57 research outputs found
Lensing in the McVittie metric
We investigate the effect of the cosmological expansion on the bending of
light due to an isolated point-like mass. We adopt McVittie metric as the model
for the geometry of the lens. Assuming a constant Hubble factor we find an
analytic expression involving the bending angle, which turns out to be
unaffected by the cosmological expansion at the leading order.Comment: 4 pages, 3 figures. Mistakes corrected. Conclusions completely
change
On the effect of the cosmological expansion on the gravitational lensing by a point mass
We analyze the effect of the cosmological expansion on the deflection of
light caused by a point mass, adopting the McVittie metric as the geometrical
description of a pointlike lens embedded in an expanding universe. In the case
of a generic, non-constant Hubble parameter we derive and approximately
solve the null geodesic equations, finding an expression for the bending angle
, which we expand in powers of the mass-to-closest approach distance
ratio and of the impact parameter-to-lens distance ratio. It turns out that the
leading order of the aforementioned expansion is the same as the one calculated
for the Schawarzschild metric and that cosmological corrections contribute to
only at sub-dominant orders. We explicitly calculate these
cosmological corrections for the case of constant and find that they
provide a correction of order on the lens mass estimate.Comment: 10 pages, 7 figures (created with tikz
Gravitational potential evolution in Unified Dark Matter Scalar Field Cosmologies: an analytical approach
We investigate the time evolution of the gravitational potential Phi for a
special class of non-adiabatic Unified Dark matter Models described by scalar
field lagrangians. These models predict the same background evolution as in the
LambdaCDM and possess a non-vanishing speed of sound. We provide a very
accurate approximation of Phi, valid after the recombination epoch, in the form
of a Bessel function of the first kind. This approximation may be useful for a
future deeper analysis of Unified Dark Matter scalar field models.Comment: 5 pages, 6 figure
Scalar-Tensor gravity with system-dependent potential and its relation with Renormalization Group extended General Relativity
We show that Renormalization Group extensions of the Einstein-Hilbert action
for large scale physics are not, in general, a particular case of standard
Scalar-Tensor (ST) gravity. We present a new class of ST actions, in which the
potential is not necessarily fixed at the action level, and show that this
extended ST theory formally contains the Renormalization Group case. We also
propose here a Renormalization Group scale setting identification that is
explicitly covariant and valid for arbitrary relativistic fluids.Comment: 29 pages, 2 figs. v2: small changes in text and ref's. v3: further
details on the relation between this work and others on the Renormalization
Group. Version to appear in JCA
Scalar models for the unification of the dark sector
We review the difficulties of the generalized Chaplygin gas model to fit
observational data, due to the tension between background and perturbative
tests. We argue that such issues may be circumvented by means of a
self-interacting scalar field representation of the model. However, this
proposal seems to be successful only if the self-interacting scalar field has a
non-canonical form. The latter can be implemented in Rastall's theory of
gravity.Comment: Latex file, 8 pages, 3 figures in eps format. To appear in the
proceedings of the CosmoSul conference, held in Rio de Janeiro, Brazil, 01-05
august of 201
Unified Dark Matter scalar field models with fast transition
We investigate the general properties of Unified Dark Matter (UDM) scalar
field models with Lagrangians with a non-canonical kinetic term, looking
specifically for models that can produce a fast transition between an early
Einstein-de Sitter CDM-like era and a later Dark Energy like phase, similarly
to the barotropic fluid UDM models in JCAP1001(2010)014. However, while the
background evolution can be very similar in the two cases, the perturbations
are naturally adiabatic in fluid models, while in the scalar field case they
are necessarily non-adiabatic. The new approach to building UDM Lagrangians
proposed here allows to escape the common problem of the fine-tuning of the
parameters which plague many UDM models. We analyse the properties of
perturbations in our model, focusing on the the evolution of the effective
speed of sound and that of the Jeans length. With this insight, we can set
theoretical constraints on the parameters of the model, predicting sufficient
conditions for the model to be viable. An interesting feature of our models is
that what can be interpreted as w_{DE} can be <-1 without violating the null
energy conditions.Comment: Slightly revised version accepted for publication in JCAP, with a few
added references; 27 pages, 13 figure
Gravitational lensing in a universe with matter and a cosmological constant
We extend the results obtained in previous works by Piattella and Park for gravitational lensing in the McVittie metric by including the effect of the transition from the matter-dominated epoch of the Universe to the
Λ
-dominated era. We derive a formula that agrees with the previous results for the McVittie metric at lowest order and compare the lensing angle predictions obtained from the Schwarzschild approximation, the McVittie model, and higher order corrections to the McVittie model. In doing this, we test if, beyond the correction from the accelerated expansion of the Universe, there is a need for including the matter content of the Universe in modeling lens systems at the redshifts observed in lens systems. We investigate if there is a need for a modification of the lens equation from these corrections and, if so, to which order and whether it is measurable. We find that, while the effect is of the same order as the one calculated previously, there is no significant contribution to the bending angle, as the first order effect is already of order
O
(
θ
4
O
)
in the observed angle
A method for evaluating models that use galaxy rotation curves to derive the density profiles
There are some approaches, either based on General Relativity (GR) or
modified gravity, that use galaxy rotation curves to derive the matter density
of the corresponding galaxy, and this procedure would either indicate a partial
or a complete elimination of dark matter in galaxies. Here we review these
approaches, clarify the difficulties on this inverted procedure, present a
method for evaluating them, and use it to test two specific approaches that are
based on GR: the Cooperstock-Tieu (CT) and the Balasin-Grumiller (BG)
approaches. Using this new method, we find that neither of the tested
approaches can satisfactorily fit the observational data without dark matter.
The CT approach results can be significantly improved if some dark matter is
considered, while for the BG approach no usual dark matter halo can improve its
results.Comment: 11 pages, 2 figures, 4 tables. v2: diverse text improvements, no
changes in the conclusions. Version accepted in MNRA
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