Form Of Scale Factor Research Articles

Comprehensive study of bouncing cosmological models in f(Q, T) theory

The main objective of this article is to investigate the viability of bouncing cosmological scenarios using different forms of scale factors with perfect matter configuration in the framework of extended symmetric teleparallel theory. This modified proposal is defined by the function f(Q, T), where Q characterizes non-metricity and T denotes the trace of energy-momentum tensor. We investigate the modified field equations of this theory using different parametric values of the Hubble parameter and non-metricity to derive viable solutions. These solutions are relevant in various cosmological bounce models such as symmetric-bounce, super-bounce, oscillatory-bounce, matter-bounce and exponential-bounce models. Furthermore, we examine the behavior of energy density and pressure to analyze the characteristics of dark energy. A comprehensive analysis is also conducted to explore the behavior of the equation of state parameter and deceleration parameter to examine the evolutionary eras of the cosmos. Our findings show that the f(Q, T) gravity describes the cosmic expansion in the vicinity of the bouncing point during the early and late times of cosmic evolution.

Behaviour of Bianchi Type-V Dark Energy Model in f(R,T) Gravity with a Specific Form of Hubble Parameter

Abstract: In this paper, we have constructed a Bianchi type V cosmological model, in the presence of bulk viscous fluid and within the framework of f(R,T) theory of gravity with an appropriate choice of the functional f(R,T)in the form f(R,T )=R+2f(T), where R and T are respectively Ricci scalar and trace of energy momentum tensor. In order to obtain a deterministic solution, we have considered two general forms of hyperbolic scale factors. The different forms of scale factors considered here produce time-varying deceleration parameters in all the cases that simulate the cosmic transition. The state finder diagnostic pair is found to be in the acceptable range. The physical parameters are constrained from different representative values to build up a realistic cosmological model aligned with the observational behaviour. Keywords: Bianchi type V, Dark energy, f(R,T) gravity, Variable deceleration parameter.

Anisotropic Multiverse with Varying c and G and Study of Its Thermodynamics

We assume the anisotropic model of the Universe in the framework of a varying speed of light c and a varying gravitational constant G theories and study different types of singularities. We write the scale factors for the singularity models in terms of cosmic time and find some conditions for possible singularities. For future singularities, we assume the forms of a varying speed of light and varying gravitational constant. For regularizing the Big Bang singularity, we assume two forms of scale factors: the sine model and the tangent model. For both models, we examine the validity of null and strong energy conditions. Starting from the first law of thermodynamics, we study the thermodynamic behaviors of a number n of universes (i.e., multiverse) for (i) varying c, (ii) varying G and (iii) varying both c and G models. We find the total entropies for all the cases in the anisotropic multiverse model. We also find the nature of the multiverse if the total entropy is constant.

FRW cosmological models with cosmological constant in f(R, T) theory of gravity

In the present paper, we have generalized the behaviors of the transit-decelerating to accelerating FRW cosmological model in f(R, T) gravity theory, where R and T are Ricci scalar and trace of the energy–momentum tensor, respectively. The solution of the corresponding field equations is obtained by assuming a linear function of the Hubble parameter H (i.e., q = c1 + c2H), which gives a time-dependent deceleration parameter [Formula: see text], where c3 and c2 are arbitrary integrating constants (Tiwari et al. Eur. Phys. J. Plus, 131, 447 (2016); Ibid. 132, 126 (2017)). There are two scenarios in which we explain the particular form of scale factor thus obtained: (i) by using the recent constraints from observational Hubble data (OHD) and joint light curves (JLA) data, which show a cosmic deceleration to acceleration and (ii) by using new constraints from supernovae type Ia union data, which show accelerating expansion of the universe (q < 0) throughout its evolution. We have observed that EoS parameter, energy density parameters, and important cosmological planes yield results compatible with the modern observational data. For the derived models, we have calculated various physical parameters as luminosity distance, distance modulus, and apparent magnitude versus redshift for both current supporting observations.

Variable Chaplygin gas cosmologies in f(R, T) gravity with particle creation

A flat FLRW (Friedmann–Lemaitre–Robertson–Walker) cosmological model with perfect fluid comprising of variable Chaplygin gas (VCG) has been studied in the context of f(R, T) gravity with particle creation. The solutions of the modified field equations are obtained through three different considered form of scale factors. The effective pressure is negative throughout the evolution of universe, which leads to accelerated expansion of the universe. In addition to that we have also discussed the importance of particle creation pressure on the cosmological parameters, energy conditions and state-finder diagnostic parameters. It is noticed that the time evolution of source function yields almost constant particle production at late times.

Can an interacting varying Chaplygin gas and tachyonic matter accelerate universe?

