An Analysis for Conservative and Non-conservative f(R, T) Gravity Models (original) (raw)

f(R,T)=f(R)+λ T gravity models as alternatives to cosmic acceleration

2018

This article presents cosmological models that arise in a subclass of f(R,T)=f(R)+f(T) gravity models, with different f(R) functions and fixed T-dependence. That is, the gravitational lagrangian is considered as f(R,T)=f(R)+λ T, with constant λ. Here R and T represent the Ricci scalar and trace of the stress-energy tensor, respectively. The modified gravitational field equations are obtained through the metric formalism for the Friedmann-Lemaître-Robertson-Walker metric with signature (+,-,-,-). We work with f(R)=R+α R^2-μ^4/R, f(R)=R+k(γ R) and f(R)=R+me^[-nR], with α, μ, k, γ, m and n all free parameters, which lead to three different cosmological models for our Universe. For the choice of λ=0, this reduces to widely discussed f(R) gravity models. This manuscript clearly describes the effects of adding the trace of the energy-momentum tensor in the f(R) lagrangian. The exact solution of the modified field equations are obtained under the hybrid expansion law. Also we present the O...

Some aspects of generalized modified gravity models

2013

In the present work, we review some general aspects of modified gravity theories, investigating mathematical and physical properties and, more specifically, the feature of viable and realistic models able to reproduce the dark energy epoch and the early-time inflation. We will discuss the black hole solutions in generalized theories of gravity: it is of fundamental interest to understand how properties and laws of black holes in General Relativity can be addressed in the framework of modified theories. In particular, we will discuss the energy issue and the possibility to derive the First Law of thermodynamics from the field equations. Then, in the analysis of cosmological solutions, we will pay a particular attention to the occurrence of finite-time future singularities and to the possibility to avoid them in F (R, G)-gravity. Furthermore, realistic models of F (R)-gravity will be analyzed in the detail. A general feature occurring in matter era will be shown, namely the high derivatives of Hubble parameter may be influenced by the high frequency oscillation of the dark energy and some correction term is required in order to stabilize the theory at high redshift. The inflationary scenario is also carefully analyzed and an unified description of the universe is risen. In the final part of the work, we will have a look at the last developments in modified gravity, namely we will investigate cosmological and black hole solutions in a covariant field theory of gravity and we will introduce the extended "teleparallel" F (T )-gravity theories. A nice application to the dark matter problem will be presented.

A constrained cosmological model in f(R,Lm) gravity

New Astronomy

In this article, we study the expanding nature of universe in the contest of f (R, Lm) gravity theory, here R represents the Ricci scalar and Lm is the matter Lagrangian density. With a specific form of f (R, Lm), we obtain the field equations for flat FLRW metric. We parametrize the deceleration parameter in terms of the Hubble parameter and from here we find four free parameters, which are constraints and estimated by using H(z), P antheon, and their joint data sets. Further, we investigate the evolution of the deceleration parameter which depicts a transition from the deceleration to acceleration phases of the universe. The evolution behaviour of energy density, pressure, and EoS parameters shows that the present model is an accelerated quintessence dark energy model. To compare our model with the ΛCDM model we use some of the diagnostic techniques. Thus, we find that our model in f (R, Lm) gravity supports the recent standard observational studies and delineates the late-time cosmic acceleration.

CONSTRAINING ALTERNATIVE THEORIES OF GRAVITY WITH THE ENERGY CONDITIONS

The Eleventh Marcel Grossmann Meeting, 2008

Solar System tests give nowadays constraints on the estimated value of the cosmological constant, which can be accurately derived from different experiments regarding gravitational redshift, light deflection, gravitational time-delay and geodesic precession. Assuming that each reasonable theory of gravitation should satisfy Solar System tests we use this limits on the estimated value of the cosmological constant to constrain alternative theories of Gravity, which are nowadays studied as possible theories for cosmological models and provide viable solutions to the cosmological constant problem and the explanation of the present acceleration of the Universe. We obtain that the estimated values, from Solar System tests, for the parameters appearing in the alternative theories of Gravity are orders of magnitude bigger than the values obtained in the framework of cosmologically relevant theories.

