Showing papers on "Deceleration parameter published in 2014"

Cosmographic bounds on the cosmological deceleration-acceleration transition redshift in f(R) gravity

Abstract: We examine the observational viability of a class of $f(\mathcal{R})$ gravity cosmological models. Particular attention is devoted to constraints from the recent observational determination of the redshift of the cosmological deceleration-acceleration transition. Making use of the fact that the Ricci scalar is a function of redshift $z$ in these models, $\mathcal{R}=\mathcal{R}(z)$, and so is $f(z)$, we use cosmography to relate a $f(z)$ test function evaluated at higher $z$ to late-time cosmographic bounds. First, we consider a model-independent procedure to build up a numerical $f(z)$ by requiring that at $z=0$ the corresponding cosmological model reduces to standard $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$. We then infer late-time observational constraints on $f(z)$ in terms of bounds on the Taylor expansion cosmographic coefficients. In doing so we parametrize possible departures from the standard $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model in terms of a two-parameter logarithmic correction. The physical meaning of the two parameters is also discussed in terms of the post-Newtonian approximation. Second, we provide numerical estimates of the cosmographic series terms by using type Ia supernova apparent magnitude data and Hubble parameter measurements. Finally, we use these estimates to bound the two parameters of the logarithmic correction. We find that the deceleration parameter in our model changes sign at a redshift consistent with what is observed.

Bianchi Type- V Cosmology in f( R, T) Gravity with Λ( T)

Abstract: A new class of cosmological models in f(R,T) modified theories of gravity proposed by Harko et al. (Phys. Rev. D 84:024020, 2011), where the gravitational Lagrangian is given by an arbitrary function of Ricci scalar R and the trace of the stress-energy tensor T, have been investigated for a specific choice of f(R,T)=f 1(R)+f 2(T) by considering time dependent deceleration parameter. The concept of time dependent deceleration parameter (DP) with some proper assumptions yield the average scale factor $a(t) = \sinh^{\frac{1}{n}}(\alpha t)$ , where n and α are positive constants. For 0 1, the models of universe exhibit phase transition from early decelerating phase to present accelerating phase which is in good agreement with the results from recent astrophysical observations. Our intention is to reconstruct f(R,T) models inspired by this special law for the deceleration parameter in connection with the theories of modified gravity. In the present study we consider the cosmological constant Λ as a function of the trace of the stress energy-momentum-tensor, and dub such a model “Λ(T) gravity” where we have specified a certain form of Λ(T). Such models may display better uniformity with the cosmological observations. The statefinder diagnostic pair {r,s} parameter has been embraced to characterize different phases of the universe. We also discuss the physical consequences of the derived models.

Friedmann model with viscous cosmology in modified f(R,T) gravity theory

Abstract: In this paper, we introduce the bulk viscosity in the formalism of modified gravity theory in which the gravitational action contains a general function $$f(R,T)$$ , where $$R$$ and $$T$$ denote the curvature scalar and the trace of the energy–momentum tensor, respectively, within the framework of a flat Friedmann–Robertson–Walker model. As an equation of state for a prefect fluid, we take $$p=(\gamma -1)\rho $$ , where $$0 \le \gamma \le 2$$ and a viscous term as a bulk viscosity due to the isotropic model, of the form $$\zeta =\zeta _{0}+\zeta _{1}H$$ , where $$\zeta _{0}$$ and $$\zeta _{1}$$ are constants, and $$H$$ is the Hubble parameter. The exact non-singular solutions to the corresponding field equations are obtained with non-viscous and viscous fluids, respectively, by assuming a simplest particular model of the form of $$f(R,T) = R+2f(T)$$ , where $$f(T)=\alpha T$$ ( $$\alpha $$ is a constant). A big-rip singularity is also observed for $$\gamma <0$$ at a finite value of cosmic time under certain constraints. We study all possible scenarios with the possible positive and negative ranges of $$\alpha $$ to analyze the expansion history of the universe. It is observed that the universe accelerates or exhibits a transition from a decelerated phase to an accelerated phase under certain constraints of $$\zeta _0$$ and $$\zeta _1$$ . We compare the viscous models with the non-viscous one through the graph plotted between the scale factor and cosmic time and find that the bulk viscosity plays a major role in the expansion of the universe. A similar graph is plotted for the deceleration parameter with non-viscous and viscous fluids and we find a transition from decelerated to accelerated phase with some form of bulk viscosity.