In this paper, a cosmological model involving interacting varying Chaplygin gas of a specific type and a tachyonic matter is considered to address the problem known as the accelerated expansion of the universe. The study shows that a specific form of the potential and appropriate values of the parameters allows a transition from early tachyonic to late-time tachyonic dark matter with [Formula: see text]. This point, up to our knowledge, makes the considered cosmological scenario unique in the literature. Moreover, discussion on future finite-time singularities for the considered models finalizes the paper. The study shows that in case of non-interacting model, the fate of universe will be either Type II (The Sudden Singularity), or Type IV (“Generalized Sudden Singularity”) singularity. On the other hand, the consideration of [Formula: see text] non-gravitational interaction in addition to Type II and Type IV singularities will induce also Type V (“[Formula: see text]-singularity”) singularity, while with [Formula: see text] only Type IV singularity will be observed. Moreover, two forms of scale factor have been considered allowing to analytically obtain the behavior of the cosmological parameters without specifying an explicit form of the potential for tachyonic matter.

Inflation Driven by q-de Sitter

We propose a generalised de Sitter scale factor for the cosmology of early and late time universe, including single scalar field is called as inflaton. This form of scale factor has a free parameter $q$ is called as nonextensivity parameter. When $q=1$, the scale factor is de Sitter. This scale factor is an intermediate form between power-law and de Sitter. We study cosmology of such families. We show that both kinds of dark components, dark energy and dark matter simultaneously are described by this family of solutions. As a motivated idea, we investigate inflation in the framework of $q$-de Sitter. We consider three types of scenarios for inflation. In a single inflation scenario, we observe that, inflation ended without any specific ending inflation $\phi_{end}$, the spectral index and the associated running of the spectral index are %$ n_\mathrm{s} - 1 \sim -2\epsilon, \quad \alpha_\mathrm{s} \equiv 0 $. To end the inflation: we should have $q=\frac{3}{4}$. We deduce that the inflation ends when the evolution of the scale factor is $a (t) =e_ {3/4} (t) $. With this scale factor there is no need to specify $\phi_{end}$. As an alternative to have inflation with ending point, We will study q-inflation model in the context of warm inflation. We propose two forms of damping term $\Gamma$. In the first case when $\Gamma=\Gamma_0$, we show the scale invariant spectrum, (Harrison-Zeldovich spectrum, i.e. $n_s=1$) may be approximately presented by ($q=\frac{9}{10},~~N=70$). Also there is a range of values of $R$ and $n_s$ which is compatible with the BICEP2 data where $q=\frac{9}{10}$. In case $\Gamma=\Gamma_1V(\phi)$, it is observed that small values of a number of e-folds are assured for small values of $q$ parameter. For $q=\frac{9}{10}$ a range of values of $R$ and $n_s$ is compatible with the BICEP2 data.

New version of viscous Chaplygin gas cosmology with varying gravitational constant

In this paper we consider a new version of Chaplygin gas cosmology that includes bulk viscosity and a varying gravitational constant. This new model of Chaplygin gas allows us to recover a barotropic fluid with a quadratic equation of state. Bulk viscosity is assumed to be a power law function of the energy density. We study the evolution of dark energy density numerically for several forms of scale factor and analytically under some assumptions corresponding to a late epoch of the universe. We discuss some cosmological parameters and fix them using recent observational data, such as those from the Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP2) telescope.

Reconstruction of modified $$f(R,T)$$ f ( R , T ) gravity with perfect fluid cosmological models

In this paper we present the cosmological viability of reconstruction of an alternative gravitational theory, namely, the modified \(f(R,T)\) gravity, where \(R\) is the Ricci scalar curvature and \(T\) the trace of stress energy momentum tensor. A functional form of \(f(R,T)= R + 2f(T)\) is chosen for the reconstruction in perfect fluid flat Friedmann–Robertson–Walker model. The gravitational field equations contain two fluid sources, one is perfect fluid and other is due to modified \(f(R,T)\) gravity which is to be considered as an exotic fluid. This allows us for derivation and analysis of a set of new cosmological solutions for \(f(R,T)\) gravity by considering these two fluids as a non-interacting. Two known forms of scale factor (de Sitter and power-law) are considered for the explicit and successful reconstruction. The equation of state parameter (EoS) of exotic matter and the effective EoS parameter have been discussed. In de Sitter solution we find that the \(f(R,T)\) fluid behaves as phantom dark energy when the usual matter (perfect fluid) shows the behavior between decelerated phase to accelerated phase. In the absence of usual matter it behaves as a cosmological constant. In case of power -law cosmology two different cases are discussed and analyzed the behavior of different phases of the universe accordingly through the equation of state and density parameters.

Viscous New Varying Modified Cosmic Chaplygin Gas with Cosmological Constant in Non-flat Universe

In this paper we study new varying modified cosmic Chaplygin gas which has viscosity in presence of cosmological constant and space curvature. By using well-known forms of scale factor in Friedmann equation we obtain behavior of dark energy density numerically. We use observational data to fix solution and discuss about stability of our system.

Extended Chaplygin gas equation of state with bulk and shear viscosities

In this note extended Chaplygin gas equation of state includes bulk and shear viscosities suggested. Bulk viscosity assumed as power law form of density and shear viscosity considered as a constant. We study evolution of dark energy density numerically for several forms of scale factor, and analytically under some assumptions corresponding to early universe. We found our model is stable for infinitesimal viscous parameters.

