential and dynamics of the quintessence scalar field are reconstructed, which describe accelerated expansion of the universe. The statefinder diagnostic pair {r,s} is adopted to characterize different phases of the universe.     

Holographic Dark Energy Model with Quintessence in Bianchi Type-I Space-Time

In this paper, we have studied a homogeneous and anisotropic universe filled with matter and holographic dark energy components. Assuming deceleration parameter to be a constant, an exact solution to Einstein’s field equations in axially symmetric Bianchi type-I line element is obtained. A correspondence between the holographic dark energy models with the quintessence dark energy models is also established. Quintessence potential and the dynamics of the quintessence scalar field are reconstructed, which describe accelerated expansion of the universe.     

Phase Space Analysis and Anisotropic Universe Model in f(T) Gravity

In this paper, we study the stability of locally rotationally symmetric(LRS) Bianchi I universe model in f(T) gravity through phase space analysis. We assume that the f(T) gravity can be treated as effective dark energy behaving like perfect fluid, and suggest that there are interactions between pressureless matter as well as dark energy.We construct the corresponding autonomous system of equations to check the stability of the model for non phantom,vacuum and phantom phases. It is concluded that critical points remain more stable in phantom phase as compared to non phantom and vacuum cases. Finally, we discuss the cosmological behavior of the model through some cosmological parameters.     

Exact Kantowski-Sachs Anisotropic Dark Energy Cosmological Models in Brans Dicke Theory of Gravitation

The main purpose of this paper is to explore the solution of Exact Kantowski-Sach cosmological models by using the Brans Dicke Theory of gravitation in the background of anisotropic dark energy. In order to obtain different physically variable models of the universe we have assumed the special law of variation of Hubbles parameter proposed by Berman (Nuovo Cimento B 74:182, 1983) which yields constant deceleration parameter and power law relation between average scale factor R and scalar field f, which has already been used by Johri and Desikan in RW Brans Dicke models. Some physical and geometrical consequences of the models have been carried out by using some physical quantities.     

Study of Thermodynamic Quantities in Generalized Gravity Theories

In this work, we have studied the thermodynamic quantities like temperature of the universe, heat capacity and squared speed of sound in generalized gravity theories like Brans-Dicke, Hořava-Lifshitz and f(R) gravities. We have considered the universe filled with dark matter and dark energy. Also we have considered the equation of state parameters for open, closed and flat models. We have observed that in all cases the equation of state behaves like quintessence. The temperature and heat capacity of the universe are found to decrease with the expansion of the universe in all cases. In Brans-Dicke and f(R) gravity theories the squared speed of sound is found to exhibit increasing behavior for open, closed and flat models and in Hořava-Lifshitz gravity theory it is found to exhibit decreasing behavior for open and closed models with the evolution of the universe. However, for flat universe, the squared speed of sound remains constant in Hořava-Lifshitz gravity.     

Bianchi -V Space -Time with Anisotropic Dark Energy in General Relativity

In a spatially homogeneous and anisotropic Bianchi type-V space-time the consequences of the presence of dynamically anisotropic dark energy and perfect fluid with heat-conduction are studied. We assume that dark energy is minimally interacting with matter and has an equation of state which is modified in a consistent way with the conservation of energy momentum tensor. Exact solutions of Einstein field equations are obtained by taking constant value of deceleration parameter. We find that this assumption is reasonable for the observation of the present day universe. The physical and geometrical properties of the models, the behavior of the anisotropy of dark energy and the thermodynamical relations that govern such solutions are discussed in detail.