Entropic-force dark energy reconsidered

Abstract: We reconsider the entropic-force model in which both kinds of Hubble terms, $\stackrel{\ifmmode \dot{}\else \textperiodcentered \fi{}}{H}$ and ${H}^{2}$, appear in the effective dark energy (DE) density affecting the evolution of the main cosmological functions, namely, the scale factor, deceleration parameter, matter density, and growth of linear matter perturbations. However, we find that the entropic-force model is not viable at the background and perturbation levels due to the fact that the entropic formulation does not add a constant term in the Friedmann equations. On the other hand, if on mere phenomenological grounds we replace the $\stackrel{\ifmmode \dot{}\else \textperiodcentered \fi{}}{H}$ dependence of the effective DE density with a linear term $H$ without including a constant additive term, we find that the transition from deceleration to acceleration becomes possible, but the recent structure formation data strongly disfavor this cosmological scenario. Finally, we briefly compare the entropic-force models with some related DE models (based on dynamical vacuum energy) which overcome these difficulties and are compatible with the present observations.

Cosmic slowing down of acceleration for several dark energy parametrizations

Abstract: We further investigate slowing down of acceleration of the universe scenario for five parametrizations of the equation of state of dark energy using four sets of Type Ia supernovae data. In a maximal probability analysis we also use the baryon acoustic oscillation and cosmic microwave background observations. We found the low redshift transition of the deceleration parameter appears, independently of the parametrization, using supernovae data alone except for the Union 2.1 sample. This feature disappears once we combine the Type Ia supernovae data with high redshift data. We conclude that the rapid variation of the deceleration parameter is independent of the parametrization. We also found more evidence for a tension among the supernovae samples, as well as for the low and high redshift data.

Effect of Shear and Bulk Viscosities on Interacting Modified Chaplygin Gas Cosmology

Abstract: In this paper, we study interacting modified Chaplygin gas (MCG) which has shear and bulk viscosities. We consider sign-changeable interaction between MCG and matter, then investigate the effects of shear and bulk viscosities on the cosmological parameters such as energy, density, Hubble expansion parameter, scale factor and deceleration parameter.

FRW Cosmology with the Extended Chaplygin Gas

Abstract: We propose extended Chaplygin gas equation of state for which it recovers barotropic fluid with quadratic equation of state. We use numerical method to investigate the behavior of some cosmological parameters such as scale factor, Hubble expansion parameter, energy density, and deceleration parameter. We also discuss the resulting effective equation of state parameter. Using density perturbations we investigate the stability of the theory.

Holographic dark energy model with linearly varying deceleration parameter and generalised Chaplygin gas dark energy model in Bianchi type-I universe

Abstract: The paper deals with a spatially homogeneous and anisotropic Bianchi type-I universe filled with two minimally interacting fluids; matter and holographic dark energy components. The nature of the holographic dark energy for Bianchi type-I space time is discussed. An exact solution to Einstein’s field equations in Bianchi type-I line element is obtained using the assumption of linearly varying deceleration parameter. Under the suitable condition, it is observed that the anisotropy parameter of the universe approaches to zero for large cosmic time and the coincidence parameter increases with increasing time. We established a correspondence between the holographic dark energy models with the generalised Chaplygin gas dark energy model. We also reconstructed the potential and dynamics of the scalar field which describes the Chaplygin cosmology. Solution of the field equations shows that a big rip type future singularity will occur for this model. It has been observed that the solutions are compatible with the results of recent observations.

Probing kinematics and fate of the Universe with linearly time-varying deceleration parameter

Abstract: The parametrizations q = q 0+q 1 z and q = q 0+q 1(1 − a/a 0) (Chevallier-Polarski-Linder parametrization) of the deceleration parameter, which are linear in cosmic redshift z and scale factor a , have been frequently utilized in the literature to study the kinematics of the Universe. In this paper, we follow a strategy that leads to these two well-known parametrizations of the deceleration parameter as well as an additional new parametrization, q = q 0+q 1(1 − t/t 0), which is linear in cosmic time t. We study the features of this linearly time-varying deceleration parameter in contrast with the other two linear parametrizations. We investigate in detail the kinematics of the Universe by confronting the three models with the latest observational data. We further study the dynamics of the Universe by considering the linearly time-varying deceleration parameter model in comparison with the standard ΛCDM model. We also discuss the future of the Universe in the context of the models under consideration.

