Flat Friedmann Robertson Walker Research Articles

Observational constraints on the wet dark fluid model in the fractal gravity

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 ΛCDM 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 ztr≈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.

Analysis of cosmic evolution admitting Garcia-Salcedo ghost and generalized ghost dark energy models

This study aims to explore the Garcia-Salcedo ghost dark energy and generalized ghost dark energy models in the context of f(R,T2) theory, where R is the Ricci scalar and T2 is the self-contraction of stress–energy tensor. We investigate the non-interacting case only corresponding to flat Friedmann–Robertson–Walker universe. We reconstruct the corresponding f(R,T2) gravity models using the considered dark energy models by taking two particular models of this gravity. The stability analysis is performed for all the cases. The behavior of the equation of state parameter is also checked. It is found that some of the reconstructed f(R,T2) models successfully describe both the phantom and quintessence epochs of the universe, which support the current cosmic accelerated expansion. This exploration reveals the intricate connections between dark energy models and modified gravitational theory, providing significant understanding into the dynamics of the cosmos on a large scale.

Analysis of reconstructed modified [formula omitted] gravity

This paper studies the reconstruction criterion in the framework of f(Q,T) gravity using a ghost dark energy model, where Q represents the non-metricity and T is the trace of energy–momentum tensor. In this regard, the correspondence scenario for a non-interacting scheme is used to construct the ghost dark energy f(Q,T) model. We consider the flat Friedmann–Robertson–Walker universe with a power-law scale factor and pressure-less matter. We investigate the behavior of the equation of state parameter and examine the stability of the ghost dark energy model via the squared speed of sound parameter. The equation of state parameter depicts the phantom era, while the squared speed of sound reveals a stable ghost dark energy model for the entire cosmic evolutionary paradigm. Finally, we study the cosmography of the ωGDE−ωGDE′ and r−s planes that correspond to a freezing region and Chaplygin gas model, respectively. It is concluded that the reconstructed ghost dark energy model represents the evolution of the cosmos for suitable choices of the parametric values.

Non-Commutative Classical and Quantum Fractionary Cosmology: FRW Case

In this work, we will explore the effects of non-commutativity in fractional classical and quantum schemes using the flat Friedmann–Robertson–Walker (FRW) cosmological model coupled to a scalar field in the K-essence formalism. In previous work, we have obtained the commutative solutions in both regimes in the fractional framework. Here, we introduce non-commutative variables, considering that all minisuperspace variables qnci do not commute, so the symplectic structure was modified. In the quantum regime, the probability density presents a new structure in the scalar field corresponding to the value of the non-commutative parameter, in the sense that this probability density undergoes a shift back to the direction of the scale factor, causing classical evolution to arise earlier than in the commutative world.

A coupled-fluid approach to explore bounce and inflationary cosmology with Barrow holographic as the driving dark fluid

Assuming that viscous fluid in the form of Barrow holographic dark energy (BHDE) (a particular case of more generalized version of HDE elaborated in Nojiri et al. in Phys Lett B 825:136844, 2022; Symmetry 13(6):928, 2021) and pressure-less dark matter are present across the flat Friedmann–Robertson–Walker universe, we find the precise solutions of viscous BHDE models in non-interacting and interacting scenarios for different choices of scale factor. We show the evolution of the EoS parameters in the viscous non-interacting and interacting situations together with the deceleration parameters for observing the transition time frame. Afterwards, we investigate a bounce inflation model using the analytical results of model’s slow-roll parameters, scalar spectral index, and tensor-to-scalar ratio. We investigated an association between BHDE and scalar field models since inflation is typically attributed to the existence of scalar fields. The evolution of the generated potential from the scalar fields are plotted against time. Finally, we investigated the GSL of the thermodynamics.

Constraining Rastall parameter for baryon asymmetry factor using isotropic and anisotropic universe models

This paper investigates the mystery of baryogenesis for isotropic and anisotropic universe models in the context of Rastall’s theory of gravity (RTG). In the first part, we utilize the Rastall field equations (FEs) for the flat Friedmann Robertson Walker (FRW) metric to find the solutions of density [Formula: see text] and Hubble parameter [Formula: see text]. Additionally, Rastall parameter plays a crucial role to discuss baryogenesis and fulfilling its criteria on the observational ground. It is found that very large positive/negative values of Rastall parameter like [Formula: see text] (for [Formula: see text]) and [Formula: see text] (for [Formula: see text]) are required to satisfy the observational bounds on baryon asymmetry factor (BAF), see Fig. 1. In the second part, we develop the FEs for RTG using the Locally Rotationally Symmetric (LRS) Bianchi type-1 line-element. Then we find the solutions of the FEs and use them to find the BAF using Rastall parameter as [Formula: see text] and anisotropic parameter [Formula: see text] to meet the consistency with the experimental limit, see Fig. 3. For [Formula: see text], the anisotropic model reduces to the isotropic FRW model, where we have a zero BAF. Finally, we conclude that RTG with both universe models is a good candidate to study baryogenesis in the early universe and the variable [Formula: see text] has a considerable impact on the BAF.

