1,069 research outputs found

### Reconstructing the Equation of State for Dark Energy In the Double Complex Symmetric Gravitational Theory

We propose to study the accelerating expansion of the universe in the double
complex symmetric gravitational theory (DCSGT). The universe we live in is
taken as the real part of the whole spacetime ${\cal M}^4_C(J)$ which is double
complex. By introducing the spatially flat FRW metric, not only the double
Friedmann Equations but also the two constraint conditions $p_J=0$ and $J^2=1$
are obtained. Furthermore, using parametric $D_L(z)$ ansatz, we reconstruct the
$\omega^{'}(z)$ and $V(\phi)$ for dark energy from real observational data. We
find that in the two cases of $J=i,p_J=0$ and $J=\epsilon,p_J\neq 0$, the
corresponding equations of state $\omega^{'}(z)$ remain close to -1 at present
($z=0$) and change from below -1 to above -1. The results illustrate that the
whole spacetime, i.e. the double complex spacetime ${\cal M}^4_C(J)$, may be
either ordinary complex ($J=i,p_J=0$) or hyperbolic complex
($J=\epsilon,p_J\neq 0$). And the fate of the universe would be Big Rip in the
future.Comment: 5 pages, 5 figures, accepted by Commun. Theor. Phy

### Unexorcized ghost in DGP brane world

The braneworld model of Dvali-Gabadadze-Porrati realizes the
self-accelerating universe. However, it is known that this cosmological
solution contains a spin-2 ghost. We study the possibility of avoiding the
appearance of the ghost by slightly modifying the model, introducing the second
brane. First we consider a simple model without stabilization of the separation
of the brane. By changing the separation between the branes, we find we can
erase the spin-2 ghost. However, this can be done only at the expense of the
appearance of a spin-0 ghost instead. We discuss why these two different types
of ghosts are correlated. Then, we examine a model with stabilization of the
brane separation. Even in this case, we find that the correlation between
spin-0 and spin-2 ghosts remains. As a result we find we cannot avoid the
appearance of ghost by two-branes model.Comment: 19 pages, 1 figur

### Spherically symmetric vacuum solutions of modified gravity theory in higher dimensions

In this paper we investigate spherically symmetric vacuum solutions of $f(R)$
gravity in a higher dimensional spacetime. With this objective we construct a
system of non-linear differential equations, whose solutions depend on the
explicit form assumed for the function $F(R)=\frac{df(R)}{dR}$. We explicit
show that for specific classes of this function exact solutions from the field
equations are obtained; also we find approximated results for the metric tensor
for more general cases admitting $F(R)$ close to the unity.Comment: 14 pages, no figure. New version accepted for publication in EPJ

### Cosmological model with interactions in the dark sector

A cosmological model is proposed for the current Universe consisted of
non-interacting baryonic matter and interacting dark components. The dark
energy and dark matter are coupled through their effective barotropic indexes,
which are considered as functions of the ratio between their energy densities.
It is investigated two cases where the ratio is asymptotically stable and their
parameters are adjusted by considering best fits to Hubble function data. It is
shown that the deceleration parameter, the densities parameters, and the
luminosity distance have the correct behavior which is expected for a viable
present scenario of the Universe.Comment: 6 pages, 8 figure

### Cosmological expansion and local systems: a Lema\^{i}tre-Tolman-Bondi model

We propose a Lema\^{i}tre-Tolman-Bondi system mimicking a two-body system to
address the problem of the cosmological expansion versus local dynamics. This
system is strongly bound but participates in the cosmic expansion and is
exactly comoving with the cosmic substratum

### Dark Viscous Fluid coupled with Dark Matter and future singularity

We study effects of viscous fluid coupled with dark matter in our universe.
We consider bulk viscosity in the cosmic fluid and we suppose the existence of
a coupling between fluid and dark matter, in order to reproduce a stable de
Sitter universe protected against future-time singularities. More general
inhomogeneous fluids are studied related to future singularities.Comment: 11 page