Kaluza-Klein cosmological model in $f(R,T)$ gravity with $\Lambda(T)$

Abstract: A class of Kaluza-Klein cosmological models in $f(R,T)$ theory of gravity have been investigated. In the work, we have considered the functional $f(R,T)$ to be in the form $f(R,T)=f(R)+f(T)$ with $f(R)=\lambda R$ and $f(T)=\lambda T$. Such a choice of the functional $f(R,T)$ leads to an evolving effective cosmological constant $\Lambda$ which depends on the stress energy tensor. The source of the matter field is taken to be a perfect cosmic fluid. The exact solutions of the field equations are obtained by considering a constant deceleration parameter which leads two different aspects of the volumetric expansion namely a power law and an exponential volumetric expansion. Keeping an eye on the accelerating nature of the universe in the present epoch, the dynamics and physical behaviour of the models have been discussed. From statefinder diagnostic pair we found that the model with exponential volumetric expansion behaves more like a $\Lambda$CDM model.

Bianchi type-I transit cosmological models with time dependent gravitational and cosmological constants: reexamined

Abstract: The Einstein’s field equations with variable gravitational and cosmological “constants” for a spatially homogeneous and anisotropic Bianchi type-I space-time are obtained in present study. To study the transit behaviour of Universe, we consider a law of variation of scale factor $$a(t) = \left( t^{k} e^{t}\right) ^{\frac{1}{n}}$$ , which yields a time dependent deceleration parameter $$q = -1 + \frac{nk}{(k + t)^{2}}$$ , comprising a class of models that depicts a transition of the universe from the early decelerated phase to the recent accelerating phase. We find that the time dependent deceleration parameter is reasonable for the present day Universe and gives an appropriate description of the evolution of the universe. For $$n = 0.27k$$ , we obtain $$q_{0} = -0.73$$ , which is similar to observed value of deceleration parameter at present epoch. It is also observed that for $$n \ge 2$$ and k = 1, we obtain a class of transit models of the universe from early decelerating to present accelerating phase. For k = 0, the universe has non-singular origin. In these models, we arrive at the decision that, from the structure of the field equations, the behaviour of cosmological and gravitational constants and are related. Taking into consideration the observational data, we conclude that the cosmological constant behaves as a positive decreasing function of time, whereas gravitational constant is increasing and tends to a constant value at late time. $$H(z)/(1+z)$$ data (32 points) and model prediction as a function of redshift for different k and n are successfully presented by using recent data. Some physical and geometric properties of the models are also discussed.

Cosmic slowing down of acceleration for several dark energy parametrizations

Abstract: We further investigate slowing down of acceleration of the universe scenario for five parametrizations of the equation of state of dark energy using four sets of supernovae data. In a maximal probability analysis we also use the baryon acoustic oscillation and cosmic microwave background observations. We found the low redshift transition of the deceleration parameter appears, independently of the parametrization, using supernovae data alone except for the Union 2.1 sample. This feature disappears once we combine the supernova data with high redshift data. We conclude that the rapid variation of the deceleration parameter is independent of the parametrization. We also found more evidence for a tension among the supernovae samples, as well as for the low and high redshift data.

Constraints on cosmological parameters in power-law cosmology

Abstract: In this paper, we examine observational constraints on the power law cosmology; essentially dependent on two parameters $H_0$ (Hubble constant) and $q$ (deceleration parameter). We investigate the constraints on these parameters using the latest 28 points of H(z) data and 580 points of Union2.1 compilation data and, compare the results with the results of $\Lambda$CDM. We also forecast constraints using a simulated data set for the future JDEM, supernovae survey. Our studies give better insight into power law cosmology than the earlier done analysis by Kumar [arXiv:1109.6924] indicating it tuning well with Union2.1 compilation data but not with H(z) data. However, the constraints obtained on $ $ and $ $ i.e. $H_0$ average and $q$ average using the simulated data set for the future JDEM, supernovae survey are found to be inconsistent with the values obtained from the H(z) and Union2.1 compilation data. We also perform the statefinder analysis and find that the power-law cosmological models approach the standard $\Lambda$CDM model as $q\rightarrow -1$. Finally, we observe that although the power law cosmology explains several prominent features of evolution of the Universe, it fails in details.

