f(R) gravity and crossing the phantom divide barrier (original) (raw)

Crossing the phantom boundary in f(R) modified gravity: Jordan vs. Einstein frames

Gravitation and Cosmology, 2012

We study capability of f (R) gravity models to allow crossing the phantom boundary in both Jordan and Einstein conformal frames. In Einstein frame, these models are equivalent to Einstein gravity together with a scalar field minimally coupled to gravity. This scalar degree of freedom appears as a quintessence field with a coupling with the matter sector. We investigate evolution of the equation of sate parameter for some cosmologically viable f (R) gravity models in both conformal frames. This investigation (beyond mere theoretical arguments) acts as an operational tool to distinguish physical status of the two conformal frames. It shows that the two conformal frames have not the same physical status.

Future oscillations around phantom divide in f ( R ) gravity

Journal of Cosmology and Astroparticle Physics, 2011

It is known that scalar-tensor theory of gravity admits regular crossing of the phantom divide line w DE = −1 for dark energy, and existing viable models of present dark energy for its particular casef (R) gravity-possess one such crossing in the recent past, after the end of the matter dominated stage. It was recently noted that during the future evolution of these models the dark energy equation of state w DE may oscillate with an infinite number of phantom divide crossings. In this paper we present an analytical condition for the existence of this effect and investigate it numerically. With the increase of the present mass of the scalaron (the scalar particle appearing in f (R) gravity) beyond the border of the existence of such oscillations, their amplitude is shown to decrease very fast, so the effect quickly becomes very small even in the infinite future.

Phantom-like behavior in -gravity

Physics Letters B, 2009

We investigate possible realization of the phantom-like behavior in the framework of f (R)-gravity models where there are no phantom fields in the matter sector of the theory. By adopting some observationally reliable ansatz for f (R), we show that it is possible to realize phantom-like behavior in f (R)-gravity without introduction of phantom fields that suffer from instabilities and violation of the null energy condition. Depending on the choice of f (R), the null energy condition is fulfilled in some subspaces of each model parameter space. PACS: 04.50. Kd, 95.36.+x

Nonsingular phantom cosmology in Five Dimensional f(R,T) Gravity

We obtain exact solutions to the field equations for 5 dimensional LRS Bianchi type-I spacetime in f(R,T)f(R,T)f(R,T) theory of gravity where specifically the following three cases are considered: (i) f(R,T)=mu(R+T)f(R,T)=\mu(R+T)f(R,T)=mu(R+T), (ii) f(R,T)=Rmu+RTmu2f(R,T)=R \mu + R T \mu^2f(R,T)=Rmu+RTmu2 and (iii) f(R,T)=R+muR2+muTf(R,T)=R+\mu R^2+\mu Tf(R,T)=R+muR2+muT where RRR and TTT respectively the Ricci scalar and trace of the energy-momentum tensor. It is found that the equation of state (EOS) parameter www is governed by the parameter mu\mumu involved in the f(R,T)f(R,T)f(R,T) expressions. We fine-tune the parameter mu\mumu to obtain effect of phantom energy in the model, however we also restrict this parameter to obtain a stable model of the universe. It is noted that the model isotropizes at finite cosmic time.

On the Necessity of Phantom Fields for Solving the Horizon Problem in Scalar Cosmologies

On the Necessity of Phantom Fields for Solving the Horizon Problem in Scalar Cosmologies, 2019

We discuss the particle horizon problem in the framework of spatially homogeneous and isotropic scalar cosmologies. To this purpose we consider a Friedmann-Lemaître-Robertson-Walker (FLRW) spacetime with possibly non-zero spatial sectional curvature (and arbitrary dimension), and assume that the content of the universe is a family of perfect fluids, plus a scalar field that can be a quintessence or a phantom (depending on the sign of the kinetic part in its action functional). We show that the occurrence of a particle horizon is unavoidable if the field is a quintessence, the spatial curvature is non-positive and the usual energy conditions are fulfilled by the perfect fluids. As a partial converse, we present three solvable models where a phantom is present in addition to a perfect fluid, and no particle horizon appears.

$f({\sf R})$ Gravity Wormholes sourced by a Phantom Scalar Field

2021

We derive an exact wormhole spacetime supported by a phantom scalar field in the context of f(sfR)f({\sf R})f(sfR) gravity. Without specifying the form of the f(sfR)f({\sf R})f(sfR) function, the scalar field self-interacts with a mass term potential that is derived from the scalar equation and the resulting f(sfR)f({\sf R})f(sfR) model is purely supported by the scalar field and it is free of ghosts and avoids the tachyonic instability.

Behavior of Phantom Scalar Fields near Black Holes

2010

We present the accretion of a phantom scalar field into a black hole for various scalar field potentials in the full non-linear regime. Our results are based on the use of numerical methods and show that for all the cases studied the black hole's apparent horizon mass decreases. We explore a particular subset of the parameter space and from our results we conclude that this is a very efficient black hole shrinking process because the time scales of the area reduction of the horizon are short. We show that the radial equation of state of the scalar field depends strongly on the space and time, with the condition ω = p/ρ > −1, as opposed to a phantom fluid at cosmic scales that allows ω < −1.

Dynamical collapse of charged scalar field in phantom gravity

Physical Review D, 2012

We investigate the problem of the dynamical collapse of a self-gravitating complex charged scalar field in Einstein-Maxwell-dilaton theory with a phantom coupling for the adequate fields in the system under consideration. We also consider two simplifications, i.e., the separate collapses of phantom Maxwell and phantom scalar fields under the influence of Einstein gravity. One starts with the regular spacetime and leads the evolution through the formation of the horizons and the final singularity. We discuss the structures of spacetimes emerging in the process of the dynamical collapse and comment on the role of the considered fields in its course.

Fate of the phantom dark energy universe in semiclassical gravity. II. Scalar phantom fields

Physical Review D, 2012

Quantum corrections coming from massless fields conformally coupled with gravity are studied, in order to see if they can lead to avoidance of the annoying Big Rip singularity which shows up in a flat Friedmann-Robertson-Walker universe filled with dark energy and modeled by a scalar phantom field. The dynamics of the model are discussed for all values of the two parameters, named α > 0 and β < 0, corresponding to the regularization process. The new results are compared with the ones obtained in [1] previously, where dark energy was modeled by means of a phantom fluid with equation of state P = ωρ, with ω < −1.

A study of phantom scalar field cosmology using Lie and Noether symmetries

International Journal of Modern Physics D, 2016

The paper deals with phantom scalar field cosmology in Einstein gravity. At first, using Lie symmetry, the coupling function to the kinetic term and the potential function of the scalar field and the equation of state parameter of the matter field are determined and a simple solution is obtained. Subsequently, Noether symmetry is imposed on the Lagrangian of the system. The symmetry vector is obtained and the potential takes a very general form from which potential using Lie symmetry can be obtained as a particular case. Then, we choose a point transformation [Formula: see text] such that one of the transformed variables (say [Formula: see text]) is a cyclic for the Lagrangian. Using conserved charge (corresponding to the cyclic coordinate) and the constant of motion, solutions are obtained.