118 research outputs found
Testing an exact -gravity model at Galactic and local scales
The weak field limit for a pointlike source of a -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
-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 is shown to be expressed as , 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
Exact -cosmological model coming from the request of the existence of a Noether symmetry
We present an -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
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
Q-plates for Switchable Excitation of Fiber OAM Modes
We demonstrate that a |q|=1/2 plate plus polarization optics can tunably excite all linear combinations of |l|=1 fiber OAM modes with up to ~30 dB purity, enabling switch fabrics in fiber-OAM networks and disentangling of degenerate mode mixing effects in long fibers
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 . We
verify that the approximate relation f ~ Omega_M^(alpha) also holds in our
models.Comment: in press on A&
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
On exact solutions for quintessential (inflationary) cosmological models with exponential potentials
We first study dark energy models with a minimally-coupled scalar field and
exponential potentials, admitting exact solutions for the cosmological
equations: actually, it turns out that for this class of potentials the
Einstein field equations exhibit alternative Lagrangians, and are completely
integrable and separable (i.e. it is possible to integrate the system
analytically, at least by quadratures). We analyze such solutions, especially
discussing when they are compatible with a late time quintessential expansion
of the universe. As a further issue, we discuss how such quintessential scalar
fields can be connected to the inflationary phase, building up, for this class
of potentials, a quintessential inflationary scenario: actually, it turns out
that the transition from inflation toward late-time exponential quintessential
tail admits a kination period, which is an indispensable ingredient of this
kind of theoretical models. All such considerations have also been done by
including radiation into the model.Comment: Revtex4, 10 figure
Constraining scalar-tensor quintessence by cosmic clocks
Scalar-tensor quintessence models can be constrained by identifying suitable
cosmic clocks which allow to select confidence regions for cosmological
parameters. In particular, we constrain the characterizing parameters of
non-minimally coupled scalar-tensor cosmological models which admit exact
solutions of the Einstein field equations. Lookback time to galaxy clusters at
low intermediate, and high redshifts is considered. The high redshift
time-scale problem is also discussed in order to select other cosmic clocks
such as quasars.Comment: 13 pages, 8 figures. to be published in Astron & Astrop
Cosmological models in scalar tensor theories of gravity and observations: a class of general solutions
We consider cosmological models in scalar tensor theories of gravity that
describe an accelerating universe, and we study a family of inverse power law
potentials, for which exact solutions of the Einstein equations are known. We
also compare theoretical predictions of our models with observations. For this
we use the following data: the publicly available catalogs of type Ia
supernovae and high redshift Gamma Ray Bursts, the parameters of large scale
structure determined by the 2-degree Field Galaxy Redshift Survey (2dFGRS), and
measurements of cosmological distances based on the Sunyaev-Zel'dovich effect,
among others.Comment: 26 pages,23 figures, accepted for publication in A&
Exponential Potentials for Tracker Fields
We show that a general, exact cosmological solution, where dynamics of scalar
field is assigned by an exponential potential, fulfils all the issues of dark
energy approach, both from a theoretical point of view and in comparison with
available observational data. Moreover, tracking conditions are discussed, with
a new treatment of the well known condition . We prove that the
currently used expression for is wrong.Comment: 29 pages,12 figures; contact [email protected]; revised version, to
appear in Physical Review
- âŠ