Flat Friedmann Robertson Walker Research Articles

Tachyonic matter cosmology with exponential and hyperbolic potentials

In this paper, we consider tachyonic matter in spatially flat Friedmann–Robertson–Walker (FRW) universe, and obtain behavior of some important cosmological parameters for two special cases of potentials. First, we assume the exponential potential and then consider hyperbolic cosine type potential. In both cases, we obtain behavior of the Hubble, deceleration and EoS parameters. Comparison with observational data suggest the model with hyperbolic cosine type scalar field potentials has good model to describe universe.

Extended scalar–tensor theory and thermodynamics in teleparallel framework

We present here a new modified gravitation theory for the galactic dark energy effect by using a general Lagrangian density which is represented by an arbitrary function f(T, [Formula: see text], X) where T describes the torsion scalar in teleparallel gravity while X shows the kinetic scalar field energy. While the function is in general form, once reduced, the model can be transformed into some of the other well-known gravitation theories. After deriving the corresponding field equations and considering the flat Friedmann–Robertson–Walker type universe which is filled with ordinary cosmic matter, we discuss both the non-equilibrium and equilibrium profiles of galactic thermodynamics. We find that there exists an equilibrium picture of thermodynamics. Additionally, we also generalize ordinary f(T, [Formula: see text], X) model’s action to the case in which there exists an interaction between the chameleon and scalar fields.

Stability Analysis of the Viscous Polytropic Dark Energy Model in Einstein Cosmology**Supported by the National Natural Science Foundation of China under Grant No 10873004, the State Key Development Program for Basic Research Program of China under Grant No 2010CB832803, and the Program for Changjiang Scholars and Innovative Research Team in University under Grant No IRT0964.

The viscous polytropic gas model as one model of dark energy is hot-spot and keystone to the modern cosmology. We study the evolution of the viscous polytropic dark energy model interacting with the dark matter in the Einstein cosmology. Setting the autonomous dynamical system for the interacting viscous polytropic dark energy with dark matter and using the phase space analysis method to investigate the dynamical evolution and its critical stability, we find that the viscosity property of the dark energy creates a benefit for the stable critical dynamical evolution of the interaction model between dark matter and dark energy in the flat Friedmann—Robertson—Walker universe and the viscosity of dark energy will soften the coincidence problem just like the interacting dark energy model.

Generalized ghost pilgrim dark energy in F(T,TG) cosmology

This paper is devoted to study the generalized ghost pilgrim dark energy (PDE) model in [Formula: see text] gravity with flat Friedmann–Robertson–Walker (FRW) universe. In this scenario, we reconstruct [Formula: see text] models and evaluate the corresponding equation of state (EoS) parameter for different choices of the scale factors. We assume power-law scale factor, scale factor for unification of two phases, intermediate and bouncing scale factor. We study the behavior of reconstructed models and EoS parameters graphically. It is found that all the reconstructed models show decreasing behavior for PDE parameter [Formula: see text]. On the other hand, the EoS parameter indicates transition from dust-like matter to phantom era for all choices of the scale factor except intermediate for which this is less than [Formula: see text]. We conclude that all the results are in agreement with PDE phenomenon.

Bouncing cosmologies in Brans–Dicke theory

In this paper it is shown that in Brans–Dicke theory, if one considers a non-minimal coupling between the matter and scalar field, can give rise to a bouncing universe (i.e., an expanding universe preceded by a contracting universe). Two examples of such universes have been considered in a spatially flat Friedmann–Robertson–Walker universe and their physical properties have been studied.

