160 research outputs found
Can higher order curvature theories explain rotation curves of galaxies?
Higher order curvature gravity has recently received a lot of attention due
to the fact that it gives rise to cosmological models which seem capable of
solving dark energy and quintessence issues without using "ad hoc" scalar
fields. In this letter, a gravitational potential is obtained which differs
from the Newtonian one because of a repulsive correction increasing with
distance. We evaluate the rotation curve of our Galaxy and compare it with the
observed data in order both to test the viability of these theories and to
estimate the scalelength of the correction. It is remarkable that the Milky Way
rotation curve is well fitted without the need of any dark matter halo and a
similar result tentatively holds also for other galaxies.Comment: 8 pages, to appear in Phys. Lett.
The inverse problem in microlensing: from the optical depth to the galaxy models parameters
We present in this paper a simple method to obtain informations on galaxy
models parameters using the measured value of the microlensing optical depth.
Assuming a 100 percent MACHO's dark halo, we ask the predicted optical depth
for a given model to be the same as the observed one, in a given direction.
Writing the optical depth in terms of the given halo model parameters and
inverting this relation with respect to one of them, it is possible to get
information on it, fixing under reasonable hypothesis the other parameters.
This is what we call the "inverse problem in microlensing. We apply this
technique to the class of power-law models with flat rotation curves,
determining the range for the core radius R_c compatible with the constraints
on the halo flattening q and the measures of the optical depth towards LMC.
Next, we apply the same method to a simple triaxial model, evaluating the axial
ratios.Comment: 23 LaTex Pages, including 5 Postscript Figures, uses astron.sty,
submitted to A&
Unified dark energy models : a phenomenological approach
A phenomenological approach is proposed to the problem of universe
accelerated expansion and of the dark energy nature. A general class of models
is introduced whose energy density depends on the redshift in such a way
that a smooth transition among the three main phases of the universe evolution
(radiation era, matter domination, asymptotical de Sitter state) is naturally
achieved. We use the estimated age of the universe, the Hubble diagram of Type
Ia Supernovae and the angular size - redshift relation for compact and
ultracompact radio structures to test whether the model is in agreement with
astrophysical observation and to constrain its main parameters. Although
phenomenologically motivated, the model may be straightforwardly interpreted as
a two fluids scenario in which the quintessence is generated by a suitably
chosen scalar field potential. On the other hand, the same model may also be
read in the context of unified dark energy models or in the framework of
modified Friedmann equation theories.Comment: 12 pages, 10 figures, accepted for publication on Physical Review
Holographic Dark Energy Scenario and Variable Modified Chaplygin Gas
In this letter, we have considered that the universe is filled with normal
matter and variable modified Chaplygin gas. Also we have considered the
interaction between normal matter and variable modified Chaplygin gas in FRW
universe. Then we have considered a correspondence between the holographic dark
energy density and interacting variable modified Chaplygin gas energy density.
Then we have reconstructed the potential of the scalar field which describes
the variable modified Chaplygin cosmology.Comment: 4 latex pages, no figures, RevTeX styl
Quinstant Dark Energy Predictions for Structure Formation
We explore the predictions of a class of dark energy models, quinstant dark
energy, concerning the structure formation in the Universe, both in the linear
and non-linear regimes. Quinstant dark energy is considered to be formed by
quintessence and a negative cosmological constant. We conclude that these
models give good predictions for structure formation in the linear regime, but
fail to do so in the non-linear one, for redshifts larger than one.Comment: 9 pages, 14 figures, "Accepted for publication in Astrophysics &
Space Science
f(R) theories of gravity in Palatini approach matched with observations
We investigate the viability of f(R) theories in the framework of the
Palatini approach as solutions to the problem of the observed accelerated
expansion of the universe. Two physically motivated popular choices for f(R)
are considered: power law, f(R) = \beta R^n, and logarithmic, f(R) = \alpha
\ln{R}. Under the Palatini approach, both Lagrangians give rise to cosmological
models comprising only standard matter and undergoing a present phase of
accelerated expansion. We use the Hubble diagram of type Ia Supernovae and the
data on the gas mass fraction in relaxed galaxy clusters to see whether these
models are able to reproduce what is observed and to constrain their
parameters. It turns out that they are indeed able to fit the data with values