The Raychaudhuri equation in homogeneous cosmologies

Abstract: In this work we address the issue of studying the conditions required to guarantee the Focusing Theorem for both null and time like geodesic congruences by using the Raychaudhuri equation. In particular we study the case of Friedmann-Robertson-Walker as well as more general Bianchi Type I spacetimes. The fulfillment of the Focusing Theorem is mandatory in small scales since it accounts for the attractive character of gravity. However, the Focusing Theorem is not satisfied at cosmological scales due to the measured negative deceleration parameter. The study of the conditions needed for congruences convergence is not only relevant at the fundamental level but also to derive the viability conditions to be imposed on extended theories of gravity describing the different expansion regimes of the universe. We illustrate this idea for f (R) gravity theories.

Generalized second law of thermodynamics in scalar-tensor gravity

Abstract: Within the context of scalar-tensor gravity, we explore the generalized second law (GSL) of gravitational thermodynamics. We extend the action of ordinary scalar-tensor gravity theory to the case in which there is a nonminimal coupling between the scalar field and the matter field (as a chameleon field). Then we derive the field equations governing the gravity and the scalar field. For a Friedmann-Robertson-Walker universe filled only with ordinary matter, we obtain the modified Friedmann equations as well as the evolution equation of the scalar field. Furthermore, we assume the boundary of the Universe to be enclosed by the dynamical apparent horizon that is in thermal equilibrium with the Hawking temperature. We obtain a general expression for the GSL of thermodynamics in the scalar-tensor gravity model. For some viable scalar-tensor models, we first obtain the evolutionary behaviors of the matter density, the scale factor, the Hubble parameter, the scalar field, and the deceleration parameter, as well as the effective equation of state (EoS) parameter. We conclude that in most of the models, the deceleration parameter approaches a de Sitter regime at late times, as expected. Also, the effective EoS parameter acts like the $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model at late times. Finally, we examine the validity of the GSL for the selected models.

Bianchi Type-II Dark Energy Model in f(R,T) Gravity

Abstract: The spatially homogeneous and totally anisotropic Bianchi Type-II space-time dark energy model with EoS parameter is considered in the presence of a perfect fluid source in the framework of f(R,T) gravity proposed by Harko et al. (Phys. Rev. D, 84:024020, 2011). With the help of special law of variation for Hubble’s parameter proposed by Berman (Nuovo Cimento B, 74:182, 1983) a dark energy cosmological model is obtained in this theory. We consider f(R,T) model and investigate the modification R+f(T) in Bianchi type-II cosmology with an appropriate choice of a function f(T)=λT. We use the power law relation between average Hubble parameter H and average scale factor R to find the solution. The assumption of constant deceleration parameter leads to two models of universe, i.e. power law model and exponential model. Some physical and kinematical properties of the model are also discussed.

The Mass of Graviton and Its Relation to the Number of Information according to the Holographic Principle.

Abstract: We investigate the relation of the mass of the graviton to the number of information N in a flat universe. As a result we find that the mass of the graviton scales as [Formula: see text]. Furthermore, we find that the number of gravitons contained inside the observable horizon is directly proportional to the number of information N; that is, N gr ∝ N. Similarly, the total mass of gravitons that exist in the universe is proportional to the number of information N; that is, [Formula: see text]. In an effort to establish a relation between the graviton mass and the basic parameters of the universe, we find that the mass of the graviton is simply twice the Hubble mass m H as it is defined by Gerstein et al. (2003), times the square root of the quantity q - 1/2, where q is the deceleration parameter of the universe. In relation to the geometry of the universe we find that the mass of the graviton varies according to the relation [Formula: see text], and therefore m gr obviously controls the geometry of the space time through a deviation of the geodesic spheres from the spheres of Euclidean metric.

