Flat FLRW Universe Research Articles

Effects of particle creation rate in two-fluid interacting cosmologies

ABSTRACT In this work, a two-fluid interacting model in a flat FLRW universe has been studied considering particle creation mechanism with a particular form of particle creation rate $\Gamma =\Gamma _0 H+\frac{\Gamma _1}{H}$ from different aspects. Statistical analysis with a combined data set of SNe Ia (Supernovae Type Ia) and Hubble data is performed to achieve the best-fitting values of the model parameters, and the model is compatible with current observational data. We also perform a dynamical analysis of this model to get an overall qualitative description of the cosmological evolution by converting the governing equations into a system of ordinary differential equations considering a proper transformation of variables. We find some non-isolated sets of critical points, among which some usually are normally hyperbolic sets of points that describe the present acceleration of the universe dominated by dark energy mimicking cosmological constant or phantom fluid. Scaling solutions are also obtained from this analysis, and they can alleviate the coincidence problem successfully. Statefinder diagnosis is also carried out for this model to compare it with the ΛCDM, and any other dark energy models byfinding various statefinder parameters. Finally, the thermodynamic analysis shows that the generalized second law of thermodynamics is valid in an irreversible thermodynamic context.

Phase‐space analysis of the anisotropies in Weyl integrable spacetime: Kantowski–Sachs cosmology

We consider anisotropic Kantowski–Sachs cosmology in Weyl integrable spacetime with an ideal gas. We perform a detailed analysis of the dynamics by investigating the stationary points and reconstruct the cosmological history. This analysis gives important information for the initial value problem in Weyl integrable spacetime. We study three cases, in which the scalar field mass depends only on the density of the ideal gas, with or without cosmological constant term, and when there exists an exponential scalar field potential. We conclude that the existence of the nonzero potential function is essential in order that a spatially flat FLRW universe which can describe inflation can be a future attractor.

Statefinder diagnosis for Barrow agegraphic dark energy

Here, by employing a Barrow entropy and the standard holographic method at a cosmic framework, we formulate Barrow agegraphic dark energy (BADE), taking the Universe age as an IR cutoff scale in a flat FLRW Universe. For evaluation of statefinder parameters in [Formula: see text] and [Formula: see text] planes, trajectories have been plotted for BADE and discovered that for various values of [Formula: see text], the model exhibits both the behavior of Chaplygin gas and quintessence. Moreover, as a supplement to the statefinder study, we looked at the BADE model without interaction in the plane [Formula: see text], which might offer us a dynamic study using the energy density BADE parameter [Formula: see text] and [Formula: see text], as per VI-[Formula: see text]CDM observational data without interaction from Planck 2018 results.

An exact solution of the higher-order gravity in standard radiation-dominated era

We report that the standard evolution of radiation-dominated era (RDE) universe a∝t1/2 is a sufficient condition for solving a sixth order gravitational field equation derived from the Lagrangian containing BRabRab+CRR;cc as well as a polynomial f(R) for a spatially flat radiation FLRW universe. By virtue of the similarity between RabRab and R2 models up to the background order and of the vanishing property of R;cc for H=1/(2t), the analytical solution can be obtained from a special case to general one. This proves that the standard cosmic evolution is valid even within modified gravitational theory involving higher-order terms. An application of this background solution to the tensor-type perturbation reduces the complicated equation to the standard second order equation of gravitational wave. We discuss the possible ways to discriminate the modified gravity model on the observations such as the gravitational wave from the disturbed universe and primordial abundances.

Scalar Field Models of Barrow Holographic Dark Energy in f(R,T) Gravity

This research study investigates Barrow holographic dark energy with an energy density of ρΛ=CH2−Δ by considering the Hubble horizon as the IR cut-off in the f(R,T) gravity framework. We employ Barrow holographic dark energy to obtain the equation of the state for the Barrow holographic energy density in a flat FLRW Universe. Concretely, we study the correspondence between quintessence, k-essence, and dilation scalar field models with the Barrow holographic dark energy in a flat f(R,T) Universe. Furthermore, we reconstruct the dynamics and potential for all these models for different values of the Barrow parameter: Δ. Via this study, we can show that for Barrow holographic quintessence, k-essence, and dilation scalar field models, if the corresponding model parameters satisfy some limitations, the accelerated expansion can be achieved.

