1,720,989 research outputs found
Cosmological Chaplygin gas as modified gravity
Full text linkIn the scramble for the understanding of the nature of dark matter and dark energy, it has recently been suggested that the change of behavior of the missing energy density might be regulated by the change in the equation of state of the background fluid. The Chaplygin Gas (CG) model in cosmology is one of the most profound candidates for this suggestion. This work aims to bring to light a geometric interpretation of the model by re-writing the difierent toy models in terms of exact f(R) gravity solutions that are generally quadratic in the Ricci scalar with appropriate ΛCDM limiting solutions
On f(R) gravity in scalar–tensor theories
No full text We study [Formula: see text] gravity models in the language of scalar–tensor (ST) theories. The correspondence between [Formula: see text] gravity and ST theories is revisited since [Formula: see text] gravity is a subclass of Brans–Dicke models, with a vanishing coupling constant ([Formula: see text]). In this treatment, four [Formula: see text] toy models are used to analyze the early-universe cosmology, when the scalar field [Formula: see text] dominates over standard matter. We have obtained solutions to the Klein–Gordon equation for those models. It is found that for the first model [Formula: see text], as time increases the scalar field decreases and decays asymptotically. For the second model [Formula: see text], it was found that the function [Formula: see text] crosses the [Formula: see text]-axis at different values for different values of [Formula: see text]. For the third model [Formula: see text], when the value of [Formula: see text] is small, the potential [Formula: see text] behaves like the standard inflationary potential. For the fourth model [Formula: see text], we show that there is a transition between [Formula: see text]. The behavior of the potentials with [Formula: see text] is totally different from those with [Formula: see text]. The slow-roll approximation is applied to each of the four [Formula: see text] models and we obtain the respective expressions for the spectral index [Formula: see text] and the tensor-to-scalar ratio [Formula: see text]. </jats:p
The Chaplygin gas as a model for modified teleparallel gravity?
Full text linkThis paper explores the possibility of treating the exotic Chaplygin-gas (CG) fluid model as some manifestation of an f(T) gravitation. To this end, we use the different cosmological CG equations of state, compare them with the equation of state for the modified teleparallel gravity and reconstruct the corresponding Lagrangian densities. We then explicitly derive the equation of state parameter of the torsion fluid and study its evolution for vacuum-torsion, radiation-torsion, dust-torsion, stiff fluid-torsion and radiation-dust-torsion multi-fluid systems. The obtained Lagrangians have, in general, matter dependence due to the matter-torsion coupling appearing in the energy density and pressure terms of the modified teleparallel gravity theory. For the simplest CG models, however, it is possibly to reconstruct f(T) Lagrangians that depend explicitly on the torsion scalar T only. The preliminary results show that, in addition to providing Chaplygin-gas-like solutions to the modified teleparallel gravitation, which naturally behave like dark matter and dark energy at early and late times respectively, the technique can be used to overcome some of the challenges attributed to the CG cosmological alternativ
Chaplygin-gas solutions of f(R) gravity
Full text linkWe explore exact f(R) gravity solutions that mimic Chaplygin-gas inspired CDM cosmology. Starting with the original, generalized and modified Chaplygin gas equations of state, we reconstruct the forms of f(R) Lagrangians. The resulting solutions are generally quadratic in the Ricci scalar, but have appropriate CDM solutions in limiting cases. These solutions, given appropriate initial conditions, can be potential candidates for scalar field-driven early universe expansion (inflation) and dark energy-driven late-time cosmic acceleration
Shear-free anisotropic cosmological models in f (R) gravity
No full textWe study a class of shear-free, homogeneous but anisotropic cosmological models with imperfect matter sources in the context of f(R) gravity. We show that the anisotropic stresses are related to the electric part of the Weyl tensor in such a way that they balance each other. We also show that within the class of orthogonal f(R) models, small perturbations of shear are damped, and that the electric part of the Weyl tensor and the anisotropic stress tensor decay with the expansion as well as the heat flux of the curvature fluid. Specializing in locally rotationally symmetric spacetimes in orthonormal frames, we examine the late-time behaviour of the de Sitter universe in f(R) gravity. For the Starobinsky model of f(R), we study the evolutionary behavior of the Universe by numerically integrating the Friedmann equation, where the initial conditions for the expansion, acceleration and jerk parameters are taken from observational data
