共查询到20条相似文献，搜索用时 765 毫秒
1.
《New Astronomy》2020
The aim of this paper is to study the Gödel type universe in modified f(R, ϕ) theory of gravity, where R stands for Ricci scalar and ϕ be the scalar potential. We investigate the modified field equations by using anisotropic and perfect fluid distributions. In particular, we consider two proposed models with some fixed values of parameters and investigate the exact solutions. The behaviour of energy conditions can be seen by a detailed graphical analysis. Furthermore, Tolman-Oppenheimer-Volkoff equation has been studied for both models in this theory. We have also discussed some exact solutions using perfect fluid. It is concluded that f(R, ϕ) theory of gravity support the phenomenon of cosmic expansion of the universe through Gödel type universe for both models. 相似文献
2.
P. H. R. S. Moraes 《Astrophysics and Space Science》2014,352(1):273-279
It is well known that the universe is undergoing a phase of accelerated expansion. Plenty of models have already been created with the purpose of describing what causes this non-expected cosmic feature. Among them, one could quote the extradimensional and the f(R,T) gravity models. In this work, in the scope of unifying Kaluza-Klein extradimensional model with f(R,T) gravity, cosmological solutions for density and pressure of the universe are obtained from the induced matter model application. Particular solutions for vacuum quantum energy and radiation are also shown. 相似文献
3.
The new class of cosmological model of the early Universe is considered with f(R,T) modified theories of gravity (Harko et al. in Phys. Rev. D 84:024020, 2011). The exact solutions to the corresponding field equations are obtained in quadrature form. The cosmological parameters have been discussed in detail. We have also discussed the well-known astrophysical phenomena, namely the Hubble parameter H(z), luminosity distance (d L ) and distance modulus μ(z) with redshift. 相似文献
4.
We study the dynamical evolution of an f(R) model of gravity in a viscous and anisotropic background which is given by a Bianchi type-I model of the Universe. We find viable forms of f(R) gravity in which one is exactly the Einsteinian model of gravity with a cosmological constant and other two are power law f(R) models. We show that these two power law models are stable with a suitable choice of parameters. We also examine three potentials which exhibit the potential effect of f(R) models in the context of scalar tensor theory. By solving different aspects of the model and finding the physical quantities in the Jordan frame, we show that the equation of state parameter satisfy the dominant energy condition. At last we show that the two power law f(R) models behave like quintessence model at late times and also the shear coefficient viscosity tends to zero at late times. 相似文献
5.
We discuss the Bianchi type I model with perfect fluid as matter content in f(R,T) gravity, where R is the Ricci scalar and T is the trace of the energy-momentum tensor. We obtain exact solutions of the field equations employing the anisotropic feature of spacetime for two expansion laws namely exponential and power expansions. The physical and kinematical quantities are examined for both cases in future evolution of the universe. We also explore the validity of null energy condition and conclude that our solutions are consistent with the current observations. 相似文献
6.
Algorithms are derived for constructing five dimensional Kaluza-Klein cosmological space-times in the presence of a perfect fluid source in the framework of f(R,T) gravity theory proposed by Harko et al. (Phys. Rev. D 84:024020, 2011). Starting from the solution of Reddy et al. (Int. J. Theor. Phys 51:3222-3227, 2012b) some classes of new solutions are generated which correspond to accelerating models of the Universe. The physical and kinematical behaviors of the models are studied. 相似文献
7.
We have studied the evolution of cosmological parameters by considering exponential harmonic field with collisional matter. A comparison has been made with the behavior of these parameters in the presence of ordinary matter and the model \(\Lambda CDM\). We have also compared the evolution of these parameters with the ones obtained in the modified gravity f(R) and f(R, T) theory case. The results are in line with those of the modified gravity so that the harmonic exponential field can be used to explain why the Universe has gone from the deceleration phase to the acceleration phase. 相似文献
8.
9.
In this paper, we study an anisotropic Bianchi-I space-time model in f(R) theory of gravity in the presence of perfect fluid as a matter contains. The aim of this paper is to find the functional form of f(R) from the field equations and hence the solution of various cosmological parameters. We assume that the deceleration parameter to be a constant, and the shear scalar proportional to the expansion scalar to obtain the power-law form of the scale factors. We find that the model describes the decelerated phases of the universe under the choice of certain constraints on the parameters. The model does not show the acceleration expansion and also transition from past deceleration to present accelerating epoch. We discuss the stability of the functional form of f(R) and find that it is completely stable for describing the decelerating phase of the universe. 相似文献
10.
K. L. Mahanta 《Astrophysics and Space Science》2014,350(2):683-689
We have constructed Locally Rotationally Symmetric Bianchi type I (LRSBI) cosmological models in the f(R,T) theory of gravity when the source of gravitation is the bulk viscous fluid. The models are constructed for f(R,T)=R+2f(T) and f(R,T)=f 1(R)+f 2(T). We found that in the first case the model degenerates into effective stiff fluid model of the universe. In the second case we obtained degenerate effective stiff fluid model as well as general bulk viscous models of the universe. Some physical and kinematical properties of the models are also discussed. 相似文献
11.