### Phenomenology of $\Lambda$-CDM model: a possibility of accelerating Universe with positive pressure

Among various phenomenological $\Lambda$ models, a time-dependent model $\dot
\Lambda\sim H^3$ is selected here to investigate the $\Lambda$-CDM cosmology.
Using this model the expressions for the time-dependent equation of state
parameter $\omega$ and other physical parameters are derived. It is shown that
in $H^3$ model accelerated expansion of the Universe takes place at negative
energy density, but with a positive pressure. It has also been possible to
obtain the change of sign of the deceleration parameter $q$ during cosmic
evolution.Comment: 16 Latex pages, 11 figures, Considerable modifications in the text;
Accepted in IJT

### Model for a Universe described by a non-minimally coupled scalar field and interacting dark matter

In this work it is investigated the evolution of a Universe where a scalar
field, non-minimally coupled to space-time curvature, plays the role of
quintessence and drives the Universe to a present accelerated expansion. A
non-relativistic dark matter constituent that interacts directly with dark
energy is also considered, where the dark matter particle mass is assumed to be
proportional to the value of the scalar field. Two models for dark matter
pressure are considered: the usual one, pressureless, and another that comes
from a thermodynamic theory and relates the pressure with the coupling between
the scalar field and the curvature scalar. Although the model has a strong
dependence on the initial conditions, it is shown that the mixture consisted of
dark components plus baryonic matter and radiation can reproduce the expected
red-shift behavior of the deceleration parameter, density parameters and
luminosity distance.Comment: 11 pages and 6 figures. To appear in GR

### Observational Constraints of Modified Chaplygin Gas in Loop Quantum Cosmology

We have considered the FRW universe in loop quantum cosmology (LQC) model
filled with the dark matter (perfect fluid with negligible pressure) and the
modified Chaplygin gas (MCG) type dark energy. We present the Hubble parameter
in terms of the observable parameters $\Omega_{m0}$, $\Omega_{x0}$ and $H_{0}$
with the redshift $z$ and the other parameters like $A$, $B$, $C$ and $\alpha$.
From Stern data set (12 points), we have obtained the bounds of the arbitrary
parameters by minimizing the $\chi^{2}$ test. The best-fit values of the
parameters are obtained by 66%, 90% and 99% confidence levels. Next due to
joint analysis with BAO and CMB observations, we have also obtained the bounds
of the parameters ($B,C$) by fixing some other parameters $\alpha$ and $A$.
From the best fit of distance modulus $\mu(z)$ for our theoretical MCG model in
LQC, we concluded that our model is in agreement with the union2 sample data.Comment: 14 pages, 10 figures, Accepted in EPJC. arXiv admin note: text
overlap with arXiv:astro-ph/0311622 by other author

### Scalar-Tensor Gravity and Quintessence

Scalar fields with inverse power-law effective potentials may provide a
negative pressure component to the energy density of the universe today, as
required by cosmological observations. In order to be cosmologically relevant
today, the scalar field should have a mass
$m_\phi = O(10^{-33} {\mathrm eV})$, thus potentially inducing sizable
violations of the equivalence principle and space-time variations of the
coupling constants. Scalar-tensor theories of gravity provide a framework for
accommodating phenomenologically acceptable ultra-light scalar fields. We
discuss non-minimally coupled scalar-tensor theories in which the scalar-matter
coupling is a dynamical quantity. Two attractor mechanisms are operative at the
same time: one towards the tracker solution, which accounts for the accelerated
expansion of the Universe, and one towards general relativity, which makes the
ultra-light scalar field phenomenologically safe today. As in usual
tracker-field models, the late-time behavior is largely independent on the
initial conditions. Strong distortions in the cosmic microwave background
anisotropy spectra as well as in the matter power spectrum are expected.Comment: 5 pages, 4 figure

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