Stability of LRS Bianchi type-I cosmological models in f(R,T)-gravity

This paper examines the stability of the transition from the early decelerating stage of the Universe to the recent accelerating stage for the perfect fluid cosmological locally rotationally symmetric(LRS) Bianchi-I model in f(R, T) theory. To determine the solution of field equations, the idea of a timevarying deceleration parameter(DP) which yields a scale factor, for which the Universe attains a phase transition scenario and is consistent with recent cosmological observations, is used. The time-dependent DP yields a scale factor a=exp■, where β and k are respectively arbitrary and integration constants. By using the recent cons_traints(H_0 _= 73.8, and q_0 =-0.54) from Type Ia Supernova(SN Ia) data in combination with Baryonic Acoustic Oscillations(BAO) and Cosmic Microwave Background(CMB) observations(Giostri et al.), we obtain the values of β = 0.0062 and k = 0.000016 for which we have derived a cosmological model from the early decelerated phase to the present accelerating phase. By applying_ other r_ecent constraints(H_0 = 73.8, q_0 =-0.73) from SNe Ia Union data(Cunha), we obtain the values of β = 0.0036 and k = 0.000084 for which we have derived a cosmological model in the accelerating phase only. We have compared both models with experimental data. The stability of the background solution has been examined also for the metric perturbations alongside the properties of future singularities in a Universe ruled by dark energy with phantom type fluid. We demonstrate the presence of a stable fixed point with a condition of state ω <-1 and numerically affirm this is really a late-time attractor in the ghost overwhelmed Universe. Some physical and geometric properties of the model are found and examined.

On the Thermodynamical Black Hole Stability in the Space-Time of a Global Monopole in f(R)-Gravity

In this work, we re-assess a class of black hole solutions in a global monopole spacetime in the framework of an f(R)-gravity model. Our main line of investigation consists in considering a region close enough to the black hole, but such that the weak field approximation is still valid. The stability of the black hole is studied in terms of its thermodynamical properties, with the radial coordinate written as a power-law function with the status of the main factor underneath the stability of the model. We obtain the explicit expressions for the thermodynamical quantities of the black hole as functions of the event horizon, by considering both the Hawking and the local temperatures. The phase transitions that may occur in this system, including the Hawking-Page phase transition, are inspected with particular attention. We work out and contemplate a solution of special interest in which one of the parameters is related to the cosmological constant. Our main result sets out to establish a comparison between both the Hawking and the local formalisms for the black hole in the framework of the f(R)-gravity in the particular space-time adopted here.

Phantom and Quintessence Fields Coupled to Scalar Curvature in General <i>f(R)</i>Gravity Theory

This paper reviews the development of f(R) gravity theory and Phantom and Quintessence fields. Specifically, we present a new general action of f(R) gravity and Phantom and Quintessence fields coupled to scalar curvature. Then, we deduce Euler-Lagrange Equations of different fields, matter tensor and effective matter tensor. Additionally, this paper obtains the general pressure, density and speed sound of the new general field action, and investigates different cosmological evolutions with inflation. Further, this paper investigates a general f(R) gravity theory with a general matter action and obtains the different field equations, general matter tensor and effective matter tensor. Besides, this paper obtains the effective Strong Energy Condition (SEC) and effective Null Energy Condition (NEC). Then, we prove that when f(R) approaches to R, the effective SEC and the effective NEC approach to the usual SEC and the usual NEC, respectively. Finally, this paper presents a general action of f(R) gravity, Quintessence and Phantom fields and their applications.

f(R)引力理论及其解的研究

首先介绍了f(R)引力理论的发展,然后引入了f(R)引力度规形式及引力场方程,重点分析了f(R)引力理论与标量—张量理论及其联系.最后我们推导了度规f(R)理论的局部解与宇宙解.

Brans-Dicke理论和f(R)理论中的可穿越虫洞

Brans-Dicke理论和f(R)理论均为不同于Einstein广义相对论的修正引力理论,并且在Brans-Dicke理论参数ω=0时能够回到f(R)理论.人们在Brans-Dicke理论框架下研究得出只有-3/2<ω<-4/3才存在可穿越虫洞,ω=0时没有;然而,f(R)理论中已经证明确实也存在可穿越虫洞.为了解决上述所谓的矛盾,有文献修改了Brans-Dicke理论中的某些函数,从而改变ω的取值范围.仔细分析发现,文献中探讨的是对应不同Φ(r),b(r)的可穿越虫洞,自然会给出不一致的结论.本文对2种修正引力理论中的可穿越虫洞问题进行了澄清,明确了各自虫洞解的讨论前提条件.

Kerr-Ads和KN-Ads解在f(R)引力中的解析推广

本文将Carter计算旋转黑洞的方法推广到f(R)引力,并解析计算得到了Kerr-Ads和KN-Ads解.Carter认为,测试粒子在黑洞时空背景中运动的过程要求Klein-Gordon方程是可分离变量的.该物理条件会反过来约束时空度规的形式,使其成为可分离正则度规形式.基于该度规形式并假定Ricci标量为常数,本文分别计算得到了f(R)引力中的旋转不带电黑洞和旋转带电黑洞.本文的所有计算过程都是严格的解析推导.在计算Einstein张量时,我们采用了活动标架法,并利用了Cartan结构方程.为了能够快速简洁地得到解析解,我们主要求解了Einstein方程中的3个分量,并给出了这3个方程的尝试解.值得注意的是,本文没有选取特定的f(R)模型,计算是在最一般的f(R)理论中进行.该解析计算过程是普适的,能够应用于其他修正引力.

