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This article explores the analysis of modified holographic Ricci dark energy (MHRDE) in the context of f (T) theory of gravity, with a focus on the validity of thermodynamics for homogenous and isotropic Friedmann-Robertson-Walker (FRW) universe by using hybrid expansion law (HEL) represented as a (t) = e mt t n , where m , n are constants. The study investigates the dynamics of the accelerating universe across the established f (T) model, f (T) = ξ 1 T + ξ 2 T ξ 3 log T , where ξ 1 , ξ 2 , ξ 3 are constants. The fundamental equations of thermodynamics features of the model have been deliberated. Additionally, the entropy density and temperature of the model are determined. We use the energy conditions in our solutions to identify a range of values for free parameters of the model, ensuring a stable and well-behaved scenario. Furthermore, a detailed examination of the physical and geometrical characteristics of the model has been conducted. [ABSTRACT FROM AUTHOR]

The object of the present paper is to investigate conharmonically flat spacetime in the framework of FRW model. In this regard we have considered perfect fluid as a source of matter distribution of the universe. We solved Einstein's field equations with variable cosmological term using special variational law - = -H2, where - is a constant and H is the Hubble's parameter. Moreover, we have described a cosmological scenario of the universe with these solutions and shown that this model is in accordance with the recent day observations. [ABSTRACT FROM AUTHOR]

The early Universe has been a subject of research for many cosmologists and it is reviewed and analyzed rigorously in cosmology recently. Thermal history and theory of phase transition coordinate us to elucidate the evolution of the Universe at its early stages and is interesting to get a gist of early universe behavior. At phase transition, effective potential plays a significant role for the kinds of transition that occurred during the evolution. The temperature, at which phase transition occurs, can be determined at minimum effective potential. It is also known fact that temperature of the Universe has changed considerably with its evolution. The present work investigates the time-temperature relation in four-dimensional and fivedimensional cosmological models. A comparison of time-temperature relation in FRW model with that of Kaluza-Klein (K-K) model demonstrates that temperature decreases faster in Kaluza-Klein model. The investigation demonstrates the important role played by extra dimension in the study of time-temperature relation at the early Universe scenario. [ABSTRACT FROM AUTHOR]

The object of the present paper is to investigate W2 -flat spacetime in the framework of FRW model. In this regard we have considered perfect fluid as a source of matter distribution of the universe. We have solved Einstein's field equations with variable cosmological term using special variational lawΛ = 3βH², where β is a constant and H is the Hubble's parameter. Moreover, we have described a cosmological scenario of the universe with these solutions and shown that this model is in accordance with the recent day observations. [ABSTRACT FROM AUTHOR]

In this work, Friedmann–Robertson–Walker model is investigated in f (R , T) = R + α T + β T 2 gravity for a perfect fluid by a numerical analysis. In order to solve the modified field equations, we use a new solution method based on to propose the functional forms of the energy density and the pressure. In this context, in keeping with the recent indications of the observed universe, we consider two different propositions. We show for both propositions that the behaviors of the Hubble and the deceleration parameters are consistent with astrophysical observations and the model depicts de Sitter universe. It is also shown that the model incorporates the initial radiation-dominated era, and it is dominated by a quintessence like fluid at late era for both the propositions. [ABSTRACT FROM AUTHOR]

A flat Friedmann–Robertson–Walker (FRW) cosmological model with Wet Dark Fluid has been studied based on fractal gravity. The exact solution of the field equations is obtained using linear time-varying deceleration parameter (Akars̈u and Dereli, 2012) and assuming fractal parameter ξ ≠ 0. The model parameters involved in the model have been constrained using the Hubble datasets H (z) of 77 data points, known as cosmic chronometers (CC), recently published Pantheon, and the joint data (CC+Pantheon samples). Additionally, we compare our model with Λ CDM in standard cosmology via error bar trajectories. We study the physical and the cosmographic parameters, such as the Hubble parameter H (z) , deceleration parameter q (z) , and the higher derivatives of the deceleration parameter q (z) , etc. under cosmic observations. It is observed that our model transits from a decelerating state to an accelerating state at z t r ≈ 2. 764. The other astrophysical parameters such as the jerk parameter and snap parameter are also discussed. Finally, we conclude that our model is an accelerating quintessence dark energy model. [ABSTRACT FROM AUTHOR]