Alternative cosmologies, based on extensions of General Relativity, predict modified thermal histories in the early universe during the pre Big Bang Nucleosynthesis (BBN) era. When the expansion rate is enhanced with respect to the standard case, thermal relics typically decouple with larger relic abundances. In this paper, we study the dynamical evolution of an f(R) model of gravity in a homogeneous and anisotropic background which is given by a Bianchi type-I model of the universe filled with dark matter, which is described by a perfect fluid with a barotropic equation of state. As an example of a consistent analysis of modified gravity, we apply the formalism to a simple background solution of R+βR n gravity. Our analysis shows that f(R) cosmology allows dark matter masses lesser than 100 GeV, in the regime ρ c ?ρ m . We finally discuss how these limits apply to some specific realizations of standard cosmologies: an f(R) gravity model, Einstein frame model. 相似文献
12.
M. Farasat Shamir 《Astrophysics and Space Science》2010,330(1):183-189
The modified theories of gravity, especially the f(R) gravity, have attracted much attention in the last decade. In this context, we study the exact vacuum solutions of Bianchi
type I, III and Kantowski-Sachs spacetimes in the metric version of f(R) gravity. The field equations are solved by taking expansion scalar θ proportional to shear scalar σ which gives A=B
n
, where A and B are the metric coefficients. The physical behavior of the solutions has been discussed using some physical quantities. Also,
the function of the Ricci scalar is evaluated in each case. 相似文献
13.
14.
In this paper, we search the existence of Bianchi type I cosmological model in f(R,T) gravity, where the gravitational Lagrangian is given by an arbitrary function of the Ricci scalar R and of the trace of the stress-energy tensor T. We obtain the gravitational field equations in the metric formalism, and reconstruct the corresponding f(R,T) functions. Attention is attached to the special case, f(R,T)=f 1(R)+f 2(T) and two examples are assumed for this model. In the first example, we consider the unification of matter dominated and accelerated phases with f(R) gravity in anisotropic universe, and in the second instance, model of f(R,T) gravity with transition of matter dominated phase to the acceleration phase is obtained. In both cases, f(R,T) is proportional to a power of R with exponents depending on the input parameters. 相似文献
15.
Evolution of the universe is discussed in the framework of f(R) theory of gravity. The deceleration parameter is used to interpret various phases of the universe. We investigate the future evolution of the flat FRW universe by using observationally viable f(R) models. A numerical technique is applied to solve the evolution equation in terms of Hubble parameter which is used to explore late time acceleration of the universe. Some novel and interesting results based on the choice of coupling parameters in gravitational action are obtained. We can conclude that the considered f(R) models imply unification of matter dominated epoch with present accelerating phase of the universe. 相似文献
16.
《New Astronomy》2020
In the present article we propose a new hybrid shape function for wormhole (WH)s in the modified f(R, T) gravity. The proposed shape function satisfied the conditions of WH geometry. Geometrical behavior of WH solutions are discussed in both anisotropic and isotropic cases respectively. Also, the stability of this model is obtained by determining the equilibrium condition. The radial null energy condition and weak energy condition are validated in the proposed shape function indicating the absence of exotic matter in modified f(R, T) gravity. 相似文献
17.
18.
In this study, we consider a flat Friedmann-Robertson-Walker (FRW) universe in the context of Palatini f(R) theory of gravity. Using the dynamical equivalence between f(R) gravity and scalar-tensor theories, we construct a point Lagrangian in the flat FRW spacetime. Applying Noether gauge symmetry approach for this f(R) Lagrangian we find out the form of f(R) and the exact solution for cosmic scale factor. It is shown that the resulting form of f(R) yield a power-law expansion for the scale factor of the universe. 相似文献
19.
We consider a collapsing sphere and discuss its evolution under the vanishing expansion scalar in the framework of f(R) gravity. The fluid is assumed to be locally anisotropic which evolves adiabatically. To study the dynamics of the collapsing
fluid, Newtonian and post Newtonian regimes are taken into account. The field equations are investigated for a well-known
f(R) model of the form R+δR
2 admitting Schwarzschild solution. The perturbation scheme is used on the dynamical equations to explore the instability conditions
of expansionfree fluid evolution. We conclude that instability conditions depend upon pressure anisotropy, energy density
and some constraints arising from this theory. 相似文献
20.
In this paper, we have investigated the plane symmetric space-time with wet dark fluid (WDF), which is a candidate for dark energy, in the framework of f (R,T) gravity Harko et al. 2011, Phys. Rev. D, 84, 024020), where R and T denote the Ricci scalar and the trace of the energy–momentum tensor respectively. We have used the equation of state in the form of WDF for the dark energy component of the Universe. It is modeled on the equation of state p = ω(ρ ? ρ ?). The exact solutions to the corresponding field equations are obtained for power-law and exponential volumetric expansion. The geometrical and physical parameters for both the models are studied. Also, we have discussed the well-known astrophysical phenomena, namely the look-back time, proper distance, the luminosity distance and angular diameter distance with red shift. 相似文献