Using the Hayward-Kodama temperature for the apparent horizon, it is found that matter content in the Universe is not thermodynamically stable, and the entry to the late accelerated expansion is actually a second order phase transition. The cosmological model used for the purpose is one that imitates the $Λ$CDM model, the favoured model for the present Universe.
The paper deals with non-equilibrium thermodynamics based on adiabatic particle creation mechanism with the motivation of considering it as an alternative choice to explain the recent observed accelerating phase of the universe. Using Friedmann equations, it is shown that the deceleration parameter ($q$) can be obtained from the knowledge of the particle production rate ($Γ$). Motivated from thermodynamical point of view, cosmological solutions are evaluated for the particle creation rates in three cosmic phases, namely, inflation, matter dominated and present late time acceleration. The decel…
We propose, and constrain with the latest observational data, three model-independent parametrizations of the cosmic deceleration parameter $q(z)$. They are well behaved and stay finite at all redshifts. We construct them by fixing the value of $q$ at high redshift, $q(z \gg 1) = 1/2$ (as demanded by cosmic structure formation), and at the far future, $q(z = -1) = -1$, and smoothly interpolating $q(z)$ between them. The fixed point at $z = -1$ is not arbitrarily chosen; it readily follows from the second law of thermodynamics. This fairly reduces the ample latitude in parameterizing $q(z)$.
The phenomenon of accelerated expansion of the present universe and a cosmic transit aspect is explored in the framework of a modified gravity theory known as $f(R,T)$ gravity (where $R$ is the Ricci scalar and $T$ is the trace of the energy momentum tensor of the matter content). The cosmic transit phenomenon signifies a signature flipping behaviour of the deceleration parameter. We employ a periodic varying deceleration parameter and obtained the exact solution of field equations. The dynamical features of the model including the oscillatory behaviour of the EOS parameter are studied. We hav…
In this paper, a stable flat cosmological model has been constructed and the evolution of dark energy has been investigated in the framework of the recently suggested $κ(R,T)$ gravity. The empirical approach we adopt in the current work reveals some interesting cosmological features consistent with observations and the standard $Λ$CDM model. The evolution of cosmic pressure shows a positive-to-negative transition corresponding to the cosmic deceleration-acceleration transition, both the deceleration parameter and the cosmic pressure have the positive-to-negative sign flipping. While this behav…
In this article, we investigate the modified symmetric teleparallel gravity or $f(Q)$ gravity, where $Q$ is the non-metricity, to study the evolutionary history of the universe by considering the functional form of $f(Q)=αQ^n$, where $α$ and $n$ are constants. Here, we consider the parametrization form of the deceleration parameter as $q=q_0+\frac{q_1\,z}{(1+z)^2}$ which provides the desired property for sign flip from a decelerating to an accelerating phase. We get the solution of the Hubble parameter by examining the mentioned parametric form of $q$, and then we impose the solution in Friedm…
Author of Ref. [1], M.R. Setare (JCAP 01, 023, 2007, arXiv:hep-th/0701242), by redefining the event horizon measured from the sphere of the horizon as the system's IR cut-off for an interacting holographic dark energy model in a non-flat universe, showed that the generalized second law of thermodynamics is satisfied for the special range of the deceleration parameter. His paper includes an erroneous calculation of the entropy of the cold dark matter. Also there are some missing terms and some misprints in the equations of his paper. Here we present that his conclusion is not true and the gener…
In the present work we discuss a third alternative to explain the latest observational data concerning the accelerating Universe and its different stages. The particle creation mechanism in the framework of non-equilibrium thermodynamics is considered as a basic cosmic mechanism acting on the flat FRW geometry. By assuming that the gravitationally induced particle production occurs under "adiabatic" conditions, the deceleration parameter is expressed in terms of the particle creation rate which is chosen as a truncated power series of the Hubble parameter. The model shows the evolution of the …
The present study is based on a generalized form of Brans-Dicke (BD) theory where, the dimensionless BD parameter is regarded as a function of the scalar field, which is reciprocal of the gravitational constant. The field equations have been solved by incorporating an empirical function f(t) in the expression representing the conservation of matter. This function f(t) has been chosen to account for a conversion of matter (both dark and baryonic) into some other form, possibly dark energy, which is known to be responsible for the accelerated expansion of universe. The requirement of a signature…
The main goal of this work is investigation of NADE in the cyclic universe scenario. Since, cyclic universe is explained by a phantom phase ($ω<-1$), it is shown when there is no interaction between matter and dark energy, ADE and NADE do not produce a phantom phase, then can not describe cyclic universe. Therefore, we study interacting models of ADE and NADE in the modified Friedmann equation. We find out that, in the high energy regime, which it is a necessary part of cyclic universe evolution, only NADE can describe this phantom phase era for cyclic universe. Considering deceleration parame…
In this paper we investigate the quintessence model with a minimally coupled scalar field in the context of recent supernovae observations. By choosing a particular form of the deceleration parameter q, which gives an early deceleration and late time acceleration for dust dominated model, we show that this sign flip in q can be obtained by a simple trigonometric patential. The early matter dominated model expands with q=1/2 as desired and enters a negative q phase quite late during the evolution.
