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Register a new userA cosmological scenario from the Starobinsky model within the $f(R,T)$ formalism
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Pardyumn Kumar Sahoo
Publication date
2017
fields
Physics
and research's language is
English
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In this paper we derive a novel cosmological model from the $f(R,T)$ theory of gravitation, for which $R$ is the Ricci scalar and $T$ is the trace of the energy-momentum tensor. We consider the functional form $f(R,T)=f(R)+f(T)$, with $f(R)$ being the Starobinksy model, named $R+alpha R^{2}$, and $f(T)=2gamma T$, with $alpha$ and $gamma$ being constants. We show that a hybrid expansion law form for the scale factor is a solution for the derived Friedmann-like equations. In this way, the model is able to predict both the decelerated and the accelerated regimes of expansion of the universe, with the transition redshift between these stages being in accordance with recent observations. We also apply the energy conditions to our material content solutions. Such an application makes us able to obtain the range of acceptability for the free parameters of the model, named $alpha$ and $gamma$.
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Recent elaborated by T. Harko and collaborators, the $f(R,T)$ theories of gravity contemplate an optimistic alternative to dark energy, for which $R$ and $T$ stand for the Ricci scalar and the trace of the energy-momentum tensor, respectively. Although the literature has shown that the $T$ dependence on the gravitational part of the action - which is due to the consideration of quantum effects - may induce some novel features in the scope of late-time cosmological dynamics, in the radiation-dominated universe, when $T=0$, no contributions seem to rise from such theories. Apparently, $f(R,T)$ contributions to a radiation-dominated universe may rise only from the $f(R,T^varphi)$ approach, which is nothing but the $f(R,T)$ gravity in the case of a self-interacting scalar field whose trace of the energy-momentum tensor is $T^varphi$. We intend, in this article, to show how $f(R,T^varphi)$ theories of gravity can contribute to the study of the primordial stages of the universe. Our results predict a graceful exit from inflationary stage to a radiation-dominated era. They also predict a late-time cosmic acceleration after a matter-dominated phase, making the $f(R,T^varphi)$ theories able to describe, in a self-consistent way, all the different stages of the universe dynamics.
We classify singularities in FRW cosmologies, which dynamics can be reduced to the dynamical system of the Newtonian type. This classification is performed in terms of geometry of a potential function if it has poles. At the sewn singularity, which is of a finite scale factor type, the singularity in the past meets the singularity in the future. We show, that such singularities appear in the Starobinsky model in $f(hat{R})=hat{R}+gamma hat{R}^2$ in the Palatini formalism, when dynamics is determined by the corresponding piece-wise smooth dynamical system. As an effect we obtain a degenerated singularity. Analytical calculations are given for the cosmological model with matter and the cosmological constant. The dynamics of model is also studied using dynamical system methods. From the phase portraits we find generic evolutionary scenarios of the evolution of the Universe. For this model, the best fit value of $Omega_gamma=3gamma H_0^2$ is equal $9.70times 10^{-11}$. We consider model in both Jordan and Einstein frames. We show that after transition to the Einstein frame we obtain both form of the potential of the scalar field and the decaying Lambda term.
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 $ Lambda(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= left(sinh(beta t)right)^{1/n} $ has been considered, where $ beta$ & $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.
The recent article entitled Cosmological inviability of $f(R,T)$ gravity [Phys. Rev. D 95 (2017) 123536], by H. Velten and T.R.P. Caram^es, claims that the reference A transition from a decelerated to an accelerated phase of the universe expansion from the simplest non-trivial polynomial function of T in the f(R,T) formalism by P.H.R.S. Moraes, G. Ribeiro and R.A.C. Correa [Astrophys. Space Sci. 361 (2016) 227] has problematic points concerning its mathematical approach and observable consequences. Velten and Caram^es argue that the equation of the scale factor evolution in time in the $f(R,T)=R+alpha T+beta T^{2}$ cosmology was erroneously calculated. One crucial consequence of the supposed corrected version of such an equation, presented by the authors in [Phys. Rev. D 95 (2017) 123536], would be the absence of the transition from a decelerated to an accelerated phase of the expansion of the universe, an outcome originally predicted by Moraes, Ribeiro and Correa. We show that the above claim is incorrect and that there are no inconsistencies with the results by Moraes, Ribeiro and Correa in the referred work. In particular, we show that Velten and Caram^es have incorrectly performed their calculations, invalidating all their mathematical and physical criticism regarding the article by Moraes, Ribeiro and Correa. In addition, we quote that the solutions obtained by Velten and Caram^es are unfeasible in view of their mathematical misunderstanding.
A plane symmetric Bianchi-I model is explored in $f(R,T)$ gravity, where $R$ is the Ricci scalar and $T$ is the trace of energy-momentum tensor. The solutions are obtained with the consideration of a specific Hubble parameter which yields a constant deceleration parameter. The various evolutionary phases are identified under the constraints obtained for physically viable cosmological scenarios. Although a single (primary) matter source is taken, due to the coupling between matter and $f(R,T)$ gravity, an additional matter source appears, which mimics a perfect fluid or exotic matter. The solutions are also extended to the case of a scalar field model. The kinematical behavior of the model remains independent of $f(R,T)$ gravity. The physical behavior of the effective matter also remains the same as in general relativity. It is found that $f(R,T)$ gravity can be a good alternative to the hypothetical candidates of dark energy to describe the present accelerating expansion of the universe.
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