Integrability conditions of quasi-Newtonian cosmologies in modified gravity
No full textWe investigate the integrability conditions of a class of shear-free perfect-fluid cosmological models within the framework of anisotropic fluid sources, applying our results to f(R) dark energy models. Generalizing earlier general relativistic results for timelike geodesics, we extend the potential and acceleration terms of the quasi-Newtonian formulation of integrable dust cosmological models about a linearized Friedmann–Lemaître–Robertson–Walker background and derive the equations that describe their dynamical evolutions. We show that in general, models with an anisotropic fluid source are not consistent, but because of the particular form the anisotropic stress πab takes in f(R) gravity, the general integrability conditions in this case are satisfied
Anisotropic solutions in modified gravity
Full text linkAnisotropic but homogeneous, shear-free cosmological models with imperfect matter sources in f(R) gravity are investigated. The relationship between the anisotropic stresses and the electric part of the Weyl tensor, as well as their evolutions in orthogonal f(R) models, is explored. The late-time behaviour of the de Sitter universe (as an example of a locally rotationally symmetric spacetimes in orthonormal frames) in f(R) gravity is examined. By taking initial conditions for the expansion, acceleration and jerk parameters from observational data, numerical integrations for the evolutionary behavior of the Universe in the Starobinsky model of f(R) have been carried out
A dynamical systems analysis of interacting dark energy models
Full text linkMSc (Astrophysical Sciences), North-West University, Potchefstroom CampusWe investigate using dynamical system analysis; the impacts of
various interaction models whereby dark energy is coupled with dark
matter. Examination on the nature of critical points for each interaction
model introduced was conducted in order to obtain the cosmological
consequence of each choice of interaction, with all the components
of the universe considered, namely, the radiation, matter, and
dark energy dominated universes. The existence of unstable radiation
epoch, unstable dark matter epoch, and stable dark energy epoch
will be shown for models displaying cosmologically acceptable results.
Using the value of the scale factor at equality we determine
the time at which the matter-dark energy equality occurred for each
model. Constraints on the coupling constant b of the interacting dark
energy models were placed in order to determine the phantom or
quintessence behaviour of the equation of state for dark energy. We
do model comparison and also compare with the LCDM, so as to filter
our models for the best possible form of interaction term between
dark matter and dark energy.Master
Dark interactions beyond the Lambda-CDM model
Full text linkMSc (Astrophysical Sciences), North-West University, Potchefstroom CampusIn this study, cosmological models are considered, where dark matter and dark energy are coupled
and may exchange energy through non-gravitational interactions with one other. These interacting
dark energy (IDE) models are introduced to address problems with the standard ΛCDM model of
cosmology, in which dark energy is assumed to be a cosmological constant. The central problem
addressed in this study is the cosmic coincidence problem (regarding the presently measured coin-
cidental ratio of dark matter to dark energy). Assuming two different linear dark energy couplings,
Q1 = δHρdm and Q2 = δHρde, we find that interacting dark energy models may alleviate and even
solve the cosmic coincidence problem by stabilising the ratio of dark matter to dark energy in both
the past and future. Furthermore, we examine how these dark interactions affect crucial events in
the expansion history of the universe. These events include the big bang and cosmic acceleration,
as well as the radiation-matter and matter-dark energy equality.
Besides studying the cosmological consequences of an interaction between the dark sectors, we
also investigate the viability of IDE models on both theoretical and observational grounds. For
both models considered, we find that negative energy densities are inevitable if energy flows from
dark matter to dark energy and that consequently we should only seriously consider models where
energy flows from dark energy to dark matter. To additionally ensure that these models are free
from early time instabilities, we need to require that dark energy is in the ‘phantom’ (ω < −1)
regime. This has the consequence that model Q1 = δHρdm will end with a future big rip singularity,
while Q2 = δHρde may avoid this fate with the right choice of cosmological parameters. Cosmo-
logical parameters for these models are obtained from type-Ia supernovae data using a previously
developed Markov Chain Monte-Carlo (MCMC) simulation. The predicted expansion history from
these models are then statistically compared to the supernovae data and the ΛCDM model, where
we find that Q1 = δHρdm is statistically rejected, while Q2 = δHρde may be considered viable.Master
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