Non-minimally coupled scalar field and scaling symmetry in a cosmological background

We study Noether symmetries of a class of non-minimally coupled scalar field in a background spatially flat Friedmann-Robertson-Walker (FRW) spacetime. We explore the model symmetries and its conserved currents and charges. Especially, the scaling symmetry, its possible break down and outcomes of such a symmetry breaking are treated in details. A suitable potential of the non-minimally coupled scalar field is adopted which is necessary to get a symmetric Lagrangian of the system including gravity, scalar field and ordinary matter density. We use the obtained charge and the adopted potential in the equations of motions to see the role of the non-minimal coupling (NMC) on the cosmic expansion. We study evolution of the scalar field in the phase space of the model and explore the stability of the obtained critical point. In this manner we derive a relation that relates the cosmological constant and gravitational constant via a unique identity which reflects the scaling symmetry breaking in the space (a, φ).

Holographic description of the dissipative unified dark fluid model with axion field

In this paper, we extend an axion [Formula: see text] gravity model, and apply the holographic principle to describe in a unifying manner the early and the late-time universe when the general equation-of-state (EoS) contains a bulk viscosity. We assume a spatially flat Friedmann–Robertson–Walker (FRW) universe model. We use a description based on the generalized infrared-cutoff holographic dark energy proposed by Nojiri and Odintsov [Eur. Phys. J. C 77 (2017) 528; Gen. Relativ. Gravit. 38 (2006) 1285], and explore the evolution of the universe when the EoS describes the asymptotic behavior between the dust in the early universe and the late universe. We explore various forms of the bulk viscosity, and calculate analytical expressions for the infrared cutoffs in terms of the particle horizon. In this way, we obtain a unifying description of the early and the late-time universe in the presence of axion matter, via a viscous holographic fluid model.

Warm inflation driven by complex quintessence field within rastall gravity

This manuscript is devoted to examining the role of the “complex quintessence field” (CQF) in driving warm inflation (WI) within the Rastall theory of gravity (RTG). Recently, the accelerating cosmic expansion has been described by CQF, which is linked to several other cosmic scenarios. In this approach, we construct the corresponding dynamical equations for inflation using the flat Friedmann–Robertson–Walker (FRW) spacetime. We begin with a fundamental approach of selecting intermediate, logamediate, and power-law models to study how inflation is realized from these models. For each model, we find analytic solutions of the scalar field and corresponding effective potential necessary to produce inflation under slow-roll (SR) conditions. These solutions allow us to develop inflationary observable in the strong dissipative regime, including the SR parameters, e-folding number, scalar/tensor power spectra, scalar spectral index (ns) and its running (αs) and finally tensor to scalar ratio (r). We checked the impact of additional theory parameter λ on the evolutionary trajectories in ns−r and ns−αs planes for the underlying inflationary models. For all three models, it is concluded that CQF-inspired inflation in RTG provides interesting and feasible results which fully satisfy the Planck constraints.

Attractor solutions in interacting dark energy models

We investigate a cosmological model in which dark energy, represented by a quintessential scalar field, is coupled to a dark-matter perfect fluid in the spatially flat Friedmann–Robertson–Walker Universe. This allows an energy exchange in the dark sector which could happen both at early times before recombination era or at late times. We use the coupling function Q=γρdmφ̇ which is induced by conformal transforming scalar-tensor and f(R) gravity theories to Einstein frame. It is argued that there is a connection between this coupling function and Q∝ρdmH. A dynamical analysis is used to show that there are early- and late-time attracting solutions for which the system evolves for a wide range of initial conditions. These attractors generalize the scaling solutions which have been already found in the non-interacting case.