Exploring physical properties of compact stars in f(R,T)-gravity:An embedding approach

Solving field equations exactly in f(R,T)−gravity is a challenging task.To do so,many authors have adopted different methods such as assuming both the metric functions and an equation of state(EoS)and a metric function.However,such methods may not always lead to well-behaved solutions,and the solutions may even be rejected after complete calculations.Nevertheless,very recent studies on embedding class-one methods suggest that the chances of arriving at a well-behaved solution are very high,which is inspiring.In the class-one approach,one of the metric potentials is estimated and the other can be obtained using the Karmarkar condition.In this study,a new class-one solution is proposed that is well-behaved from all physical points of view.The nature of the solution is analyzed by tuning the f(R,T)−coupling parameterχ,and it is found that the solution leads to a stiffer EoS forχ=−1 than that forχ=1.This is because for small values ofχ,the velocity of sound is higher,leading to higher values of Mmax in the M−R curve and the EoS parameterω.The solution satisfies the causality condition and energy conditions and remains stable and static under radial perturbations(static stability criterion)and in equilibrium(modified TOV equation).The resulting M−R diagram is well-fitted with observed values from a few compact stars such as PSR J1614-2230,Vela X-1,Cen X-3,and SAX J1808.4-3658.Therefore,for different values ofχ,the corresponding radii and their respective moments of inertia have been predicted from the M−I curve.

Effect of arbitrary matter-geometry coupling on thermodynamics in f(R)theories of gravity

In this paper,the thermodynamics of the Friedmann–Lemait re–Robertson–Walker universe have been explored in f(R)theories of gravity with arbitrary matter-geometry coupling.The equivalence between the modified Friedmann equations with any spatial curvature and the first law of thermodynamics is confirmed,where the assumption of the entropy plays a crucial role.Then laws of thermodynamics in our considering case are obtained.They can reduce to the ones given in Einstein's general theory of relativity under certain conditions.Moreover,a particular model is investigated through the obtained generalized second law of thermodynamics with observational results of cosmographic parameters.

弗雷德里克·詹姆逊的文学观和文学批评理论

追溯了"文化批评家"、"后现代理论家"弗雷德里克.詹姆逊的文学思想发展,从他的系列文化研究经典论述中挖掘其卓具特色的文学观点和文学批评理论,指出他的文学观是建立在马克思的辩证法基础上的开放、多元、具有包容性的阐释体系,他的文学批评理论辟出了一条文学与社会,尤其是与生产方式相联系的新途径,对当前的文学批评和文化研究都有很大的启示。

f(R)引力理论中水星进动方程求解

f(R)引力理论是对爱因斯坦引力理论进行修正的最热门理论之一,水星进动是对引力理论考察的经典方式.通过四维速度的归一化条件,得到特定f(R)引力的运动方程,并对方程各项进行数量级估计,再对该方程做各种简化处理,最后由计算机编程求出数值解.结果表明,由f(R)引力理论水星进动方程给出的进动值与实验观测值相吻合.

Comprehensive analysis of relativistic embedded class-I exponential compact spheres in f(R,φ)gravity via Karmarkar condition

In this article,we use the prominent Karmarkar condition to investigate some novel features of astronomical objects in the f(R,φ)gravity;R andφrepresent the Ricci curvature and the scalar field,respectively.It is worth noting that we classify the exclusive set of modified field equations using the exponential type model of the f(R,φ)theory of gravity f(R,φ)=φ(R+α(eβR-1)).We show the embedded class-I approach via a static,spherically symmetric spacetime with an anisotropic distribution.To accomplish our objective,we use a particular interpretation of metric potential(grr)that has already been given in the literature and then presume the Karmarkar condition to derive the second metric potential.We employ distinct compact stars to determine the values of unknown parameters emerging in metric potentials.To ensure the viability and consistency of our exponential model,we execute distinct physical evolutions,i.e.the graphical structure of energy density and pressure evolution,mass function,adiabatic index,stability,equilibrium,and energy conditions.Our investigation reveals that the observed anisotropic findings are physically appropriate and have the highest level of precision.

Instability of a black hole with f(R) global monopole under extended uncertainty principle

We consider the evolution of a black hole involving an f(R)global monopole based on the Extended Uncertainty Principle(EUP).The black hole evolutions refer to the instability due to the Parikh-Kraus-Wilczeck tunneling radiation or fragmentation.It is found that the EUP corrections make the entropy difference larger to encourage the black hole to radiate more significantly.We also show that the appearance of the EUP effects results in the black hole’s division.The influence from the global monopole and the revision of general relativity can also adjust the black hole evolution simultaneously but cannot change the final result that the black hole will not be stable because of the EUP’s effects.

从利维斯对艾略特诗歌的批评看利维斯的诗学观

相对于利维斯的小说批评,其诗歌批评被讨论得较少。事实上,利维斯的诗歌批评也描绘出了英诗的"伟大的传统",特别是他对以艾略特为代表的现代主义诗歌的甄别和评价,推动了一批新诗人被认可和接受的进程。利维斯一方面继承传统,另一方面又极具现代性意识,他对艾略特诗歌的批评不仅最早树立了艾略特在诗坛上的不朽地位,而且也玉成了他本人在现代主义诗歌批评史上的特殊地位。本文考察了利维斯对艾略特诗歌近半个世纪的研究与批评,进而揭示利维斯的现代主义诗学观。这样的尝试是为了更深刻全面地理解利维斯的整个诗学思想体系和文学观,也是艾略特研究不可忽视的一环。