Time-Dependent Density of Modified Cosmic Chaplygin Gas with Variable Cosmological Constant in Non-Flat Universe

In this paper we study modified cosmic Chaplygin cosmology with non-zero cosmological constant in non-flat Universe. By using well-known forms of scale factor we obtain time-dependent dark energy density by numerical analysis of non-linear differential equation and fitting curves. We use observational data to fix solution and discuss about stability of our system. First of all we consider cosmological constant as a constant in Einstein equation, and then study possibility of variable cosmological constant.

New Agegraphic Dark Energy model in chameleon Brans-Dicke cosmology for different forms of the scale factor

In this work, we study the New Agegraphic Dark Energy (NADE) model (which contains the conformal time η as infrared cut-off) in the framework of Brans-Dicke cosmology with chameleon scalar field which is non-minimally coupled to the matter field. Considering interacting Dark Energy and Dark Matter (DM), we calculate some relevant cosmological parameters, i.e. the equation of state (EoS) parameter, the deceleration parameter q and the evolution of the energy density parameter $\varOmega_{D}'$ for different forms of scale factors, i.e. the power-law, the emergent, the intermediate and the logamediate ones, which leads to different expressions of η.

Roles of Different Forms of Scale Factor in Non-linear Electrodynamics for Accelerating Universe

In this work, we have assumed the modified Lagrangian of non-linear electrodynamics for accelerated universe. The energy density and pressure for non-linear electromagnetic theory have been considered in terms of both electric and magnetic fields. The Einstein’s filed equations have been considered in FRW universe for Hořava-Lifshitz gravity. Since we are considering the non-linear form of Lagrangian for accelerating universe, so four forms of scale factors like logamediate, intermediate, emergent and power law forms are chosen in our investigation. For every expansion, the natures of electric field and magnetic field have been shown through graphical representation. The electric and magnetic fields increase for logamediate, intermediate and emergent expansion and decrease in power law expansion.

Thermodynamical Properties of Quark-Gluon Plasma and Effects of Bulk Viscosity

In this paper, we use equation of state, temperature and bulk viscosity of the quark gluon plasma and calculate some cosmological and thermodynamical quantities. In order to obtain such quantities we choose special case of energy density in terms of Hubble parameter. We apply the approximation solution and obtain the exact form of scale factor, temperature and bulk viscous stress. Finally, we draw some figures for the geometric and thermodynamic quantities in terms of time.

Generalized Second Law of Thermodynamics in Emergent Universe

In this letter, we have considered the FRW model of the emergent universe, which was presented in our previous work (Debnath, in Class. Quantum Gravity 25:205019, 2008). We have chosen one of the form of scale factor in such a way that the emergent scenario is possible in the universe. We have also considered the universe as a thermodynamical system with the horizon surface as a boundary of the system. The entropy and the radius of the event horizon have been calculated in the emergent scenario. When the emergent scenario occurs, we have shown that the generalized second law of thermodynamics is always satisfied for open, flat and closed models of the universe.

Emergent Scenario in Anisotropic Universe

In this work, we have considered that the anisotropic universe is filled with normal matter and phantom field (or tachyonic field). We have chosen the exponential forms of scale factors a and b in such a way that there is no singularity for evolution of the anisotropic universe. Here we have shown that the emergent scenario is possible for open, closed or flat universe if the universe contains phantom field or tachyonic field or phantom tachyonic field. From recently developed statefinder parameters, the behaviour of different stages of the evolution of the emergent universe have been generated.

THE ROLE OF A CHAMELEON FIELD IN AN ANISOTROPIC UNIVERSE WITH LOGAMEDIATE AND INTERMEDIATE SCENARIOS

In this work, we consider the Universe is being filled with matter composed of a chameleon-type dark energy scalar field. Employing a particular form of potential, we discuss the field's role in the accelerating phase of the Universe for an anisotropic model using the logamediate and intermediate forms of scale factors. The natures of statefinder and slow-roll parameters are discussed diagrammatically.

Valence force fields and the lattice dynamics of beryllium oxide

The lattice dynamics of beryllium oxide have been studied using a rigid-ion model, with short-range forces represented by a valence force field. Various existing calculations on group-IV elements using such a field have been examined as a prelude to transference of force constants from diamond to beryllium oxide. The effects of ionicity on the force constants have been included in the form of scale factors. It is shown that no satisfactory fit to the long-wavelength data on BeO can be found with transferred force constants. However, adequate least-squares fits can be found both with four- and six-parameter valence force fields, the discrepancy with experiment being large only for one optical mode at the Brillouin-zone center. Dispersion curves along $\ensuremath{\Delta}$ and $\ensuremath{\Sigma}$ are presented and are in fair agreement with experiment, deviations arising essentially from the quality of the fit to the long-wavelength data. The bond-bending interactions are found to play a significant role and arguments have been presented to show that the inclusion of further angle-angle interactions would yield a very satisfactory picture of the dynamics.