Acceleration of the universe in f(R) gravity models

A general formalism for the investigation of the late time dynamics of the universe for any analytic f(R) gravity model, along with a cold dark matter, has been discussed in the present work. The formalism is then elucidated with two examples. The values of the parameters of the models are chosen in such a way that they are consistent with the basic observational requirement.     

Kaluza-Klein universe with cosmic strings and bulk viscosity in f(R,T) gravity

A five dimensional Kaluza-Klein cosmological model is considered in the frame work of f(R,T) gravity proposed by Harko et al. (Phys. Rev. D 84:024020, 2011) when the source for energy momentum tensor is a bulk viscous fluid containing one dimensional cosmic strings. A barotropic equation of state is assumed to get a determinate solution of the field equations. Also, the bulk viscous pressure is assumed to be proportional to the energy density. The physical behavior of the model is also discussed.     

Reconstruction of f(R,T) gravity in anisotropic cosmological models of accelerating universe

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.     

A stable flat universe with variable cosmological constant in f(R,T) gravity

In this paper, a general FRW cosmological model has been constructed in f(R,T) gravity reconstruction with variable cosmological constant. A number of solutions to the field equations has been generated by utilizing a form for the Hubble parameter that leads to Berman's law of constant deceleration parameter q = m-1. The possible decelerating and accelerating solutions have been investigated. For(q 0) we get a stable flat decelerating radiation-dominated universe at q = 1. For(q 0) we get a stable accelerating solution describing a flat universe with positive energy density and negative cosmological constant. Nonconventional mechanisms that are expected to address the late-time acceleration with negative cosmological constant have been discussed.     

Energy conditions for Bianchi type I universe in f(G) gravity

In this paper, we discuss energy conditions in modified Gauss-Bonnet gravity for locally rotationally symmetric Bianchi type I universe model with perfect fluid. The matter contents are constructed to discuss the energy conditions bounds. We take two specific f(G) models along with present day values of Hubble, deceleration, jerk and snap parameters. It is found that weak and null energy conditions are satisfied while strong energy conditions are violated for both models which represents the accelerated expansion of the universe.     

Cylindrical thin-shell wormholes in f(R) gravity

In this paper, we employ cut and paste scheme to construct thin-shell wormhole of a charged black string with f(R) terms. We consider f(R) model as an exotic matter source at wormhole throat. The stability of the respective solutions are analyzed under radial perturbations in the context of R+δR ² model. It is concluded that both stable as well as unstable solutions do exist for different values of δ. In the limit δ→0, all our results reduce to general relativity.     

Neutron stars in generalized f(R) gravity

A gravity theory is considered with the Einstein-Hilbert Lagrangean R+aR ²+bR _μν R ^μν , R _μν being Ricci’s tensor and R the curvature scalar. The parameters a and b are taken of order 1 km². Arguments are given which suggest that the effective theory so obtained might be a fair approximation of a viable theory. A numerical integration is performed of the field equations for a free neutron gas. The result is that the star mass increases with increasing central density until about 1 solar mass and then decreases. The baryon number increases monotonically, which suggests that the theory allows stars in equilibrium with arbitrary baryon number, no matter how large.     

Static wormhole solutions in f(R) gravity

In this work, we study static spherically symmetric wormhole solutions in f(R) gravity. We explore wormhole solutions for anisotropic and isotropic fluids as well as barotropic equation of state with radial pressure. The behavior of weak and null energy conditions is investigated in each case. It is found that these energy conditions are violated for both the anisotropic and isotropic case but are satisfied for barotropic fluids in particular regions. This confirms the existence of wormholes obeying the energy conditions in these regions.     

Evolution of the information in the universe

Information in the universe evolves according to a nonlinear law, which results from a combination of nonlinear dynamics and quantum theory. The Kolmogoroff-Sinai entropy rate of the Universe evolves in inverse proportion to the temperature. Since the evolution of temperature is known from standard cosmology, the time dependence of the universal entropy rate can be determined and the temporal evolution of the universal information content can be calculated. Information starts growing at the enormous rate of 10⁴⁴ bits per second from an initial value of at least 2 (seven) bits at Planck time. However, it reaches its contemporary value only if one or more inflationary phases have been passed by the Universe in the course of its evolution.     

Noether gauge symmetry approach in f(R) gravity

We discuss the f(R) gravity model in which the origin of dark energy is identified as a modification of gravity. The Noether symmetry with gauge term is investigated for the f(R) cosmological model. By utilization of the Noether Gauge Symmetry (NGS) approach, we obtain two exact forms f(R) for which such symmetries exist. Further it is shown that these forms of f(R) are stable.     

Inflation in mimetic f(R,T) gravity

In this paper, we employ mimetic $f(R,T)$ gravity coupled with Lagrange multiplier and mimetic potential to yield viable inflationary cosmological solutions consistent with latest Planck and BICEP2/Keck Array data. We present here three viable inflationary solutions of the Hubble parameter ($H$) represented by $H\left(N\right)={(A\exp \beta N+B{\alpha }^N)}^{\gamma }$, $H\left(N\right)={(A{\alpha }^N+B\log N)}^{\gamma }$, and $H\left(N\right)={(A{e}^{\beta N}+B\log N)}^{\gamma }$, where $A$, $\beta$, $B$, $\alpha$, $\gamma$ are free parameters, and $N$ represents the number of e-foldings. We carry out the analysis with the simplest minimal $f(R,T)$ function of the form $f(R,T)=R+\chi T$, where $\chi$ is the model parameter. We report that for the chosen $f(R,T)$ gravity model, viable cosmologies are obtained compatible with observations by conveniently setting the Lagrange multiplier and the mimetic potential.