Cosmological Boundary Flux Parameter

The {\it{Cosmological Boundary Flux Parameter}} is a novel proposal that attempts to explain the origin of the cosmological parameter $\Lambda$ purely by geometric nature. Then we implement this new approach to a flat FLRW universe along with a barotropic fluid. We present an ansatz in which $\Lambda$ is straightforwardly coupled to the matter sector; therefore, only one additional parameter was introduced: $\lambda$. Also, through a statistical analysis, using late-time data of observational Hubble and type Ia Supernovae, we computed the joint best-fit value of the free parameters by means of the affine-invariant MCMC. We want to emphasise that the joint analysis produces a smaller $H_{0}^{\rm CBFP}=69.80\rm\,\, Km \,s^{-1}\,Mpc^{-1}$ in contrast to the flat $\Lambda$CDM result $H_{0}^{\Lambda\rm CDM}=70.53\rm\,\, Km \,s^{-1}\,Mpc^{-1}$. The work presented here seeks to contribute to the discussion of the possible explanation for the cosmos' acceleration, together with tackling other important questions in modern cosmology.

Some features of Kaniadakis holographic dark energy model

In this study, the Kaniadakis holographic dark energy (KHDE) model is investigated within the framework of a flat FLRW universe by using an infrared cut-off as Hubble horizon. For the different values of the Kaniadakis parameter [Formula: see text] and [Formula: see text], we perform an analysis on the deceleration, the equation of state and the energy density parameters. The current phase shift of the Cosmos may be rationalized by the change in sign of the deceleration parameter in the KHDE model, which went from [Formula: see text] to [Formula: see text], signifying a move from a phase of deceleration to one of acceleration. In order to get an understanding of the geometrical behavior shown by the KHDE model, we compute and show the statefinder evolutionary trajectory for the pairs [Formula: see text] and [Formula: see text]. The evolutionary trajectory of [Formula: see text] reveals that the [Formula: see text] crosses the phantom divide line [Formula: see text] at a late period in the evolutionary process. It has also been determined, by examination of the KHDE models stability analysis, that the model has behaved in an unstable manner during the whole course of the history of the universe. In addition to this, we optimized the model parameters by making use of the distance modulus measurement that was taken from the most current Union 2.1 dataset of supernovae.

Flat FLRW Universe in logarithmic symmetric teleparallel gravity with observational constraints

In this paper, we investigate the homogeneous and isotropic flat Friedmann–Lemaître–Robertson–Walker Universe in the logarithmic form of gravity, where Q is the non-metricity scalar, specifically, , where α and β are free model parameters. In this study, we consider a parametrization of the Hubble parameter as , where γ and η are model/free parameters which are constrained by an R 2-test from 57 points of the Hubble datasets in the redshift range 0.07 < z < 2.36. Further, we investigate the physical properties of the model. We analyze the energy conditions to check the compatibility of the model. We found the strong energy condition is violated for the logarithmic form of gravity due to the reality that the Universe in an accelerating phase. Finally, we discuss some important cosmological parameters in this context to compare our model with dark energy models such as jerk parameter and statefinder parameters.