Inflationary cosmology leading to a soft type singularity

A remarkable property of modern cosmology is that it allows for a special case of symmetry, consisting in the possibility of describing the early-time acceleration (inflation) and the late-time acceleration using the same theoretical framework. In this paper, we consider various cosmological models corresponding to a generalized form for the equation of state for the fluid in a flat Friedmann–Robertson–Walker (FRW) universe, emphasizing cases where the so-called type IV singular inflation is encountered in the future. This is a soft (non-crushing) kind of singularity. Parameter values for an inhomogeneous equation of state leading to singular inflation are obtained. We present models for which there are two type IV singularities, the first corresponding to the end of the inflationary era and the second to a late-time event. We also study the correspondence between the theoretical slow-roll parameters leading to type IV singular inflation and the recent results observed by the Planck satellite.

Kantowski–Sachs cosmological solutions in the generalized teleparallel gravity via Noether symmetry

We study the f(T) theory as an extension of teleparallel gravity and consider the Noether symmetry of Kantowski–Sachs (KS) anisotropic model for this theory. We specify the explicit teleparallel form of f(T) and find the corresponding exact cosmological solutions under the assumption that the Lagrangian admits the Noether symmetry. It is found that the universe experiences a power law expansion for the scale factors in the context of f(T) theory. By deriving equation of state (EOS) parameter, we show that the universe passes through the phantom and [Formula: see text]CDM theoretical scenarios. In this way, we estimate a lower limit age for the universe in excellent agreement with the value reported from recent observations. When KS model reduces to the flat Friedmann–Robertson–Walker (FRW) metric, our results are properly transformed into the corresponding values.

Hybrid Expansion Law with Interacting Cosmic Fluids

A theoretical model for hybrid expansion law in a spatially flat Friedmann- Robertson-Walker (FRW) model filled with two interacting perfect fluid is developed. We obtain the behaviour of the energy densities, pressures, and the energy transfer as functions of the cosmological time. This provides a new family of solutions for hybrid expansion law with interacting cosmic fluids.

String cloud and domain walls with quark matter for a higher dimensional FRW universe in self creation cosmology

In this study, we research a higher dimensional flat Friedmann-Robertson-Walker (FRW) universe in Barber–s second theory when strange quark matter (SQM) and normal matter (NM) are attached to the string cloud and domain walls. We obtain zero string tension density for this model. We obtain dust quark matter solutions. This result agrees with Kiran and Reddy, Krori et al, Sahoo and Mishra and Reddy. In our solutions the quark matter transforms to other particles over time. We also obtain two different solutions for domain walls with quark and normal matters by using a deceleration parameter. Also, the features of the obtained solutions are discussed and some physical and kinematical quantities are generalized and discussed. Our results are consistent with Yılmaz, Adcox et al and Back et al in four and five dimensions.

The van der Waals fluid and its role in cosmology

We analyze the properties of a generic cosmological fluid described by the van der Waals equation-of-state. Exact solutions for the energy-density evolution are found as implicit functions of the scale factor for a flat Friedmann–Robertson–Walker (FRW) spacetime. The possible values of the free parameter in the van der Waals equation are selected a posteriori, in accordance with asymptotic behaviors that are physically relevant. The stability of the model against small perturbations is studied through the hydrodynamic perturbations of the fluid for the relevant cases. It is found that a van der Waals fluid seems appropriate to describe noneternal inflationary scenarios.

Late cosmic acceleration in a vector-Gauss–Bonnet gravity model

In this work, we study a general vector–tensor model of dark energy (DE) with a Gauss–Bonnet term coupled to a vector field and without explicit potential terms. Considering a spatially flat Friedmann–Robertson–Walker (FRW) type universe and a vector field without spatial components, the cosmological evolution is analyzed from the field equations of this model considering two sets of parameters. In this context, we have shown that it is possible to obtain an accelerated expansion phase of the universe since the equation state parameter [Formula: see text] satisfies the restriction [Formula: see text] (for suitable values of model parameters). Further, analytical expressions for the Hubble parameter [Formula: see text], equation state parameter [Formula: see text] and the invariant scalar [Formula: see text] are obtained. We also find that the square of the speed of sound is negative for all values of redshift, therefore, the model presented here shows a sign of instability under small perturbations. We finally perform an analysis using [Formula: see text] observational data and we find that for the free parameter [Formula: see text] in the interval [Formula: see text], at 99.73% C.L. (and fixing [Formula: see text] and [Formula: see text]), the model has a good fit to the data.