Evaluation of cosmological models in f(R,T) gravity in different dark energy scenario

New Astronomy, 2022

In present paper, we search the existence of dark energy scalar field models within in f (R, T) gravity theory established by Harko et al. (Phys. Rev. D 84, 024020, 2011) in a flat FRW universe. The correspondence between scalar field models have been examined by employing new generalized dynamical cosmological term Λ(t). In this regards, the best fit observational values of parameters from three distinct sets data are applied. To decide the solution to field equations, a scale factor a = (sinh(βt)) 1/n has been considered, where β & n are constants. Here, we employ the recent ensues (H 0 = 69.2 and q 0 = −0.52) from (OHD+JLA) observation (Yu et al., Astrophys. J. 856, 3, 2018). Through the numerical estimation and graphical assessing of various cosmological parameters, it has been experienced that findings are comparable with kinematics and physical properties of universe and compatible with recent cosmological ensues. The dynamics and potentials of scalar fields are clarified in FRW scenario in the present model. Potentials reconstruction is highly reasonable and shows a periodic establishment and in agreement with latest observations.

Testing Some f(R,T) Gravity Models from Energy Conditions

Journal of Modern Physics, 2013

and are input parameters. We observe that by adjusting suitably these input parameters, energy conditions can be satisfied. Moreover, an analysis of the perturbations and stabilities of de Sitter solutions and power-law solutions is performed with the use of the two models. The results show that for some values of the input parameters, for which energy conditions are satisfied, de Sitter solutions and power-law solutions may be stables.

Some FRW models of accelerating universe with dark energy

Astrophysics and Space Science 332 (2011) 449-454 [arXiv:1010.0672v1 [gr-qc]]

The paper deals with a spatially homogeneous and isotropic FRW space-time filled with perfect fluid and dark energy components. The two sources are assumed to interact minimally, and therefore their energy momentum tensors are conserved separately. A special law of variation for the Hubble parameter proposed by Berman (Nuovo Cimento B 74:182, 1983) has been utilized to solve the field equations. The Berman’s law yields two explicit forms of the scale factor governing the FRW space-time and constant values of deceleration parameter. The role of dark energy with variable equation of state parameter has been studied in detail in the evolution of FRW universe. It has been found that dark energy dominates the universe at the present epoch, which is consistent with the observations. The physical behavior of the universe has been discussed in detail.

Sketch of a cosmological model based on the law of energy conservation

The European Physical Journal Plus, 2013

We suggest an approach for the description of gravitation fields based on the weak equivalence principle, where the rest mass of an object is decreased by the field as much as its static binding energy. The law of modification of the rest mass we introduce, in fact, serves as the energy conservation law in the gravitation field, as pointed out by the first author in his previous publications. Thus, this approach allows avoiding known ambiguities of the General Theory of Relativity (GTR) with respect to the energy of the gravitation field. We further indicate ways toward a covariant formulation of our approach; however, in the present contribution we use the limit of a weak gravitation field, in order to describe the evolution of the Universe at times sufficiently far from the classically presumed "Big Bang". Even along with this limitation, we demonstrate the efficiency of our approach, and determine a number of essential properties of the behavior of the Universe, which are thence based on just the law of energy conservation, and have a general character. In particular, we find out a very small positive (outward) acceleration for the expansion of the Modern Universe, which therefore constitutes a clue for the dark energy quest without involving the cosmological constant (whose value in quantum gravity has anyway a huge difference from that furnished by GTR). We also show that at the earlier stages of the Universe evolution, the acceleration might be negative. In addition, we find mainly a radially non-uniform exfoliation of the Universe (versus the classically assumed directionally uniform expanding Universe); thus we come out with a way of formation of galaxies along directions perpendicular to the direction of the expansion of the Universe. Finally, we derive the Hubble law along with a satisfactory calculation of the Hubble constant, and show that the linear dependence of the velocity v on a distance R represents, in effect, an approximation, which is not fulfilled for very large R. We also come up with an explanation, regarding the so-far-unanswered dispersion of data around the classical Hubble plot approximations.

Theoretical and observational constraints of viablef(R)theories of gravity

Physical Review D, 2016

Modified gravity has attracted much attention over the last few years and remains a potential candidate for dark energy. In particular, the so-called viable f (R) gravity theories, which are able to both recover General Relativity (GR) and produce late-time cosmic acceleration, have been widely studied in recent literature. Nevertheless, extended theories of gravity suffer from several shortcomings which compromise their ability to provide realistic alternatives to the standard cosmological ΛCDM Concordance model. We address the existence of cosmological singularities and the conditions that guarantee late-time acceleration, assuming reasonable energy conditions for standard matter in the so-called Hu-Sawicki f (R) model, currently among the most widely studied modifications to General Relativity. Then using the Supernovae Ia Union 2.1 catalogue, we further constrain the free parameters of this model. The combined analysis of both theoretical and observational constraints sheds some light on the viable parameter space of these models and the form of the underlying effective theory of gravity.