29 research outputs found

### Observational Cosmology in Macroscopic Gravity

We discuss the construction of cosmological models within the framework of
Macroscopic Gravity (MG), which is a theory that models the effects of
averaging the geometry of space-time on large scales. We find new exact
spatially homogeneous and isotropic FLRW solutions to the MG field equations,
and investigate large-scale perturbations around them. We find that any
inhomogeneous perturbations to the averaged geometry are severely restricted,
but that possible anisotropies in the correlation tensor can have dramatic
consequences for the measurement of distances. These calculations are a first
step within the MG approach toward developing averaged cosmological models to a
point where they can be used to interpret real cosmological data, and hence to
provide a working alternative to the "concordance" LCDM model.Comment: 22 page

### Are braneworlds born isotropic?

It has recently been suggested that an isotropic singularity may be a generic
feature of brane cosmologies, even in the inhomogeneous case. Using the
covariant and gauge-invariant approach we present a detailed analysis of linear
perturbations of the isotropic model ${\cal F}_b$ which is a past attractor in
the phase space of homogeneous Bianchi models on the brane. We find that for
matter with an equation of state parameter $\gamma > 1$, the dimensionless
variables representing generic anisotropic and inhomogeneous perturbations
decay as $t\to 0$, showing that the model ${\cal F}_b$ is asymptotically stable
in the past. We conclude that brane universes are born with isotropy naturally
built-in, contrary to standard cosmology. The observed large-scale homogeneity
and isotropy of the universe can therefore be explained as a consequence of the
initial conditions if the brane-world paradigm represents a description of the
very early universe.Comment: Changed to match published versio

### Irreversible Processes in Inflationary Cosmological Models

By using the thermodynamic theory of irreversible processes and Einstein
general relativity, a cosmological model is proposed where the early universe
is considered as a mixture of a scalar field with a matter field. The scalar
field refers to the inflaton while the matter field to the classical particles.
The irreversibility is related to a particle production process at the expense
of the gravitational energy and of the inflaton energy. The particle production
process is represented by a non-equilibrium pressure in the energy-momentum
tensor. The non-equilibrium pressure is proportional to the Hubble parameter
and its proportionality factor is identified with the coefficient of bulk
viscosity. The dynamic equations of the inflaton and the Einstein field
equations determine the time evolution of the cosmic scale factor, the Hubble
parameter, the acceleration and of the energy densities of the inflaton and
matter. Among other results it is shown that in some regimes the acceleration
is positive which simulates an inflation. Moreover, the acceleration decreases
and tends to zero in the instant of time where the energy density of matter
attains its maximum value.Comment: 13 pages, 2 figures, to appear in PR

### Renormalization Group Approach to Generalized Cosmological models

We revisit here the problem of generalized cosmology using renormalization
group approach. A complete analysis of these cosmologies, where specific models
appear as asymptotic fixed-points, is given here along with their linearized
stability analysis.Comment: 10 pages, to appear in the International Journal of Theoretical
Physic

### Spherically Symmetric Solutions in Macroscopic Gravity

Schwarzschild's solution to the Einstein Field Equations was one of the first
and most important solutions that lead to the understanding and important
experimental tests of Einstein's theory of General Relativity. However,
Schwarzschild's solution is essentially based on an ideal theory of
gravitation, where all inhomogeneities are ignored. Therefore, any
generalization of the Schwarzschild solution should take into account the
effects of small perturbations that may be present in the gravitational field.
The theory of Macroscopic Gravity characterizes the effects of the
inhomogeneities through a non-perturbative and covariant averaging procedure.
With similar assumptions on the geometry and matter content, a solution to the
averaged field equations as dictated by Macroscopic Gravity are derived. The
resulting solution provides a possible explanation for the flattening of
galactic rotation curves, illustrating that Dark Matter is not real but may
only be the result of averaging inhomogeneities in a spherically symmetric
background.Comment: 14 pages, added and updated references, some paragraphs rewritten for
clarity, typographical errors fixed, results have not change