Dynamics of a planar thin shell at a Taub–FRW junction

We address the problem of stitching together the vacuum, static, planar-symmetric Taub spacetime and the flat Friedmann–Robertson–Walker (FRW) cosmology using the Israel thin shell formalism. The joining of Taub and FRW spacetimes is reminiscent of the Oppenheimer–Snyder collapse used in modeling the formation of a singularity from a collapsing spherical ball of dust. A possible mechanism for the formation of a planar singularity is provided. It is hoped that tackling such example will improve our intuition on planar-symmetric systems in Einstein’s general relativity (GR).

Generalized Second Law of Thermodynamics in Parabolic LTB Inhomogeneous Cosmology**Supported financially by Research Institute for Astronomy & Astrophysics of Maragha (RIAAM), Iran

We study thermodynamics of the parabolic Lemaitre–Tolman–Bondi (LTB) cosmology supported by a perfect fluid source. This model is the natural generalization of the flat Friedmann–Robertson–Walker (FRW) universe, and describes an inhomogeneous universe with spherical symmetry. After reviewing some basic equations in the parabolic LTB cosmology, we obtain a relation for the deceleration parameter in this model. We also obtain a condition for which the universe undergoes an accelerating phase at the present time. We use the first law of thermodynamics on the apparent horizon together with the Einstein field equations to get a relation for the apparent horizon entropy in LTB cosmology. We find out that in LTB model of cosmology, the apparent horizon's entropy could be feeded by a term, which incorporates the effects of the inhomogeneity. We consider this result and get a relation for the total entropy evolution, which is used to examine the generalized second law of thermodynamics for an accelerating universe. We also verify the validity of the second law and the generalized second law of thermodynamics for a universe filled with some kinds of matters bounded by the event horizon in the framework of the parabolic LTB model.

Non-Minimally Conformally Coupling Cosmology with Multiple Vacua Potential with Cubic-Quintic-Septic Duffing Oscillator Properties

Abstract The dynamics of a flat Friedmann–Robertson–Walker (FRW) cosmological model with a barotropic matter is studied, which is dominated by an oscillating scalar field conformally coupled to the gravity with a scalar potential characterised by multiple vacua. Several motivating consequences are observed and discussed accordingly.

Large-scale structure observables in general relativity

We review recent studies that rigorously define several key observables of the large-scale structure of the Universe in a general relativistic context. Specifically, we consider (i) redshift perturbation of cosmic clock events; (ii) distortion of cosmic rulers, including weak lensing shear and magnification; and (iii) observed number density of tracers of the large-scale structure. We provide covariant and gauge-invariant expressions of these observables. Our expressions are given for a linearly perturbed flat Friedmann–Robertson–Walker metric including scalar, vector, and tensor metric perturbations. While we restrict ourselves to linear order in perturbation theory, the approach can be straightforwardly generalized to higher order.

Quark and strange quark matter solutions for higher dimensional FRW universe in Lyra geometry

In this study, we have investigated the higher dimensional flat Friedmann-Robertson-Walker (FRW) universe for a cloud of string with perfect fluid attached quark and strange quark matter (SQM) in Lyra geometry. Generalized (n + 2) dimensional flat FRW universe solutions have been achieved with the aid of equation of states (EOS) and deceleration parameter (q). From the field equations, we have obtained that the cloud of string with perfect fluid does not survive. Since the string tension density vanishes (λ = 0) for this model, as a result, the cloud of string with perfect-fluid-attached quark and strange quark matter energy-momentum tensor is automatically transformed into a perfect-fluid-attached quark and strange quark matter energy-momentum tensor. Also, our solutions agree with Halford’s study. β 2 behaves like a cosmological constant. When t → ∞ the pressure of quark matter and the density go to zero, then, the quark matter transforms into another matter with time. Also our quark matter solutions are in agreement with the present idea.