In this study, Friedmann–Robertson–Walker space-time filled with a perfect fluid in f (R , T) modified theory, where R is the Ricci scalar and T is the trace of the energy–momentum tensor of matter, has been considered. The investigation of the phase transition of the universe from the decelerating expansion phase to the accelerating one has been made by adopting a special form of the varying deceleration parameter that is inversely proportional to the Hubble parameter. The exact solution of the field equations has been derived. The kinematic and dynamical quantities of the model have been obtained and their evolutions have been discussed by means of their graphs. The statefinder diagnostic has been used and the age of the universe has been computed for testing the validity of the model. It has been shown that the dominant energy of the model is ordinary matter which behaves as the SCDM model at the beginning and it is a quintessence like fluid which behaves as the Λ CDM model at late times. [ABSTRACT FROM AUTHOR]

In this paper, we present the bulk viscous phenomena to mimic new holographic dark energy behavior in modified f(R, T) gravity. We assume the bulk viscosity coefficient, ζ in the form of ζ = ζ 0 + ζ 1 H , where ζ 0 , ζ 1 are positive constants and H is the Hubble parameter. We obtain the exact solution of the scale factor and classify all the possible scenarios (deceleration, acceleration and their transition) of the evolution of the Universe. It is observed that the model transits from the decelerated phase to accelerated phase in early or late time depending on the values of viscous terms. For large values of viscous terms, the model always accelerates throughout the evolution. Furthermore, we also analyze two diagnostic parameters, statefinder r , s and Om to discriminate our model with the other existing dark energy models. The evolutions of these diagnostics are shown in r - s , r - q and Om − z planes. It is found that the behavior of trajectories in different planes depends on the bulk viscous coefficients. A small combination of ζ 0 and ζ 1 gives the quintessence-like behavior, whereas a large combination gives Chaplygin gas-like model. However, the model approaches the ΛCDM model in the late time evolution of the Universe. The value of the effective equation of state parameter comes to be − 0.9745 which is very close to the observational data. The entropy and generalized second law of thermodynamics are valid under certain constraints. The analysis shows that the dark energy phenomena may be explained in the presence of bulk viscosity in the modified theory of gravity. [ABSTRACT FROM AUTHOR]

Nojiri and Odintsov (Phys Rev D 68:123512, 2003) and Hu and Sawicki (Phys Rev D 76:064004, 2007) have studied nonlinear functions in modified gravity that explain the cosmic acceleration without cosmological constant, fulfill the conditions of local gravity and stability and pass the solar system tests. In this paper, FRW model, a best fitted and fruitful mathematical model of the physical universe (Astrophys J 82:248, 1935; Proc Natl Acad Sci 15:168, 1929; Phys Rev D 73:80, 1948; Astrophys J 142:419, 1965), is studied in the context of these nonlinear functions. The cosmological implications such as Hubble parameter, deceleration parameter, jerk parameter, matter density and the effective equation of state parameter of the universe are plotted with respect to redshift. Subsequently, the age of the universe is predicted in f(R) gravity. All are found to represent the features of present phase of the universe. [ABSTRACT FROM AUTHOR]

A flat Friedmann-Robertson-Walker (FRW) cosmological model with perfect fluid has been investigated within the framework of the particle creation mechanism. Perfect fluid isentropic particle creation rate as a function of Hubble parameter has been incorporated. By considering a simple parametrization of Hubble parameter, we discuss the cosmological scenarios through time evolution of the cosmological parameters. We examine the different regimes of the universe by studying the phase transition from decelerated to accelerated era. We also explore the classical stability of model and the state-finder diagnostic for highlighting the departure of the considered cosmological model from the Λ Cold Dark Matter (Λ CDM) model. [ABSTRACT FROM AUTHOR]

We explore a matter-dominated model with bulk viscosity in Brans-Dicke theory to interpret the observed cosmic accelerating expansion phenomena. We obtain the exact solution of the field equations by taking constant bulk viscous coefficient (which potentially could explain the present accelerated expansion of the Universe) and Brans-Dicke scalar field proportional to some power of the scale factor. The model is studied statistically using the available astronomical data and then compare using the tools taken from information criterion. Using the best-fit values of model parameters we find that the model shows transition from decelerated phase to accelerated phase. The effective equation of state parameter is within the quintessence region (− 1 ≤ ω < 0 ). Thus, the model shows quintessence behavior and approaches to Λ CDM model in late time. It does not cross the phantom divide line and hence it is free from big-rip singularity. The viscous model also alleviates the age problem. We discuss two diagnostic parameters, namely statefinder and O m (z) and compare the model with the existing models by plotting the trajectories. We also apply Akaike information criterion (AIC) and Bayesian information criterion (BIC) to discriminate the viscous model based on the penalization associated to the number of parameters. The analyses based on the AIC indicates that there is less support for the viscous model when compared to the Λ CDM model, while those based on the BIC indicates that there is strong evidence against it in favor of the Λ CDM model. [ABSTRACT FROM AUTHOR]

