20 research outputs found
Inflationary f(R) cosmologies
This paper discusses a simple procedure to reconstruct f (R)-gravity models
from exact cosmological solutions of the Einstein field equations with a
noninteracting classical scalar field-and-radiation background. From the kind
of inflationary scenario we want, we show how the potential functions can be
obtained. We then show how an f (R) gravitational Lagrangian density that
mimics the same cosmological expansion as the scalar field-driven inflation of
General Relativity can be reconstructed. As a demonstration, we calculate the
slow-roll parameters (the spectral index n s and the tensor-to-scalar ratio r)
and compare them to the Planck data.Comment: 13 pages, 10 figure
Cosmological perturbations in f(G) gravity
We explore cosmological perturbations in a modified Gauss-Bonnet f(G)
gravity, using a 1+3 covariant formalism. In such a formalism, we define
gradient variables to get perturbed linear evolution equations. We transform
these linear evolution equations into ordinary differential equations using a
spherical harmonic decomposition method. The obtained ordinary differential
equations are time-dependent and then transformed into redshift dependent.
After these transformations, we analyze energy-density perturbations for
two-fluid systems, namely for a Gauss-Bonnet field-dust system and for a
Gaus-Bonnet field-radiation system for three different pedagogical f(G) models:
trigonometric, exponential, and logarithmic. For the Gauss-Bonnet field-dust
system, energy-density perturbations decay with an increase in redshift for all
three models. For the Gauss-Bonnet field-radiation system, the energy-density
perturbations decay with an increase in redshift for all of the three f(G)
models for long-wavelength modes whereas for short-wavelength modes, the
energy-density perturbations decay with increasing redshift for the logarithmic
and exponential f(G) models and oscillate with decreasing amplitude for the
trigonometric f(G) model.Comment: 32 pages, 9 figures. arXiv admin note: text overlap with
arXiv:1801.01758 by other author
Multifluid cosmology in f (G) gravity
The treatment of 1 + 3 covariant perturbation in a multifluid cosmology with
the consideration of f (G) gravity, G being the Gauss-Bonnet term, is done in
the present paper. We define a set of covariant and gauge-invariant variables
to describe density, velocity and entropy perturbations for both the total
matter and component fluids. We then use different techniques such as scalar
decomposition, harmonic decomposition, quasi-static approximation together with
the redshift transformation to get simplified perturbation equations for
analysis. We then discuss number of interesting applications like the case
where the universe is filled with a mixture of radiation and Gauss-Bonnet
fluids as well as dust with Gauss-Bonnet fluids for both short- and
long-wavelength limits. Considering polynomial f (G) model, we get numerical
solutions of energy density perturbations and show that they decay with
increase in redshift. This feature shows that under f (G) gravity, specifically
under the considered f (G) model, one expects that the formation of the
structure in the late Universe is enhanced.Comment: 45 pages, 4 figure
Accelerating universe in modified teleparallel gravity theory
This paper studies the cosmology of accelerating expansion of the universe in
modified teleparallel gravity theory. We discuss the cosmology of
gravity theory and its implication to the new general form of the equation of
state parameter for explaining the late-time accelerating expansion of
the universe without the need for the cosmological constant scenario. We
examine the numerical value of in different paradigmatic
gravity models. In those models, the numerical result of is favored
with observations in the presence of the torsion scalar T associated with a
boundary term B and shows the accelerating expansion of the universe.Comment: Conference proceeding: Nuclear Activity in Galaxies Across Cosmic
Time" (Ethiopia) accepted for publishing under the Cambridge University
Press, eds. M. Povic, P. Marziani, J. Masegosa, H. Netzer, S. H. Negu, and S.
B. Tessem
On 1 + 3 covariant perturbations of the quasi-Newtonian space-time in modified Gauss-Bonnet gravity
The consideration of a 1 + 3 covariant approach to cold dark matter universe
with no shear cosmological dust model with irrotational flows is developed in
the context of f (G) gravity theory in the present study. This approach reveals
the existence of integrability conditions which do not appear in non-covariant
treatments. We constructed the integrability conditions in modified
Gauss-Bonnet f (G) gravity basing on the constraints and propagation equations.
These integrability conditions reveal the linearized silent nature of
quasi-Newtonian models in f (G) gravity. Finally, the linear equations for the
overdensity and velocity perturbations of the quasi-Newtonian space-time were
constructed in the context of modified f (G) gravity. The application of
harmonic decomposition and redshift transformation techniques to explore the
behaviour of the overdensity and velocity perturbations using f (G) model were
made. On the other hand we applied the quasi-static approximation to study the
approximated solutions on small scales which helps to get both analytical and
numerical results of the perturbation equations. The analysis of the energy
overdensity and velocity perturbations for both short and long wavelength modes
in a dust-Gauss-Bonnet fluids were done and we see that both energy overdensity
and velocity perturbations decay with redshift for both modes. In the limits to
{\Lambda}CDM , it means f (G) = G the considered f (G) model results coincide
with {\Lambda}CDM .Comment: This manuscript is accepted for publication in the International
Journal of Modern Physics D(IJMPD). 28 pages, 9 figure