K-essential covariant holography

The holographic principle is applied to a flat Friedmann–Robertson–Walker space-time dominated by dark energy when this is due to the presence of a k-essence scalar field, both for dark energy and phantom scenarios. In this framework, a geometrical covariant approach permits the construction of holographic hypersurfaces. The resulting covariant preferred screens, both for phantom and non-phantom regions, are then compared with those obtained by using the holographic dark energy model with the future event horizon as the infrared cut-off. In the phantom case, one of the two obtained holographic screens is placed on the big rip hypersurface, both for the covariant holographic formalism and the holographic phantom model. It is also analysed whether this covariant formalism allows a mathematically consistent formulation of fundamental theories based on the existence of a S-matrix at infinite distances.

A late time accelerated FRW model with scalar and vector fields via Noether symmetry

We study the evolution of a three-dimensional minisuperspace cosmological model by the Noether symmetry approach. The phase space variables turn out to correspond to the scale factor of a flat Friedmann–Robertson–Walker (FRW) model, a scalar field with potential function V(ϕ) with which the gravity part of the action is minimally coupled and a vector field of its kinetic energy is coupled with the scalar field by a coupling function f(ϕ). Then, the Noether symmetry of such a cosmological model is investigated by utilizing the behavior of the corresponding Lagrangian under the infinitesimal generator of the desired symmetry. We explicitly calculate the form of the coupling function between the scalar and the vector fields and also the scalar field potential function for which such symmetry exists. Finally, by means of the corresponding Noether current, we integrate the equations of motion and obtain exact solutions for the scale factor, scalar and vector fields. It is shown that the resulting cosmology is an accelerated expansion universe for which its expansion is due to the presence of the vector field in the early times, while the scalar field is responsible of its late time expansion.

Best values of parameters for interacting HDE with GO IR-cutoff in Brans–Dicke cosmology

We investigate the interacting holographic dark energy (HDE) with Granda–Oliveros (GO) infrared (IR)-cutoff in the framework of Brans–Dicke (BD) cosmology. We obtain the equation of state (EoS) parameter of HDE, wD, the effective EoS parameter w eff , the deceleration parameter q and the squared of sound speed [Formula: see text] in a flat Friedmann–Robertson–Walker (FRW) universe. We show that at late-time the cosmic coincidence problem can be alleviated. Also we show that for noninteracting case, HDE can give a unified dark matter–dark energy (DM–DE) profile in BD cosmology, except that it cannot solve the coincidence problem in the future. By studying the EoS parameter, we see that the phantom divide may be crossed. Using the latest observational data, we calculate the best values of the parameters for interacting HDE in BD framework. Computing the deceleration parameter implies that the transition from deceleration to the acceleration phase occurred for redshift z ≥ 0.5. Finally, we investigate the sound stability of the model, and find that HDE with Granda–Oliveros (GO)-cutoff in the framework of BD cosmology can lead to a stable DE-dominated universe favored by observations, provided we take β = 0.44 and b2 < 0.35. This is in contrast to HDE model in Einstein gravity which does not lead to a stable DE-dominated universe.

Observational constraints on variable equation of state parameters of dark matter and dark energy after Planck

In this paper, we study a cosmological model in general relativity within the framework of spatially flat Friedmann–Robertson–Walker space–time filled with ordinary matter (baryonic), radiation, dark matter and dark energy, where the latter two components are described by Chevallier–Polarski–Linder equation of state parameters. We utilize the observational data sets from SNLS3, BAO and Planck + WMAP9 + WiggleZ measurements of matter power spectrum to constrain the model parameters. We find that the current observational data offer tight constraints on the equation of state parameter of dark matter. We consider the perturbations and study the behavior of dark matter by observing its effects on CMB and matter power spectra. We find that the current observational data favor the cold dark matter scenario with the cosmological constant type dark energy at the present epoch.