Extensions of the gravitational framework of Brans & Dicke (BD) are studied by considering two different scenarios: (i) 'BD- Λ CDM', in which a rigid cosmological constant, Λ , is included, thus constituting a BD version of the vanilla concordance Λ CDM model (the current standard model of cosmology with flat three-dimensional geometry), and (ii) 'BD-RVM', a generalization of (i) in which the vacuum energy density (VED), ρ vac , is a running quantity evolving with the square of the Hubble rate: δ ρ vac (H) ∝ ν m Pl 2 (H 2 − H 0 2) (with | ν | ≪ 1 and H 0 being the present value of the Hubble rate). This dynamical scenario is motivated by recent studies of quantum field theory (QFT) in curved spacetime, which lead to the running vacuum model (RVM). In both cases, rigid or running ρ vac , the GR limit can be recovered smoothly. We solve the background as well as the perturbation equations for each cosmological model and test their performance against the modern wealth of cosmological data, namely a compilation of the latest SNIa+ H (z) +BAO+LSS+CMB observations. We utilize the AIC and DIC statistical information criteria in order to determine if they can fit better the observations than the concordance model. The two BD extensions are tested by considering three different datasets. According to the AIC and DIC criteria, both BD extensions (i) and (ii) are competitive, but the second one (the BD-RVM scenario) is particularly favored when it is compared with the vanilla model. This fact may indicate that the current observations favor a mild dynamical evolution of the Newtonian coupling G N as well as of the VED. While further studies will be necessary, the results presented here suggest that the Brans & Dicke theory with running vacuum could have the potential to alleviate the two tensions at the same time. [ABSTRACT FROM AUTHOR]

The motivation of this paper is to study the bulk viscosity effect in Ricci dark energy (RDE) model within the framework of modified f(R, T) gravity, where R is the Ricci scalar and T is the trace of the energy–momentum tensor. As most studies assume that the universe is filled with a perfect fluid, viscosity is expected to present at least during some stages, especially in the early stage of the evolution of the universe but it could still become significant in the future. We assume the universe is filled with viscous RDE and pressureless dark matter. We consider the total bulk viscous coefficient is in the form of ζ = ζ 0 + ζ 1 H, where ζ 0 and ζ 1 are the constants. We obtain the solutions to the modified field equations by assuming a form f(R, T) = R + λ T, where λ is a constant. We find the scale factor and deceleration parameter, and classify all possible evolutions of the universe. We briefly discuss the future finite-time singularity and show that the Big Rip singularity appears in viscous RDE model. We investigate two geometrical diagnostics, statefinder parameter and Om to analyze the dynamics of evolution of the universe. The trajectories of statefinder parameter reveal that the model behaves like quintessence for small ζ , and for large ζ it shows the Chaplygin gas-like. However, in late time both the models approach Λ CDM. The model shows a transition from decelerated phase to accelerated phase. Similarly, the Om analysis reveals that the model behaves like quintessence for small ζ and phantom-like for large ζ. We extend our study to analyze the time evolution of the total entropy and generalized second law of thermodynamics of viscous RDE model in f(R, T) theory inside the apparent horizon. Our study shows that the generalized second law of thermodynamics always preserves in viscous RDE model in a region enclosed by the apparent horizon under the suitable constraints of viscous coefficients. [ABSTRACT FROM AUTHOR]

In this paper, we study the evolution of the dark energy parameter in the spatially homogeneous and isotropic Friedmann-Robertson-Walker (FRW) model filled with barotropic fluid and dark energy in the scalar-tensor theory of gravitation proposed by Brans and Dicke (Phys. Rev. 24:925, ). A determinate solution is presented using (i) the special law of variation for Hubble's parameter proposed by Berman (Nuovo Cimento B 74:183, ) and (ii) trace free energy momentum tensor of the two fluid. Two cases of interacting and non-interacting fluid (barotropic and dark energy) scenario is considered and general results are obtained. The physical aspects of the results obtained are, also, discussed. [ABSTRACT FROM AUTHOR]

Considering a spherically symmetric nonstatic cosmological flat model of the Robertson-Walker universe, the problem of perfect fluid distribution interacting with the gravitational field in the presence of a zero-rest-mass scalar field and electromagnetic field in Brans-Dicke (B-D) theory has been investigated. Exact solutions are obtained by using a general approach of solving the partial differential equations and it has been observed that the electromagnetic field cannot survive the cosmological flat model due to the action caused by the presence of the zero-rest-mass scalar field, and the isotropic pressure p turns out to be negative. The pressure and density are found to be independent of time. [ABSTRACT FROM AUTHOR]