Horizon thermodynamics and cosmological equations: a holographic-like connection between thermostatistical quantities on a cosmological horizon and in the bulk

Horizon thermodynamics is expected to be related to the effective energy based on the energy density calculated from the Friedmann equation for a Friedmann–Robertson–Walker (FRW) universe. In the present study, the effective energy and thermostatistical quantities on a cosmological horizon are examined to clarify the holographic-like connection between them, with a focus on a de Sitter universe. To this end, the Helmholtz free energy on the horizon is derived from horizon thermodynamics. The free energy is found to be equivalent to the effective energy calculated from the Friedmann equation. This consistency is interpreted as a kind of holographic-like connection. To examine this connection, Padmanabhan’s holographic equipartition law, which is related to the origin of spacetime dynamics, is applied to a de Sitter universe. It is found that the law should lead to a holographic-like connection. The holographic-like connection is considered to be a bridge between thermostatistical quantities on the horizon and in the bulk. For example, cosmological equations for a flat FRW universe can be derived from horizon thermodynamics by accepting the connection as a viable scenario. In addition, a thermal entropy equivalent to the Bekenstein–Hawking entropy is obtained from the Friedmann equation using the concept of a canonical ensemble in statistical physics. The present study should provide new insight into the discussion of horizon thermodynamics and cosmological equations.

On the gravitational collapse in 4-dimensional Einstein–Gauss–Bonnet gravity

In this paper, we treat 4-dimensional Einstein–Gauss–Bonnet (EGB) gravity as general relativity with an effective stress-energy tensor. We will study the modified Oppenheimer–Snyder–Datt model of the gravitational collapse of a star in a 4-dimensional EGB black hole (BH). The inside geometry of the star is described by the spatially flat Friedmann–Robertson–Walker metric and the matter is distributed uniformly without any pre-assumption about its equation of state. The exterior EGB BH is smoothly matched to the interior geometry without the requirement of any thin shell. This gives the energy density, pressure, and the equation of state of collapsing matter. At the end, we study the time evolution of event and apparent horizons.

Aspects of cosmologies with complete scenario

We consider a flat Friedmann–Robertson–Walker (FRW) universe with expansion history based on a Hubble parameter. The Hubble parameter ansatz, by construction, yields de Sitter-like evolution at early and late-times. The phase transition from accelerated to decelerated era during early times and decelerated to accelerated era in the recent past provide the complete cosmological history of the universe in a natural way. In order to exhibit the dynamical evolution of universe, we explore the inflationary dynamics along with graceful exit, behaviors of cosmological parameters, energy conditions along with behavior in ω − ω ′ plane to identify the thawing and freezing regions. Statefinder diagnostic highlights that the universe will evolve to Λ cold dark matter model during late-times. We also find the scalar field description for the considered Hubble parameter ansatz. • The FRW model having expansion history based on the Hubble parameter is studied. • Cosmic parameters illustrate physically realistic cases of the universe evolution. • Model exhibits graceful exit from inflation during early era. • The model explains late-time accelerating evolution of the universe. • The model possess potential having polynomial form with field.

Modified cosmology through Barrow entropy

We investigate the cosmological consequences of the modified Friedmann equations when the entropy associated with the apparent horizon, given by Barrow entropy, $S\sim A^{1+\delta/2}$, where $0\leq\delta\leq1$, represents the amount of the quantum-gravitational deformation of the horizon. We study implications of this model in a flat Friedmann-Robertson-Walker (FRW) universe with/without cosmological constant. Taking the cosmological constant into account, this model can describe the current accelerated expansion, although the transition from deceleration phase to the acceleration phase takes place in the lower redshifts. We investigate the evolution of the scale factor and show that with increasing $\delta$, the value of the scale factor increases as well. We also estimate the age of the universe in Barrow cosmology which is smaller than the age of the universe in standard cosmology.

The effective equation of state in Rastall gravity

In this paper, we study a homogeneous and isotropic flat Friedmann–Robertson–Walker (FRW) universe in Rastall gravity, in which there exist the components of radiation and dust. That is to say, the density of fluid [Formula: see text] satisfies the barotropic equation of state [Formula: see text], where [Formula: see text] and [Formula: see text], and the equation of state [Formula: see text]. We derive the modified Friedmann equations, and the effective equation of state [Formula: see text] in this universe model. For the dust-dominated case, the effective equation of state [Formula: see text] is described by [Formula: see text], where [Formula: see text] is the Rastall parameter. As [Formula: see text], we have [Formula: see text] which means an accelerated expansion of the universe occurs. Along with the conditions for the convergent modified Friedmann equations and the positive density of dust, the Rastall parameter [Formula: see text] should be in the range of [Formula: see text]. In other words, the various [Formula: see text] for [Formula: see text] correspond to those of quintessence with the general barotropic equation of state [Formula: see text]. For the general case, the Rastall universe full of radiation and dust, the expression of [Formula: see text] is complicated involving the Rastall parameter, the density of radiation and the density of dust. As the ratio of the densities of dust and radiation is smaller than [Formula: see text], [Formula: see text] is less than [Formula: see text]. Moreover, to keep the density of dust positive, the Rastall parameter should be restricted in [Formula: see text] too. By the numerical calculation, we find that [Formula: see text] from [Formula: see text] decreases as [Formula: see text] increases. And [Formula: see text] for different [Formula: see text] ultimately tend to [Formula: see text], the value of which are in the range of [Formula: see text]. Obviously, the expansion of the cosmology filled with radiation and dust or only dust in Rastall gravity [Formula: see text] accelerates without any “exotic” energy whose equation of state [Formula: see text].