### Braneworld Dynamics of Inflationary Cosmologies with Exponential Potentials

In this work we consider Randall-Sundrum braneworld type scenarios, in which
the spacetime is described by a five-dimensional manifold with matter fields
confined in a domain wall or three-brane. We present the results of a
systematic analysis, using dynamical systems techniques, of the qualitative
behaviour of Friedmann-Lemaitre-Robertson-Walker type models, whose matter is
described by a scalar field with an exponential potential. We construct the
state spaces for these models and discuss how their structure changes with
respect to the general-relativistic case, in particular, what new critical
points appear and their nature and the occurrence of bifurcation.Comment: 15 pages, 9 figures, RevTex 4. Submitted to Physical Review

### A quasi-static approach to structure formation in black hole universes

JD and TC both acknowledge support from the STFC under grant STFC ST/N504257/1

### Modified gravity in a viscous and non-isotropic background

We study the dynamical evolution of an $f(R)$ model of gravity in a viscous
and anisotropic background which is given by a Bianchi type-I model of the
Universe. We find viable forms of $f(R)$ gravity in which one is exactly the
Einsteinian model of gravity with a cosmological constant and other two are
power law $f(R)$ models. We show that these two power law models are stable
with a suitable choice of parameters. We also examine three potentials which
exhibit the potential effect of $f(R)$ models in the context of scalar tensor
theory. By solving different aspects of the model and finding the physical
quantities in the Jordan frame, we show that the equation of state parameter
satisfy the dominant energy condition. At last we show that the two power law
$f(R)$ models behave like quintessence model at late times and also the shear
coefficient viscosity tends to zero at late times.Comment: 7 pages, 2 figure

### Scaling solution, radion stabilization, and initial condition for brane-world cosmology

We propose a new, self-consistent and dynamical scenario which gives rise to
well-defined initial conditions for five-dimensional brane-world cosmologies
with radion stabilization. At high energies, the five-dimensional effective
theory is assumed to have a scale invariance so that it admits an expanding
scaling solution as a future attractor. The system automatically approaches the
scaling solution and, hence, the initial condition for the subsequent
low-energy brane cosmology is set by the scaling solution. At low energies, the
scale invariance is broken and a radion stabilization mechanism drives the
dynamics of the brane-world system. We present an exact, analytic scaling
solution for a class of scale-invariant effective theories of five-dimensional
brane-world models which includes the five-dimensional reduction of the
Horava-Witten theory, and provide convincing evidence that the scaling solution
is a future attractor.Comment: 17 pages; version accepted for PRD, references adde

### Equation of state for Universe from similarity symmetries

In this paper we proposed to use the group of analysis of symmetries of the
dynamical system to describe the evolution of the Universe. This methods is
used in searching for the unknown equation of state. It is shown that group of
symmetries enforce the form of the equation of state for noninteracting scaling
multifluids. We showed that symmetries give rise the equation of state in the
form $p=-\Lambda+w_{1}\rho(a)+w_{2}a^{\beta}+0$ and energy density
$\rho=\Lambda+\rho_{01}a^{-3(1+w)}+\rho_{02}a^{\beta}+\rho_{03}a^{-3}$, which
is commonly used in cosmology. The FRW model filled with scaling fluid (called
homological) is confronted with the observations of distant type Ia supernovae.
We found the class of model parameters admissible by the statistical analysis
of SNIa data. We showed that the model with scaling fluid fits well to
supernovae data. We found that $\Omega_{\text{m},0} \simeq 0.4$ and $n \simeq
-1$ ($\beta = -3n$), which can correspond to (hyper) phantom fluid, and to a
high density universe. However if we assume prior that
$\Omega_{\text{m},0}=0.3$ then the favoured model is close to concordance
$\Lambda$CDM model. Our results predict that in the considered model with
scaling fluids distant type Ia supernovae should be brighter than in
$\Lambda$CDM model, while intermediate distant SNIa should be fainter than in
$\Lambda$CDM model. We also investigate whether the model with scaling fluid is
actually preferred by data over $\Lambda$CDM model. As a result we find from
the Akaike model selection criterion prefers the model with noninteracting
scaling fluid.Comment: accepted for publication versio