In this paper, the generalized second law (GSL) of thermodynamics and entropy is revisited in the context of cosmological models in Gauss-Bonnet gravity with the boundary of the universe is assumed to be enclosed by the dynamical apparent horizon. The model is best fitted with the observational data for distance modulus. The best fitted geometric and thermodynamic parameters such as equation of state parameter, deceleration parameter and entropy are derived. To link between thermodynamic and geometric parameters, the "entropy rate of change multiplied by the temperature" as a model independent…
Authors of ref. [1], M.R. Setare and S. Shafei (JCAP 09 (2006) 011), studied the thermodynamics of a holographic dark energy model in a non-flat universe enclosed by the apparent horizon $R_A$ and the event horizon measured from the sphere of the horizon named $L$. In section 3 in ref. [1], Authors showed that for $R_A$ the generalized second law of thermodynamics is respected, while for $L$ it is satisfied for the special range of the deceleration parameter. Here we present that their calculations for $R_A$ should be revised. Also we show that their conclusion for $L$ is not true and the gene…
Modified gravity and generalized second law (GSL) of thermodynamics are interesting topics in the modern cosmology. In this regard, we investigate the GSL of gravitational thermodynamics in the framework of modified Gauss-Bonnet gravity or f(G)-gravity. We consider a spatially FRW universe filled with the matter and radiation enclosed by the dynamical apparent horizon with the Hawking temperature. For two viable f(G) models, we first numerically solve the set of differential equations governing the dynamics of f(G)-gravity. Then, we obtain the evolutions of the Hubble parameter, the Gauss-Bonn…
We examine the validity of the generalized second law (GSL) of gravitational thermodynamics in the context of interacting $f(R)$ gravity. We take into account that the boundary of the universe to be confined by the dynamical apparent horizon in a flat FRW universe. We study the effective equation of state, deceleration parameter and GSL in this interaction-framework. We find that the evolution of the total entropy increases through the interaction term. As a example, we consider a $f(R)$ gravity with a power-law dependence on the curvature $R$. Here, we find exact solutions for a model in whic…
A time-dependent phenomenological model of $Λ$, viz. $\dot Λ\sim H^3$ is selected to investigate the $Λ$-CDM cosmology. Time-dependent form of the equation of state parameter $ω$ is derived and it has been possible to obtain the sought for flip of sign of the deceleration parameter q. Present age of the Universe, calculated for some specific values of the parameters agrees very well with the observational data.
