89 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
The production of matter from curvature in a particular linearized high order theory of gravity and the longitudinal response function of interferometers
The strict analogy between scalar-tensor theories of gravity and high order
gravity is well known in literature. In this paper it is shown that, from a
particular high order gravity theory known in literature, it is possible to
produce, in the linearized approch, particles which can be seen like massive
scalar modes of gravitational waves and the response of interferometers to this
type of particles is analyzed. The presence of the mass generates a
longitudinal force in addition of the transverse one which is proper of the
massless gravitational waves and the response of an arm of an interferometer to
this longitudinal effect in the frame of a local observer is computed. This
longitudinal response function is directly connected with the function of the
Ricci scalar in the particular action of this high order theory. Important
conseguences from a theoretical point of view could arise from this approach,
because it opens to the possibility of using the signals seen from
interferometers to understand which is the correct theory of gravitation.Comment: Accepted for Journal of Cosmology and Astroparticle Physic
Dynamical and gravitational lensing properties of a new phenomenological model of elliptical galaxies
Recent observations of the line of sight velocity profile of elliptical galaxies have furnished controversial results with some works favouring the presence of a large amount of dark matter in the outer regions and others arguing in favour of no dark matter at all. In order to shed new light on this controversy, we propose here a new phenomenological description of the total mass profile of galaxies. Under the hypothesis of spherical symmetry, we assume a double power-law expression for the global M/L ratio Upsilon(r)= Upsilon_0(r/r_0) ^{alpha}(1+r/r_0)^{beta}. In particular, Upsilon propto r^{alpha} for r/r_01 so that alpha1), Upsilon propto r^{alpha+beta} thus showing that models with alpha+beta=0 have an asymptotically constant M/L ratio. A wide range of possibilities is obtained by varying the slope parameters in the range we determine on the basis of physical considerations. Choosing a general expression for the luminosity density profile j(r), we work out an effective galaxy model that accounts for all the phenomenology observed in real elliptical galaxies. We derive the main dynamics and lensing properties of such an effective model. We analyze a general class of models, able to take into account different dynamical trends. We are able to obtain analytical expressions for the main dynamical and lensing quantities. We show that constraining the values of alpha+beta makes it possible to analyze the problem of the dark matter in elliptical galaxies. Indeed, positive values of alpha+beta would be a strong evidence for dark matter. Finally we indicate possible future approaches in order to face the observational data, in particular using velocity dispersion profiles and lensed quasar events
High redshift constraints on dark energy models and tension with the flat LambdaCDM model
So far large and different data sets revealed the accelerated expansion rate
of the Universe, which is usually explained in terms of dark energy. The nature
of dark energy is not yet known, and several models have been introduced: a non
zero cosmological constant, a potential energy of some scalar field, effects
related to the non homogeneous distribution of matter, or effects due to
alternative theories of gravity. In [1, 2] a tension with the flat LambdaCDM
model has been discovered using a high-redshift Hubble diagram of supernovae,
quasars, and gamma-ray bursts. Here we use Union2 type Ia supernovae (SNIa) and
Gamma Ray Bursts (GRB) Hubble diagram, and a set of direct measurements of the
Hubble parameter to explore different dark energy models. We use the
Chevallier-Polarski-Linder (CPL) parametrization of the dark energy equation of
state (EOS), a minimally coupled quintessence scalar field, and, finally, we
consider models with dark energy at early times (EDE). We perform a statistical
analysis based on the Markov chain Monte Carlo (MCMC) method, and explore the
probability distributions of the cosmological parameters for each of the
competing models. We apply the Akaike Information Criterion (AIC) to compare
these models: our analysis indicates that an evolving dark energy, described by
a scalar field with exponential potential is favoured by observational data.Comment: 27 pages, 11 figures submitted to JCA
Effects of quintessence on observations of Type Ia SuperNovae in the clumpy Universe
We discuss the amplification dispersion in the observed luminosity of
standard candles, like supernovae (SNe) of type Ia, induced by gravitational
lensing in a Universe with dark energy (quintessence). We derive the main
features of the magnification probability distribution function (pdf) of SNe in
the framework of on average Friedmann-Lemaitre-Robertson-Walker (FLRW) models
for both lensing by large-scale structures and compact objects. The
magnification pdf is strongly dependent on the equation of state, , of the
quintessence. The dispersion increases with the redshift of the source and is
maximum for dark energy with very large negative pressure; the effects of
gravitational lensing on the magnification pdf, i.e. the mode biased towards
de-amplified values and the long tail towards large magnifications, are reduced
for both microscopic DM and quintessence with an intermediate . Different
equations of state of the dark energy can deeply change the dispersion in
amplification for the projected observed samples of SNe Ia by future space-born
missions. The "noise" in the Hubble diagram due to gravitational lensing
strongly affects the determination of the cosmological parameters from SNe
data. The errors on the pressureless matter density parameter, , and
on are maximum for quintessence with not very negative pressure. The
effect of the gravitational lensing is of the same order of the other
systematics affecting observations of SNe Ia. Due to the lensing by large-scale
structures, in a flat Universe with , at a cosmological
constant () can be interpreted as dark energy with (at
2- confidence limit).Comment: 11 pages, 6 figure
Distances in inhomogeneous quintessence cosmology
We investigate the properties of cosmological distances in locally
inhomogeneous universes with pressureless matter and dark energy
(quintessence), with constant equation of state. We give exact solutions for
angular diameter distances in theempty beam approximation. In this hypothesis,
the distance-redshift equation is derived fron the multiple lens-plane theory.
The case of a flat universe is considered with particular attention. We show
how this general scheme makes distances degenerate with respect to w_X and the
smoothness parameters, alpha, accounting for the homogeneously distributed
fraction of energy of the i-components. We analyse how this degeneracy
influences the critical redshift where the angular diameter distance takes its
maximum, and put in evidence future prospects for measuring the smoothness
parameter of the pressureless matter, alpha_M.Comment: 24 pages, 9 ps 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
A new method for the estimate of H_0 from quadruply imaged gravitational lens systems
We present a new method to estimate the Hubble constant H_0 from the measured
time delays in quadruply imaged gravitational lens systems. We show how it is
possible to get an estimate of H_0 without the need to completely reconstruct
the lensing potential thus avoiding any a priori hypotheses on the expression
of the galaxy lens model. Our method only needs to assume that the lens
potential may be expressed as r^{\alpha} F(\theta), whatever the shape function
F(\theta) is, and it is thus able to fully explore the degeneracy in the mass
models taking also into account the presence of an external shear. We test the
method on simulated cases and show that it does work well in recovering the
correct value of the slope \alpha of the radial profile and of the Hubble
constant H_0. Then, we apply the same method to the real quadruple lenses
PG1115+080 and B1422+231 obtaining H_0 = 58_{-15}^{+17} km/s/Mpc (68% CL).Comment: 12 pages, 5 figures, accepted for publication on Astronomy &
Astrophysic
Slott-Agape Project
SLOTT-AGAPE (Systematic Lensing Observation at Toppo Telescope - Andromeda
Gravitational Amplification Pixel Lensing Experiment) is a new collaboration
project among international partners from England, France, Germany, Italy and
Switzerland that intends to perform microlensing observation by using M31 as
target. The MACHOs search is made thanks to the pixel lensing technique.Comment: 4 pages, 2 figures, proceeding of XLIII Congresso della Societa'
Astronomica Italiana, Napoli, 4-8 Maggio, 199
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