Geodesic deviation in Sáez–Ballester theory

We study the geodesic deviation (GD) equation in a generalized version of the Sáez–Ballester (SB) theory in arbitrary dimensions. We first establish a general formalism and then restrict to particular cases, where (i) the matter-energy distribution is that of a perfect fluid, and (ii) the spacetime geometry is described by a vanishing Weyl tensor. Furthermore, we consider the spatially flat FLRW universe as the background geometry. Based on this setup, we compute the GD equation as well as the convergence condition associated with fundamental observers and past directed null vector fields. Moreover, we extend that framework and extract the corresponding geodesic deviation in the modified Sáez–Ballester theory (MSBT), where the energy–momentum tensor and potential emerge strictly from the geometry of the extra dimensions. In order to examine our herein GD equations, we consider two novel cosmological models within the SB framework. Moreover, we discuss a few quintessential models and a suitable phantom dark energy scenario within the mentioned SB and MSBT frameworks. Noticing that our herein cosmological models can suitably include the present time of our Universe, we solve the GD equations analytically and/or numerically. By employing the correct energy conditions plus recent observational data, we consistently depict the behavior of the deviation vector η ( z ) and the observer area distance r 0 ( z ) for our models. Concerning the Hubble constant problem, we specifically focus on the observational data reported by the Planck collaboration and the SH0ES collaboration to depict η ( z ) and r 0 ( z ) for our herein phantom model. Subsequently, we contrast our results with those associated with the Λ CDM model. We argue that the MSBT can be considered as a fitting candidate for a proper description of the late evolution of the universe.

Cosmology in f(R,Lm) gravity

In this letter, we investigate the cosmic expansion scenario of the universe in the framework of f(R,Lm) gravity theory. We consider a non-linear f(R,Lm) model, specifically, f(R,Lm)=R2+Lmn+β, where n and β are free model parameters. Then we derive the motion equations for flat FLRW universe and obtain the exact solution of corresponding field equations. Then we estimate the best fit ranges of model parameters by using updated H(z) datasets consisting of 57 points and the Pantheon datasets consisting of 1048 points. Further we investigate the physical behavior of density and the deceleration parameter. The evolution of deceleration parameter depicts a transition from deceleration to acceleration phases of the universe. Moreover, we analyze the stability of the solution of our cosmological model under the observational constraint by considering a linear perturbation. Lastly, we investigate the behavior of Om diagnostic parameter and we observe that our model shows quintessence type behavior. We conclude that our f(R,Lm) cosmological model agrees with the recent observational studies and can efficiently describe the late time cosmic acceleration.

Tsallis holographic dark energy reconsidered

We consider the interacting Tsallis Holographic Dark Energy (THDE), with the Granda–Oliveros (GO) scale as the infrared (IR) cutoff, as dynamical vacuum. We analytically solved for the Hubble parameter, in a spatially flat FLRW universe with dark energy and matter as components, and the solution traces the evolutionary path from the prior decelerated to the late accelerated epoch. Without interaction, the model predicts a varLambda CDM like behavior with an effective cosmological constant. We used Pantheon Supernovae type Ia, observational Hubble data (OHD), cosmic microwave background (CMB), and baryon acoustic oscillation (BAO) data to constrain the free parameters of the model. The estimated values of the cosmological parameters were consistent with observational results. We analyzed the behavior of the model using the statefinder and omega ^prime _{e}-omega _{e} plane where omega _{e} and omega ^prime _{e} corresponds to the effective equation of state and its evolution, respectively. The model shows a quintessence behavior in general, and the model trajectory ends in a point that corresponds to the de Sitter phase. We performed a dynamical analysis of the model, concluding that the prior decelerated and late accelerated phases are unstable and stable equilibria, respectively. We also investigated the thermodynamical nature of the model and found that the generalized second law remains valid in the dynamical vacuum treatment of the model.

Dynamical system analysis of logotropic dark fluid with a power law in the rest-mass energy density

We consider a spatially flat FLRW universe. We assume that it is filled with dark energy in the form of logotropic dark fluid coupled with dark matter in the form of a perfect fluid having a barotropic equation of state. We employ dynamical system tools to obtain a complete qualitative idea of the evolution of such a universe. It is interesting to note that we ought to consider an approximation for the pressure of the logotropic dark fluid in the form of an infinite series so as to be able to construct the autonomous system required for a dynamical system study. This series form provides us with a power law in the rest-mass energy density of the logotropic dark fluid. We compute the critical points of the autonomous system and analyze these critical points by applying linear stability theory. Our analysis reveal a scenario of late-time accelerated universe dominated by the logotropic fluid which behaves as cosmological constant, preceded by an intermediate phase of the Universe dominated by logotropic fluid which behaves as dark matter in the form of perfect fluid. Moreover, it also crosses the phantom divide line.