The present study is motivated by the study of reference [1], where the generalized second law of thermodynamics has been investigated for a flat FRW universe for two viable models of $f(T)$ gravity. In the present work, we have considered a non-flat universe and accordingly studied the behaviors of equation of state parameter and deceleration parameter. Subsequently, using the first law of thermodynamics we derived the expressions for the time derivative of the total entropy of a universe enveloped by apparent horizon. In the next phase, with the choice of scale factor pertaining to an emerge…
In this paper, we study the nonlinear electrodynamics in the framework of $f(T)$ gravity for FRW universe along with dust matter, magnetic and torsion contributions. We evaluate the equation of state and deceleration parameters to explore the accelerated expansion of the universe. The validity of generalized second law of thermodynamics for Hubble and event horizons is also investigated in this scenario. For this purpose, we assume pole-like and power-law forms of scale factor and construct $f(T)$ models. The graphical behavior of the cosmological parameters versus smaller values of redshift $…
Within the context of scalar-tensor gravity, we explore the generalized second law (GSL) of gravitational thermodynamics. We extend the action of ordinary scalar-tensor gravity theory to the case in which there is a non-minimal coupling between the scalar field and the matter field (as chameleon field). Then, we derive the field equations governing the gravity and the scalar field. For a FRW universe filled only with ordinary matter, we obtain the modified Friedmann equations as well as the evolution equation of the scalar field. Furthermore, we assume the boundary of the universe to be enclos…
Some dark energy cosmological models are constructed in the framework of a generalised Brans-Dicke theory which contains a self interacting potential and a dynamical coupling parameter. The dark sector of the universe is considered through a unified linear equation of state. The parameters of the unified dark fluid have been constrained from some physical basis. Since the universe is believed to have undergone a transition from an early deceleration to a late time acceleration, the deceleration parameter should have a signature flipping behaviour at the transition redshift. We have used a hybr…
We investigate the late time acceleration with a Chaplygin type of gas in spherically symmetric inhomogeneous model. At the early phase we get Einstien-deSitter type of solution generalised to inhomogeneous spacetime. But at late stage of the evolution our solutions admit the accelerating nature of the universe. For a large scale factor our model behaves like a ?CDM model. We calculate the deceleration parameter for this anisotropic model, which, unlike its homogeneous counterpart, shows that the flip is not syn- chronous occurring early at the outer shells. This is in line with other physical…
Motivated by recent results on non-vanishing spatial curvature \cite{curve} we employ the holographic model of dark energy to investigate the validity of first and second laws of thermodynamics in non-flat (closed) universe enclosed by apparent horizon $R_A$ and the event horizon measured from the sphere of horizon named $L$. We show that for the apparent horizon the first law is roughly respected for different epochs while the second laws of thermodynamics is respected while for $L$ as the system's IR cut-off first law is broken down and second law is respected for special range of decelerati…
In this paper, bulk viscosity is introduced in new holographic dark energy (HDE) to describe the effects of cosmic non-perfect fluid on the evolution of the universe in modified $f(R,T)$ gravity. Assuming $f(R,T)=R+λT$, where $R$ is the Ricci scalar, $T$, the trace of energy-momentum tensor and $λ$ is a constant, we derive a general function of Hubble parameter with bulk viscous form to provide a procedure for the viscous new HDE model building. Especially, we assume the total bulk viscosity coefficient proportional to the velocity of the expansion of the universe in such a way that $ζ=ζ_0+ζ_1…
We investigate the validity of the generalized second law (GSL) of gravitational thermodynamics in the framework of f(T) modified teleparallel gravity. We consider a spatially flat FRW universe containing only the pressureless matter. The boundary of the universe is assumed to be enclosed by the Hubble horizon. For two viable f(T) models containing $f(T)=T+μ_1{(-T)}^n$ and $f(T)=T-μ_2 T(1-e^{β\frac{T_0}{T}})$, we first calculate the effective equation of state and deceleration parameters. Then, we investigate the null and strong energy conditions and conclude that a sudden future singularity a…