F-essence is a generalization of the usual Dirac model with the nonstandard kinetic term. In this paper, we introduce a new model of spinor cosmology containing both Ricci scalar and the non minimally coupled spinor fields in its action. We have investigated the cosmology with both isotropy and anisotropy, where the equations of motion of FRW and Bianchi type-I spacetimes have been derived and solved numerically. Finally the quantization of these models through Wheeler-De Witt (WD) wave function has been discussed. [ABSTRACT FROM AUTHOR]

The thermodynamical effect of particle creation on the early evolution of Friedmann-Robertson-Walker universe is analyzed in the context of open thermodynamical systems within the framework of Brans-Dicke theory. The field equations are solved exactly using the 'gamma-law' equation of state p=( γ−1) ρ, where the adiabatic parameter γ depends on the scale factor of the universe. We assume the functional form of γ in such a way that it describes a unified description of two early phases of the evolution of the universe, viz. inflationary phase and radiation-dominated phase. The role of Brans-Dicke scalar and creation of matter particles is investigated during the expansion of the universe. It is found that the expansion of the universe is driven by the particle creation in each phase. The physical and geometrical behaviors of the solutions are discussed in detail. [ABSTRACT FROM AUTHOR]

One of the greatest challenges in today's cosmology to determine the nature of dark energy, the sourse of the observed present acceleration of the universe. Besides the vacuum energy, various dark energy models have been suggested. The Friedmann-Robertson-Walker (FRW) spacetime plays an important role in modern cosmology. In particular, the most popular models of dark energy work in the FRW spacetime. In this work, a new class of integrable FRW cosmological models is presented. These models induced by the well-known Painlevé equations. Some nonintegrable FRW models are also considered. These last models are constructed with the help of Pinney, Schrödinger and hypergeometric equations. Scalar field description and two-dimensional generalizations of some cosmological models are presented. Finally some integrable and nonintegrable F(R) and F(G) gravity models are constructed. [ABSTRACT FROM AUTHOR]

In this paper we propose a higher dimensional Cosmology based on FRW model and brane-world scenario. We consider the warp factor in the brane-world scenario as a scale factor in 5-dimensional generalized FRW metric, which is called as bulk scale factor, and obtain the evolution of it with space-like and time-like extra dimensions. It is then showed that, additional space-like dimensions can produce exponentially bulk scale factor under repulsive strong gravitational force in the empty universe at a very early stage. [ABSTRACT FROM AUTHOR]

Using the superfield approach we construct the n = 2 supersymmetric Lagrangian for the FRW Universe with barotropic perfect fluid as matter field. The obtained supersymmetric algebra allowed us to take the square root of the Wheeler–DeWitt equation and solve the corresponding quantum constraint. This model leads to the relation between the vacuum energy density and the energy density of the dust matter. [ABSTRACT FROM AUTHOR]

The Hamilton-Jacobi analysis is applied to the dynamics of the scalar fluctuations about the Friedmann-Robertson-Walker (FRW) metric. The gauge conditions are determined from the consistency conditions. The physical degrees of freedom of the model are obtained by the symplectic projector method. The role of the linearly dependent Hamiltonians and the gauge variables in the Hamilton-Jacobi formalism is discussed. [ABSTRACT FROM AUTHOR]

The adiabatic matter creation for a spatially flat, homogeneous and isotropic universe is discussed in holographic dark energy (HDE) in the context of Brans–Dicke theory. Since the HDE density is considered as a dynamical cosmological constant, it is natural to study it in a dynamical frame of Brans–Dicke theory. We show that HDE model with matter creation, whose infrared cutoff is set by the Hubble horizon, leads to solution that unify the early and late time acceleration. The latest observational data from supernovae (Pantheon sample), Hubble H (z) and latest local H 0 by SH0ES are used for constraining the model parameters. Using the Markov chain Monte Carlo method, we obtain the best fit values of the parameters for different models. It is found that the HDE model without matter creation does not achieve transition phase when the Hubble horizon is taken as an IR cut-off. However, it achieves the phase transition from decelerated phase to accelerated phase with same IR cut-off with the inclusion of gravitationally induced matter creation. The best fit values of HDE model with matter creation are fitted very well in Hubble error data. Using AIC and BIC, and geometrical diagnostic parameters, we compare the models with Λ CDM model and discuss the viability of the model. A detailed thermodynamic analysis is also carried out. [ABSTRACT FROM AUTHOR]