The Cosmology of a Non-Minimally Coupled f(R,T) Gravitation

A non-minimally coupled cosmological scenario is considered in the context of f(R,T)=f1(R)+f2(R)f3(T) gravity (with R being the Ricci scalar and T the trace of the energy-momentum tensor) in the background of the flat Friedmann–Robertson–Walker (FRW) model. The field equations of this modified theory are solved using a time-dependent deceleration parameter for a dust. The behavior of the model is analyzed taking into account constraints from recent observed values the deceleration parameter. It is shown that the analyzed models can explain the transition from the decelerating phase to the accelerating one in the expansion of the universe, by staying true to the results of the observable universe. It is shown that the models are dominated by a quintessence-like cosmological dark fluid at the late universe.

Matter and antimatter asymmetry in a rainbow universe

In rainbow gravity, the ordinary general relativity (GR) field equations can be modified by a parametric set of equations and thus the cosmology of this scenario may give new sounds about the physics of the early universe. As we know in the radiation-dominated universe the gravitational baryogenesis mechanism (GBM) cannot occur in the GR context. In this regard, by assuming the flat Friedmann–Robertson–Walker (FRW) metric we study GBM in the rainbow gravity. We obtain that for a rainbow universe filled with radiation, the baryon-to-entropy ratio from the GBM can have a non-zero value. In this work, we study the baryon-to-entropy ratio for the corresponding cosmological models of two suggested dispersion relations and we obtain that in the first case the baryogenesis mechanism occurs and also the results are compatible with the observational constraints while the second case is not consistent with the observational constraints.

Cosmic acceleration and ekpyrotic bounce with Chameleon field

In this paper, we explore the homogeneous and isotropic flat Friedmann–Robertson–Walker (FRW) model in Chameleon cosmology. By considering a non-minimal coupling between the scalar field and matter, we present a non-singular bouncing cosmological scenario of the universe. The universe initially exhibits the ekpyrotic phase during the contracting era, undergoes a non-singular bounce, and then in expanding era, it smoothly transits to the decelerating era having matter and radiation-dominated phases. Further, this decelerating era is smoothly connected to the late-time dark energy-dominated era of the present epoch. We use numerical solution techniques to solve non-minimally coupled gravity equations for understanding the evolution of scalar field along with other quantities like effective potential in the model. The model thus unifies an ekpyrotic, non-singular, asymmetric bounce with the dark energy era of the present epoch. We study the evolution of bouncing model and confront the model with observational results on the equation of state parameter by constraining the model parameters.

FRW model with two-fluid source in fractal cosmology

Present paper deals with flat Friedmann - Robertson - Walker (FRW) model with two - fluid source in fractal cosmology. In this model one fluid represents matter content of the universe and another fluid is radiation field modeling the cosmic microwave background. To get the deterministic model, we have used the relation between pressure and density for matter through the gamma law equation of state $p_{m} = (\gamma-1)\, \rho_{m}, \,\, 1\leq \gamma\leq2$ . The solutions of fractal field equations are obtained in terms of Kummer's confluent hypergeometric function of the first kind. Some physical parameters of the models are obtained and discussed their behavior with the help of graphs in detail.

Non-minimally coupled transit cosmology in f(R, T) gravity

A non-minimally coupled cosmological model is studied in [Formula: see text] gravity for a particular choice of the function in the background of flat Friedmann–Robertson–Walker universe. The modified field equations are solved with the help of a varying deceleration parameter. The time evolution of the model is analyzed for both the dynamic and kinematic quantities. For testing the viability of the results, energy conditions and the statefinder diagnostic are used. It has been shown that this model, which we discussed to examine the phase transition in the expansion of the universe, is compatible with current astrophysical observations, and that the DE model dominating the early universe is Chaplygin gas, while the model dominating the late universe is quintessence.