Entropic-force cosmology provides, in contrast with dark energy descriptions, a concrete physical understanding of the accelerated expansion of the universe. The acceleration appears to be a consequence of the entropy associated with the information storage in the universe. Since these cosmological models are unable of explaining the different periods of acceleration and deceleration unless a correction term is considered, we study the effects of including a subdominant power-law term within a thermodynamically admissible entropic-force model. The temperature of the universe horizon is obtaine…
Here, we investigate the growth of matter density perturbations as well as the generalized second law (GSL) of thermodynamics in the framework of $f(R)$-gravity. We consider a spatially flat FRW universe filled with the pressureless matter and radiation which is enclosed by the dynamical apparent horizon with the Hawking temperature. For some viable $f(R)$ models containing the Starobinsky, Hu-Sawicki, Exponential, Tsujikawa and AB models, we first explore numerically the evolution of some cosmological parameters like the Hubble parameter, the Ricci scalar, the deceleration parameter, the dens…
In this paper, using Barrow entropy, we propose an interacting model of Barrow holographic dark energy (BHDE). In particular, we study the evolution of a spatially flat FLRW universe composed of pressureless dark matter and BHDE that interact with each other through a well-motivated interaction term. Considering the Hubble horizon as the IR cut-off, we then study the evolutionary history of important cosmological parameters, particularly, the density parameter, the equation of state parameter, and the deceleration parameter in the BHDE model and find satisfactory behaviors in the model. We per…
In this work we consider the entropy-corrected version of interacting holographic dark energy (HDE), in the non-flat universe enclosed by apparent horizon. Two corrections of entropy so-called logarithmic 'LEC' and power-law 'PLEC' in HDE model with apparent horizon as an IR-cutoff are studied. The ratio of dark matter to dark energy densities $u$, equation of state parameter $w_D$ and deceleration parameter $q$ are obtained. We show that the cosmic coincidence is satisfied for both interacting models. By studying the effect of interaction in EoS parameter, we see that the phantom divide may b…
Exact solutions are studied in the context of modified Brans-Dicke theory. The non-linearity of the modified Brans-Dicke field equations is treated with the Euler-Duarte-Moreira method of integrability of anharmonic oscillator equation. While some solutions show a forever accelerating nature, in some cases there is a signature flip in the evolution of deceleration parameter in recent past. Importance of these latter models are studied in the context of late-time acceleration of the universe. Constraints on the model parameters are obtained from Markov Chain Monte Carlo (MCMC) analysis using th…
In this work we study some aspects of the Rastall gravity, being the thermodynamics consistency of the model the core of this paper, for this purpose we will consider the dynamical equations of Rastall model in a flat FLRW geometry. Under a holographic description can be seen that this scenario for gravity contributes to the energy density of the fluid with an extra term that can be related to the deceleration parameter, providing a way to estimate the value of the Rastall parameter, termed as $ξλ$, at present time. By adopting a specific Ansatz for the $ξλ$ term it is possible to determine th…
In this paper we have considered the holographic dark energy and studied it's cosmology and thermodynamics. We have analysed the generalized second law (GSL) of thermodynamics in a flat universe consists of interacting dark energy and dark matter. We did the analysis both under thermal equilibrium and non-equilibrium conditions. If the apparent horizon is taken as the boundary of the universe, we have shown that the rate of change of the total entropy of the universe is proportional to $(1+q)^2,$ which in fact shows that the GSL is valid at the apparent horizon irrespective of the sign of $q,$…
A theoretical model of cosmic expansion has been formulated on an assumption of inter-conversion of matter and dark energy, in the framework of Brans-Dicke theory. An empirical scale factor has been used, which generates a signature flip of the deceleration parameter with time. To account for the non-conservation of matter, a function of time f(t) is incorporated into the equation representing the density of matter. Its value at any instant of time is proportional to the matter content of the universe. The functional form of f(t) has been determined from the field equations by using an empiric…