of the Hubble constant and of the matter density parameter in agreement with
some model independent estimates, but the today deceleration parameter is
higher than what is measured in the concordance LambdaCDM model.Comment: 14 pages, 8 figures, submitted to Physical Review
Probing the dark matter issue in f(R)-gravity via gravitational lensing
For a general class of analytic f(R)-gravity theories, we discuss the weak
field limit in view of gravitational lensing. Though an additional Yukawa term
in the gravitational potential modifies dynamics with respect to the standard
Newtonian limit of General Relativity, the motion of massless particles results
unaffected thanks to suitable cancellations in the post-Newtonian limit. Thus,
all the lensing observables are equal to the ones known from General
Relativity. Since f(R)-gravity is claimed, among other things, to be a possible
solution to overcome for the need of dark matter in virialized systems, we
discuss the impact of our results on the dynamical and gravitational lensing
analyses. In this framework, dynamics could, in principle, be able to reproduce
the astrophysical observations without recurring to dark matter, but in the
case of gravitational lensing we find that dark matter is an unavoidable
ingredient. Another important implication is that gravitational lensing, in the
post-Newtonian limit, is not able to constrain these extended theories, since
their predictions do not differ from General Relativity.Comment: 7 pages, accepted for publication in EPJ
Tomography from the Next Generation of Cosmic Shear Experiments for Viable f(R) Models
We present the cosmic shear signal predicted by two viable cosmological
models in the framework of modified-action f(R) theories. We use f(R) models
where the current accelerated expansion of the Universe is a direct consequence
of the modified gravitational Lagrangian rather than Dark Energy (DE), either
in the form of vacuum energy/cosmological constant or of a dynamical scalar
field (e.g. quintessence). We choose Starobinsky's (St) and Hu & Sawicki's (HS)
f(R) models, which are carefully designed to pass the Solar System gravity
tests. In order to further support - or rule out - f(R) theories as alternative
candidates to the DE hypothesis, we exploit the power of weak gravitational
lensing, specifically of cosmic shear. We calculate the tomographic shear
matrix as it would be measured by the upcoming ESA Cosmic Vision Euclid
satellite. We find that in the St model the cosmic shear signal is almost
completely degenerate with LCDM, but it is easily distinguishable in the HS
model. Moreover, we compute the corresponding Fisher matrix for both the St and
HS models, thus obtaining forecasts for their cosmological parameters. Finally,
we show that the Bayes factor for cosmic shear will definitely favour the HS
model over LCDM if Euclid measures a value larger than ~0.02 for the extra HS
parameter n_HS.Comment: 26 pages, 6 figures, 2 tables; tomographic and Bayesian analyses
updated and modified according to reviewer's suggestions; references update
Limits on decaying dark energy density models from the CMB temperature-redshift relation
The nature of the dark energy is still a mystery and several models have been
proposed to explain it. Here we consider a phenomenological model for dark
energy decay into photons and particles as proposed by Lima (J. Lima, Phys.
Rev. D 54, 2571 (1996)). He studied the thermodynamic aspects of decaying dark
energy models in particular in the case of a continuous photon creation and/or
disruption. Following his approach, we derive a temperature redshift relation
for the CMB which depends on the effective equation of state and on
the "adiabatic index" . Comparing our relation with the data on the CMB
temperature as a function of the redshift obtained from Sunyaev-Zel'dovich
observations and at higher redshift from quasar absorption line spectra, we
find , adopting for the adiabatic index ,
in good agreement with current estimates and still compatible with
, implying that the dark energy content being constant in time.Comment: 8 pages, 1 figur
New agegraphic dark energy in Horava-Lifshitz cosmology
We investigate the new agegraphic dark energy scenario in a universe governed
by Horava-Lifshitz gravity. We consider both the detailed and non-detailed
balanced version of the theory, we impose an arbitrary curvature, and we allow
for an interaction between the matter and dark energy sectors. Extracting the
differential equation for the evolution of the dark energy density parameter
and performing an expansion of the dark energy equation-of-state parameter, we
calculate its present and its low-redshift value as functions of the dark
energy and curvature density parameters at present, of the Horava-Lifshitz
running parameter , of the new agegraphic dark energy parameter ,
and of the interaction coupling . We find that
and . Although this analysis indicates that the
scenario can be compatible with observations, it does not enlighten the
discussion about the possible conceptual and theoretical problems of
Horava-Lifshitz gravity.Comment: 17 pages, no figures, version published at JCA
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