Barrow holographic phantom

In the present work, we consider the recently proposed non-interacting Barrow holographic dark energy (Int. J. Geom. Methods Mod. Phys. 18(01) (2021) 2150014, doi:10.1142/S0219887821500146) with Hubble horizon as IR cutoff in a flat FLRW universe. We examine the evolutionary behavior of deceleration parameter and equation of state parameter for the different Barrow exponent [Formula: see text], and observe suitable behavior in the model. We choose [Formula: see text] in the non-interacting Barrow holographic dark energy which is explained by a phantom scalar field, then we exhibit the phantomic narration of the Barrow holographic dark energy with [Formula: see text] and reconstruct the potential of the phantom scalar field.

Does diffusion mechanism favor the emergent scenario of the universe?

In this work, the flat FLRW Universe has been modeled with cosmic matter in the form of diffusive barotropic fluid. The diffusive fluid undergoes dissipation due to diffusion mechanism in the form of cosmological scalar field [Formula: see text]. From the perspective of non-equilibrium thermodynamics, the evolution equations of the universe have been formulated. By a suitable choice of the cosmological scalar field, emergent scenario of the universe has been obtained.

Cosmological Boundary Flux Parameter

The {\it{Cosmological Boundary Flux Parameter}} is a novel proposal that attempts to explain the origin of the cosmological parameter $\Lambda$ purely by geometric nature. Then we implement this new approach to a flat FLRW universe, along with a barotropic fluid. We present an ansatz in which $\Lambda$ couples straightforwardly to the matter sector; therefore only one extra parameter was introduced: $\lambda$. Also, through a statistical analysis, using late time data of Observational Hubble and type Ia Supernovae, we computed the best-fit value of the free parameters by means of the affine-invariant MCMC. We want to emphasize that the Joint analysis produces a smaller $H_{0}=71.51\rm\,\, Km \,s^{-1}\,Mpc^{-1}$ in contrast to the $\Lambda$CDM result $H_{0}=72.15\rm\,\, Km \,s^{-1}\,Mpc^{-1}$. Indeed, this outcome approaches more to the CMB value $H_{0}^{\rm CMB}=67.70\rm\,\, Km \,s^{-1}\,Mpc^{-1}$ (Planck + BAO). Thus, this might suggest that our propoal might be a good candidate to amend the $H_{0}$ tension. In spite of that, more statistical analysis needs to be done to be certain about this. The work here presented seeks to contribute to the discussion of the possible explanation for the cosmos' acceleration, together with tackling other important questions in modern cosmology.

Cosmological constraints of interacting phantom dark energy models

In this paper, we consider three phantom dark energy models, in the context of interaction between the dark components namely cold dark matter (CDM) and dark energy (DE). The first model, known as wdCDM can induce a big rip singularity (BR) while the two remaining induce future abrupt events known as the Little Rip (LR) and Little Sibling of the Big Rip (LSBR). These phantom DE models can be distinguished by their equation of state. We invoke a new phenomenon such as the interaction between CDM and DE given that it could solve or alleviate some of the problems encountered in standard cosmology. We aim to find out the effect of such an interaction on the cosmological parameters of the studied models, as well as, the persistence or the disappearance of the singularity and the abrupt events induced by the models under study. We choose an interaction term proportional to DE density, i.e. Q=λHρd, since the case where Q∝ρm could lead to a large scale instability at early time. We also do not claim at all that Q=λHρd is the ideal choice since it suffers from a negative CDM density in the future. By the use of a Markov Chain Monte Carlo (MCMC) approach, and by assuming a flat FLRW Universe, we constrain the cosmological parameters of each of the three phantom DE models studied. Furthermore, by the aid of the corrected Akaike Information Criterion (AICc) tool, we compare our phantom DE models. Finally, a perturbative analysis of phantom DE models under consideration is performed based on the best fit background parameters.