We study the evolution of the universe in the presence of inflaton, matter, radiation, and holographic dark energy. The time evolution of the scale factor is obtained by solving the Friedmann equation of the universe with a good approximation. We present two independent ways which determine the value of the dark energy constant $d\sim 1$ from the observational data. The two ways are measuring the deceleration parameter and measuring a universal constant depending only on $d$. The universal constant is given by a dimensionless combination of three scale factors at the equipartition times of rad…
Higher-Dimensional Cosmology in the Framework of Generalized Chaplygin Gas: An Approximation Method
Netaji Nagar Day College（奈塔吉·纳加尔日学院）；Relativity and Cosmology Research Centre, Jadavpur University（贾达普大学相对论与宇宙学研究中心）；New Alipore College (Retd.)（新阿里波尔学院（退休））；Department of Physics, University of North Bengal（北孟加拉大学物理系）
Our Universe has an arrow of time. In accordance with the second law of thermodynamics, entropy has been increasing ever since the Big Bang. The fact that matter is in thermal equilibrium in the very early Universe, as indicated by the cosmic microwave background, has led to the idea that gravitational entropy must be very low in the beginning. Penrose proposed that gravitational entropy can be quantified by the Weyl curvature, which increases as structures formed. A concrete realization of such a measure is the Clifton-Ellis-Tavakol gravitational entropy, which has been shown to be increasing…
We study the validity of the generalized second law (GSL) of gravitational thermodynamics in a non-flat FRW universe containing the interacting in $f(T)$ gravity. We consider that the boundary of the universe to be confined by the dynamical apparent horizon in FRW universe. In general, we discuss the effective equation of state, deceleration parameter and GLS in this framewok. Also, we find that the interacting-term $Q$ modifies these quantities and in particular, the evolution of the total entropy, results in an increases on the GLS of thermodynamic, by a factor $4π\,R_A^3\,Q/3$. By using a v…
We study thermodynamics of cosmological models in the Horava-Lifshitz theory of gravity, and systematically investigate the evolution of the universe filled with a perfect fluid that has the equation of state $p=wρ$, where $p$ and $ρ$ denote, respectively, the pressure and energy density of the fluid, and $w$ is an arbitrary real constant. Depending on specific values of the free parameters involved in the models, we classify all of them into various cases. In each case the main properties of the evolution are studied in detail, including the periods of deceleration and/or acceleration, and th…
In this work we investigate the evolution of a Universe consisted of a scalar field, a dark matter field and non-interacting baryonic matter and radiation. The scalar field, which plays the role of dark energy, is non-minimally coupled to space-time curvature, and drives the Universe to a present accelerated expansion. The non-relativistic dark matter field interacts directly with the dark energy and has a pressure which follows from a thermodynamic theory. We show that this model can reproduce the expected behavior of the density parameters, deceleration parameter and luminosity distance.
In the framework of Brans-Dicke (BD) theory, the first part of the present study determines the time dependence of BD parameter, energy density and equation of state (EoS) parameter of the cosmic fluid in a universe expanding with acceleration, preceded by a phase of deceleration. For this purpose, a scale factor has been chosen such that the deceleration parameter, obtained from it, shows a signature flip with time. Considering the dark energy to be responsible for the entire pressure, the time evolution of energy parameters for matter and dark energy and the EoS parameter for dark energy hav…
The paper deals with a theoretical model for interacting Tsallis holographic dark energy (THDE) whose infrared (IR) cut-off scale is set by the Hubble length. The interaction $Q$ between the dark sectors (dark energy and pressureless dark matter) of the universe has been assumed to be non-gravitational in nature. The functional form of $Q$ is chosen in such a way that it reproduces well known and most used interactions as special cases. We then study the nature of the THDE density parameter, the equation of state parameter, the deceleration parameter and the jerk parameter for this interacting…
