178 research outputs found

### Quintessence or Phoenix?

We show that it is impossible to determine the state equation of quintessence
models on the basis of pure observational SNIa data. An independent estimate of
$\Omega_{M0}$ is necessary. Also in this most favourable case the situation can
be problematic.Comment: 7 pages, 4 figures - [email protected]

### Testing a quintessence model with CMBR peaks location

We show that a model of quintessence with exponential potential, which allows
to obtain general exact solutions, can generate locations of CMBR peaks which
are fully compatible with present observational dataComment: 7 pages, no figure

### Testing an exact $f(R)$-gravity model at Galactic and local scales

The weak field limit for a pointlike source of a $f(R) \propto
R^{3/2}$-gravity model is studied. We aim to show the viability of such a model
as a valid alternative to GR + dark matter at Galactic and local scales.
Without considering dark matter, within the weak field approximation, we find
general exact solutions for gravity with standard matter, and apply them to
some astrophysical scales, recovering the consistency of the same
$f(R)$-gravity model with cosmological results.}{In particular, we show that it
is possible to obtain flat rotation curves for galaxies, [and consistency with]
Solar System tests, as in the so-called "Chameleon Approach". In fact, the
peripheral velocity $v_\infty$ is shown to be expressed as $v_\infty =
\lambda \sqrt{M}$, so that the Tully-Fisher relation is recovered. The results
point out the possibility of achieving alternative theories of gravity in which
exotic ingredients like dark matter and dark energy are not necessary, while
their coarse-grained astrophysical and cosmological effects can be related to a
geometric origin.Comment: 8 pages, 2 figures, accepted in Astron. & Astrop

### Improved Action Functionals in Non-Perturbative Quantum Gravity

Models of gravity with variable G and Lambda have acquired greater relevance
after the recent evidence in favour of the Einstein theory being
non-perturbatively renormalizable in the Weinberg sense. The present paper
builds a modified Arnowitt-Deser-Misner (ADM) action functional for such models
which leads to a power-law growth of the scale factor for pure gravity and for
a massless phi**4 theory in a Universe with Robertson-Walker symmetry, in
agreement with the recently developed fixed-point cosmology. Interestingly, the
renormalization-group flow at the fixed point is found to be compatible with a
Lagrangian description of the running quantities G and Lambda.Comment: Latex file. Record without file already exists on SLAC-SPIRES, and
hence that record and the one for the present arxiv submission should become
one record onl

### Exact $f(R)$-cosmological model coming from the request of the existence of a Noether symmetry

We present an $f(R)$-cosmological model with an exact analytic solution,
coming from the request of the existence of a Noether symmetry, which is able
to describe a dust-dominated decelerated phase before the current accelerated
phase of the universe.Comment: 4 pages, 2 figures, Contribution to the proceedings of Spanish
Relativity Meeting 2008, Salamanca, Sapin, 15-19 September 200

### Slow rolling, inflation and quintessence

We comment on the choice of the quintessence potential, examining the
slow-roll approximation in a minimally coupled theory of gravity. We make some
considerations on the potential behaviors, the related \gamma parameter, and
their relationships to phantom cosmology.Comment: 7 page

### The peculiar velocity field in a quintessence model

We investigate the evolution of matter density perturbations and some
properties of the peculiar velocity field for a special class of exponential
potentials in a scalar field model for quintessence, for which a general exact
solution is known. The data from the 2-degree Field Galaxy Redshift Survey
(2dFGRS) suggest a value of the today pressureless matter density Omega_M0 =
0.18 +- 0.05.Comment: 11 pages, 3 figure

### Coupling parameters and the form of the potential via Noether symmetry

We explore the conditions for the existence of Noether symmetries in the
dynamics of FRW metric, non minimally coupled with a scalar field, in the most
general situation, and with nonzero spatial curvature. When such symmetries are
present we find general exact solution for the Einstein equations. We also show
that non Noether symmetries can be found.
Finally,we present an extension of the procedure to the Kantowski- Sachs
metric which is particularly interesting in the case of degenerate Lagrangian.Comment: 13 pages, no figure

### Noether symmetry approach in phantom quintessence cosmology

In the framework of phantom quintessence cosmology, we use the Noether
Symmetry Approach to obtain general exact solutions for the cosmological
equations. This result is achieved by the quintessential (phantom) potential
determined by the existence of the symmetry itself. A comparison between the
theoretical model and observations is worked out. In particular, we use type Ia
supernovae and large scale structure parameters determined from the 2-degree
Field Galaxy Redshift Survey (2dFGRS)and from the Wide part of the VIMOS-VLT
Deep Survey (VVDS). It turns out that the model is compatible with the
presently available observational data. Moreover we extend the approach to
include radiation. We show that it is compatible with data derived from
recombination and it seems that quintessence do not affect nucleosynthesis
results.Comment: 26 pages, 13 figure

### Two viable quintessence models of the Universe: confrontation of theoretical predictions with observational data

We use some of the recently released observational data to test the viability
of two classes of minimally coupled scalar field models of quintessence with
exponential potentials for which exact solutions of the Einstein equations are
known. These models are very sturdy, depending on only one parameter - the
Hubble constant. To compare predictions of our models with observations we
concentrate on the following data: the power spectrum of the CMBR anisotropy as
measured by WMAP, the publicly available data on type Ia supernovae, and the
parameters of large scale structure determined by the 2-degree Field Galaxy
Redshift Survey (2dFGRS). We use the WMAP data on the age of the universe and
the Hubble constant to fix the free parameters in our models. We then show that
the predictions of our models are consistent with the observed positions and
relative heights of the first 3 peaks in the CMB power spectrum, with the
energy density of dark energy as deduced from observations of distant type Ia
supernovae, and with parameters of the large scale structure as determined by
2dFGRS, in particular with the average density of dark matter. Our models are
also consistent with the results of the Sloan Digital Sky Survey (SDSS).
Moreover, we investigate the evolution of matter density perturbations in our
quintessential models, solve exactly the evolution equation for the density
perturbations, and obtain an analytical expression for the growth index $f$. We
verify that the approximate relation f ~ Omega_M^(alpha) also holds in our
models.Comment: in press on A&

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