Growth of structures and redshift-space distortion data in scale-dependent gravity

This study is devoted to the implications of scale-dependent gravity in Cosmology. Redshift-space distortion data indicate that there is a tension between $\Lambda$CDM and available observations as far as the value of the rms density fluctuation, $\sigma_8$, is concerned. It has been pointed out that this tension may be alleviated in alternative theories in which gravity is weaker at red-shift $z \sim 1$. We study the evolution of density perturbations for non-relativistic matter on top of a spatially flat FLRW Universe, and we compute the combination $A=f \sigma_8$ in the framework of scale-dependent gravity, where both Newton's constant and the cosmological constant are allowed to vary with time. Upon comparison between available observational data (supernovae data as well as redshift-space distortion data) and theoretical predictions of the model, we determine the numerical value of $\sigma_8$ that best fits the data.

Exploring the new Tsallis agegraphic dark energy with interaction through statefinder

In this work, we explore the recently proposed new Tsallis agegraphic dark energy model in a flat FLRW Universe by taking the conformal time as IR cutoff with interaction. The deceleration parameter of the interacting new Tsallis agegraphic dark energy model provides the phase transition of the Universe from decelerated to accelerated phase. The EoS parameter of the model shows a rich behaviour as it can be quintessence-like or phantom-like depending on the interaction ($b^2$) and parameter $B$. The evolutionary trajectories of the statefinder parameters and $(\omega_D, \omega_D^{'})$ planes are plotted by considering the initial condition $\Omega_{D}^{0} =0.73$, $H_{0}= 67$ according to $\Lambda$CDM observational Planck 2018 data for different $b^2$ and $B$. The model shows both quintessence and Chaplygin gas behaviour in the statefinder $(r, s)$ and $(r, q)$ pair planes for different $b^2$ and $B$.

Barrow HDE model for statefinder diagnostic in FLRW universe

We have analyzed the Barrow holographic dark energy (BHDE) in the framework of flat FLRW universe by considering the various estimations of Barrow exponent △. Here, we define BHDE, by applying the usual holographic principle at a cosmological system, for utilizing the Barrow entropy rather than the standard Bekenstein–Hawking. To understand the recent accelerated expansion of the universe, consider the Hubble horizon as the IR cutoff. The cosmological parameters, especially the density parameter [Formula: see text], the equation of the state parameter [Formula: see text], energy density [Formula: see text] and the deceleration parameter [Formula: see text] are studied in this paper and found the satisfactory behaviors. Moreover we additionally focus on the two geometric diagnostics, the statefinder [Formula: see text] and [Formula: see text] to discriminant BHDE model from the [Formula: see text]CDM model. Here we determined and plotted the trajectories of evolution for statefinder [Formula: see text], [Formula: see text] and [Formula: see text] diagnostic plane to understand the geometrical behavior of the BHDE model by utilizing Planck 2018 observational information. Finally, we have explored the new Barrow exponent △, which strongly affects the dark energy equation of state that can lead it to lie in the quintessence regime, phantom regime and exhibits the phantom-divide line during the cosmological evolution.

Dynamics of an interacting Barrow holographic dark energy model and its thermodynamic implications

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 perform a detailed study on the dynamics of the field equations by studying the asymptotic behavior of the field equations, while we write the analytic expression for the scale factor with the use of Laurent series. Finally, we study the implications of gravitational thermodynamics in the interacting BHDE model with the dynamical apparent horizon as the cosmological boundary. In particular, we study the viability of the generalized second law by assuming that the apparent horizon is endowed with Hawking temperature and Barrow entropy.