We derive non-flat cosmological models for two cases (i.e., dust and radiation) in the context of Møller's tetradic theory (MTT) of gravitation using the tetrad that creates the non-flat Friedmann-Robertson-Walker (FRW) metric. These two models are affected by the free dimensional parameter, $λ$, that characterized MTT, which approaches zero in the flat case for both models. Using standard definitions of thermodynamics, we calculate the radius horizon, Hawking temperature, and entropy of our non-flat models in the framework of cosmology and show the effect of $λ$ on open and closed universes. …
A Nonsingular Logarithmic Bouncing Cosmology in $f(R,T)$ Gravity with Thermodynamic Viability
Maharaja Agrasen University（玛哈拉贾阿格拉森大学）；S.P.M. Science and Gilani Arts, Commerce College（SPM科学与吉拉尼艺术商业学院）；Pacif Institute of Cosmology and Selfology (PICS)（太平洋宇宙学与自我学研究所（PICS））；L N Gumilyov Eurasian National University（L.N.古米廖夫欧亚国立大学）；Sharda University（沙尔达大学）
Since the late 1990's observations of type Ia Supernova, our universe is predicted to experience a late time cosmic acceleration. Theoretical support to this observation were intended to be built via proposition of a hypothetical fluid which staying inside the universe exerts negative pressure. Cosmologists have prescribed many candidates for this exotic fluid so far. In this alley, a popular method is to choose time dependent equation of state parameter $ω= \frac{p}ρ$ and to parametrize it as a function of redshift. Again some common families of such parametrizations are constructed among whi…
A phenomenological generalized ghost dark energy model has been studied under the framework of FRW universe. In ghost dark energy model the energy density depends linearly on Hubble parameter (H) but in this dark energy model, the energy density contains a the sub-leading term which is depends on $\mathcal{O} (H^2)$, so the energy density takes the form $ρ_D=αH+ βH^2$, where $α$ and $β$ are the constants. The solutions of the Friedman equation of our model leads to a stable universe. We have fitted our model with the present observational data including Stern data set. With the help of best fi…
It was shown by Tsallis and Cirto that thermodynamical entropy of a gravitational system such as black hole must be generalized to the non-additive entropy, which is given by $S_h=γA^β$, where $A$ is the horizon area and $β$ is the nonextensive parameter \cite{Tsa}. In this paper, by taking the entropy associated with the apparent horizon of the Friedmann-Robertson-Walker (FRW) Universe in the form of Tsallis entropy, and assuming the first law of thermodynamics, $dE=T_hdS_h+WdV$, holds on the apparent horizon, we are able to derive the corresponding Friedmann equations describing the dynamics…
We have considered a spatially flat, homogeneous and isotropic FLRW Universe filled with a single fluid, known as logotropic dark fluid (LDF), whose pressure evolves through a logarithmic equation of state. We use the recent Pantheon SNIa and cosmic chronometer datasets to constrain the parameters of this model, the present fraction of dark matter $Ω_{m0}$ and the Hubble constant $H_0$. We find that the mean values of these parameters are $Ω_{m0}=0.288\pm 0.012$ and $H_{0}=69.652\pm 1.698~{\rm km/s/Mpc}$ at the $1σ$ CL. We also find that the LDF model shows a smooth transition from the deceler…
In this work it is investigated the evolution of a Universe where a scalar field, non-minimally coupled to space-time curvature, plays the role of quintessence and drives the Universe to a present accelerated expansion. A non-relativistic dark matter constituent that interacts directly with dark energy is also considered, where the dark matter particle mass is assumed to be proportional to the value of the scalar field. Two models for dark matter pressure are considered: the usual one, pressureless, and another that comes from a thermodynamic theory and relates the pressure with the coupling b…
Thermodynamic behavior of cosmological models with fractional entropy
Facultad de Física, Universidad Veracruzana（韦拉克鲁斯大学物理学院）；Instituto de Física, Pontificia Universidad Católica de Valparaíso（瓦尔帕莱索天主教大学物理研究所）；Laboratorio de investigación de Cómputo de Física, Facultad de Ciencias Naturales y Exactas, Universidad de Playa Ancha（普拉亚安查大学自然与精确科学学院计算物理研究实验室）
By assuming that a dark component (dark energy) in the universe strictly obeys the holographic principle, that is, its entropy is one fourth of the apparent horizon, we find that the existence of the other dark component (dark matter) is compulsory, as a compensation of dark energy, based on the first law of thermodynamics. By using the method of dynamical system analysis, we find that there exists a stable dark energy-dark matter scaling solution at late time, which is helpful to solve the coincidence problem. For reasonable parameters, the deceleration parameter is well consistent with curre…