FRW cosmology with the extended Chaplygin gas

Abstract: In this paper, we propose extended Chaplygin gas equation of state for which it recovers barotropic fluid with quadratic equation of state. We use numerical method to investigate the behavior of some cosmological parameters such as scale factor, Hubble expansion parameter, energy density and deceleration parameter. We also discuss about the resulting effective equation of state parameter. Using density perturbations we investigate the stability of the theory.

Bianchi Type-II String Cosmological Model with Magnetic Field in f ( R , T ) Gravity

Abstract: The spatially homogeneous and totally anisotropic Bianchi type-II cosmological solutions of massive strings have been investigated in the presence of the magnetic field in the framework of f(R,T) gravity proposed by Harko et al. (Phys Rev D 84:024020, 2011). With the help of special law of variation for Hubble’s parameter proposed by Berman (Nuovo Cimento B 74:182, 1983) cosmological model is obtained in this theory. We consider f(R,T) model and investigate the modification R+f(T) in Bianchi type-II cosmology with an appropriate choice of a function f(T)=μ T. We use the power law relation between average Hubble parameter H and average scale factor R to find the solution. The assumption of constant deceleration parameter leads to two models of universe, i.e. power law model and exponential model. Some physical and kinematical properties of the model are also discussed.

Bianchi-V string cosmology with power law expansion in f (R, T) gravity

Abstract: In this paper, we search the existence of the Bianchi-V string cosmological model in the f (R, T) gravity with power law expansion. Einstein’s field equations have been solved by taking into account the law of variation of Hubble’s parameter that yields the constant value of the deceleration parameter (DP). We observe that the massive strings dominate the early universe but they do not survive for long time and finally disappear from the universe. We examine the nature of classical potential and also discuss the physical properties of the universe.

Higher derivative corrections of $f(R)$ gravity with varying equation of state in the case of variable $G$ and $\Lambda$

Abstract: In this paper, we study three models of f(R) modified gravity including higher order terms based on different equation of state parameter. We also assume variable G and $\Lambda$. By using numerical analysis, we obtain the behavior of some important cosmological parameters, like the Hubble expansion H parameter and the deceleration parameter q. The statefinder diagnostics is also performed for all models.

Interacting dark matter and holographic dark energy in an anisotropic universe

Abstract: In this paper we have studied the anisotropic and homogeneous Bianchi type-I universe filled with interacting Dark matter and Holographic dark energy Here we discussed two models, in first model the solutions of the field equations are obtained for constant value of deceleration parameter where as in the second model the solutions of the field equations are obtained for special form of deceleration parameter It is shown that for suitable choice of interaction between dark matter and holographic dark energy there is no coincidence problem (unlike ΛCDM) Also, in all the resulting models the anisotropy of expansion dies out very quickly and attains isotropy after some finite time The Statefinder diagnostic is applied to both the models in order to distinguish between our dark energy models with other existing dark energy models The physical and geometrical aspects of the models are also discussed

On extended sign-changeable interactions in the dark sector

Abstract: We extend the cosmological couplings proposed in Sun et al. and Wei, where they suggested interactions with change of signs along the cosmological evolution. Our extension liberates the changes of sign of the interaction from the deceleration parameter and from the relation of energy densities of the dark sector and considers the presence of non interactive matter. In three cases we obtain the general solutions and the results obtained in models fitted with Hubble’s function and SNe Ia data, are analyzed regarding the problem of the cosmological coincidence, the problem of the crisis of the cosmological age and the magnitude of the energy density of dark energy at early universe. Also we graphically study the range of variation of, the actual dark matter density parameter, the effective equation of state of the dark energy and the redshift of transition to the accelerated regimen, generated by variations at order $$1\sigma $$ in the coupling parameters.

Two-fluid atmosphere from decelerating to accelerating Friedmann–Robertson–Walker dark energy models

Abstract: Evolution of dark energy parameter within scope of a spatially homogeneous and isotropic Friedmann–Robertson–Walker model filled with perfect fluid and dark energy components is studied by generalizing recent results. The two sources are claimed to interact minimally so that their energy momentum tensors are conserved separately. The conception of time-dependent deceleration parameter with some suitable assumption yields an average scale factor \(a = [\sinh (\alpha t)]^{\frac{1}{n}}\), with α and n being positive arbitrary constants. For 0 1, the models of universe exhibit phase transition from early decelerating phase to present accelerating phase which is supported by results from recent astrophysical observations. It is observed that the transition red shift (zt) for our derived model with q0 = −0.73 is ≅0.32. This is in good agreement with cosmological observations in literature. Some physical and geometric properties of this model along with physical acceptability of cosmological solutions have been discussed in detail.

Oscillating Quintom Model with Time Periodic Varying Deceleration Parameter

Abstract: We propose a new law for the deceleration parameter that varies periodically with time. According to the law, we give a model of the oscillating universe with quintom matter in the framework of a 4-dimensional Friedmann—Robertson—Walker background. We find that, in the model, the Hubble parameter oscillates and keeps positive. The universe undergoes decelerating expansion and accelerating expansion alternately without singularity

Bianchi type-I cosmology in $f(R, T)$ gravity

Abstract: We investigate the exact solutions of a Bianchi type-I space-time in the context of f(R, T) gravity [1], where f(R, T) is an arbitrary function of the Ricci scalar R and the trace of the energy-momentum tensor T. For this purpose, we find two exact solutions using the assumption of a constant deceleration parameter and the variation law of the Hubble parameter. The obtained solutions correspond to two different models of the Universe. The physical behavior of these models is also discussed.

Interacting Modified Holographic Ricci Dark Energy Model and Statefinder Diagnosis in Flat Universe

Abstract: In this paper, we have considered the modified holographic Ricci dark energy interacting with dark matter through a nongravitational coupling. We took three phenomenological forms for the interaction term Q in the model, where, in general, it is proportional to the Hubble parameter and densities of the dark sectors, that Q = ρde+ρm, ρm and ρde, respectively. We have obtained analytical solutions for the three interacting models, and studied the evolutions of equations of state parameter and deceleration parameter. The results are compared with the observationally constrained values for the best parameters of the model. In general, we have shown that the equation of state of models are showing a de Sitter type behavior, in the far future evolution of the universe. We have also done the statefinder analysis of the model to discriminate it from other standard models of dark energy and have shown that the present r–s values of the models were in the range of the r–s value of the Chaplygin gas model of dark energy.

Will there be future deceleration? A study of particle creation mechanism in non-equilibrium thermodynamics

Abstract: The paper deals with non-equilibrium thermodynamics based on adiabatic particle creation mechanism with the motivation of considering it as an alternative choice to explain the recent observed accelerating phase of the universe. Using Friedmann equations, it is shown that the deceleration parameter ($q$) can be obtained from the knowledge of the particle production rate ($\Gamma$). Motivated from thermodynamical point of view, cosmological solutions are evaluated for the particle creation rates in three cosmic phases, namely, inflation, matter dominated and present late time acceleration. The deceleration parameter ($q$) is expressed as a function of the redshift parameter ($z$), and its variation is presented graphically. Also, statefinder analysis has been presented graphically in three different phases of the universe. Finally, two non-interacting fluids with different particle creation rates are considered as cosmic substratum, and deceleration parameter ($q$) is evaluated. It is examined whether more than one transition of $q$ is possible or not by graphical representations.

Cosmology and thermodynamics of FLRW universe with bulk viscous stiff fluid

Abstract: We consider a cosmological model dominated by a stiff fluid with a constant bulk viscosity. We classify all the possible cases of the universe predicted by the model and analyze the scale factor and the density as well as the curvature scalar. We find that when the dimensionless constant bulk viscous parameter is in the range \(0 6\) the model does not have a big bang and shows an increase in the fluid density and scalar curvature as the universe expands which eventually saturates as the scale factor \(a \rightarrow \infty \) in the future. We have analyzed the model with statefinder diagnostics and find that the model is different from the \(\Lambda \)CDM model but approaches the \(\Lambda \)CDM point as \(a \rightarrow \infty \). We have also analyzed the status of the generalized second law of thermodynamics with an apparent horizon as the boundary of the universe and found that the law is generally satisfied when \(0 \le \bar{\zeta } 6\) the law is satisfied when the scale factor is larger than a minimum value.