74 research outputs found
Cosmological parameters from lenses distance ratio
Strong lensing provides popular techniques to investigate the mass
distribution of intermediate redshift galaxies, testing galaxy evolution and
formation scenarios. It especially probes the background cosmic expansion,
hence constraining cosmological parameters. The measurement of Einstein radii
and central velocity dispersions indeed allows to trace the ratio D_s/D_ls
between the distance D_s from the observer to the source and the distance D_ls
from the lens to the source. We present an improved method to explicitly
include the two - component structure in the galaxy lens modeling, in order to
analyze the role played by the redshift and the model dependence on a nuisance
parameter, F_E, which is usually marginalized in the cosmological applications.
We show how to deal with these problems and carry on a Fisher matrix analysis
to infer the accuracy on cosmological parameters achieved by this method.Comment: 8 pages, 2 figures, 2 tables, accepted for publication on Monthly
Notices of Royal Astronomical Societ
On boosted space-times with cosmological constant and their ultrarelativistic limit
The problem of deriving a shock-wave geometry with cosmological constant by
boosting a Schwarzschild-de Sitter (or anti-de Sitter) black hole is
re-examined. Unlike previous work in the literature, we deal with the exact
Schwarzschild-de Sitter (or anti-de Sitter) metric. By virtue of peculiar
cancellations in this exact calculation, where the metric does not depend
linearly on the mass parameter, we find a singularity of distributional nature
on a null hypersurface, which corresponds to a shock-wave geometry derived in a
fully non-perturbative way. The result agrees with previous calculations, where
the metric had been linearized in the mass parameter.Comment: 13 pages, Latex file. In the final version, the presentation has been
improved, and two references have been adde
Testing a quintessence model with CMBR peaks locations
We show that a model of quintessence with exponential potential,
which allows to obtain general exact solutions, can generate location of
CMBR peaks which are fully compatible with present observational data
Riemann curvature of a boosted spacetime geometry
The ultrarelativistic boosting procedure had been applied in the literature
to map the metric of Schwarzschild-de Sitter spacetime into a metric describing
de Sitter spacetime plus a shock-wave singularity located on a null
hypersurface. This paper evaluates the Riemann curvature tensor of the boosted
Schwarzschild-de Sitter metric by means of numerical calculations, which make
it possible to reach the ultrarelativistic regime gradually by letting the
boost velocity approach the speed of light. Thus, for the first time in the
literature, the singular limit of curvature through Dirac's delta distribution
and its derivatives is numerically evaluated for this class of spacetimes.
Eventually, the analysis of the Kretschmann invariant and the geodesic equation
show that the spacetime possesses a scalar curvature singularity within a
3-sphere and it is possible to define what we here call boosted horizon, a sort
of elastic wall where all particles are surprisingly pushed away, as numerical
analysis demonstrates. This seems to suggest that boosted geometries are ruled
by a sort of antigravity effect since all geodesics seem to refuse to enter the
boosted horizon, even though their initial conditions are aimed at driving the
particles towards the boosted horizon.Comment: 33 pages, 8 figures. In the new version, a section and new references
have been added, and the presentation has been amended and improve
Noether symmetry approach to scalar-field-dominated cosmology with dynamically evolving G and Lambda
This paper studies the cosmological equations for a scalar field Phi in the
framework of a quantum gravity modified Einstein--Hilbert Lagrangian where G
and Lambda are dynamical variables. It is possible to show that there exists a
Noether symmetry for the point Lagrangian describing this scheme in a FRW
universe. Our main result is that the Noether Symmetry Approach fixes both
Lambda = Lambda(G) and the potential V = V(Phi) of the scalar field. The method
does not lead, however, to easily solvable equations, by virtue of the higher
dimensionality of the reduced configuration space involved, the additional
variable being the running Newton coupling.Comment: 10 pages, Revtex
On the transition from complex to real scalar fields in modern cosmology
We study some problems arising from the introduction of a complex scalar
field in cosmology, modelling its possible behaviors in both the inflationary
and dark energy stages of the universe. Such examples contribute to show that,
while the complex nature of the scalar field can be indeed important during
inflation, it loses its meaning in the later dark-energy dominated era of
cosmology, when the phase of the complex field is practically constant, and
there is indeed a transition from complex to real scalar field. In our
considerations, the Noether symmetry approach turns out to be a useful tool
once again. We arrive eventually at a potential containing the sixth and fourth
powers of the scalar field, and the resulting semiclassical quantum cosmology
is studied to gain a better understanding of the inflationary stage.Comment: 21 pages, 6 figures. In the new version, sections I, IV and VI have
been improved, and two words have been added at the beginning of the titl
Spherically symmetric ADM gravity with variable G and Lambda(c)
This paper investigates the Arnowitt--Deser--Misner (hereafter ADM) form of
spherically symmetric gravity with variable Newton parameter G and cosmological
term Lambda(c). The Newton parameter is here treated as a dynamical variable,
rather than being merely an external parameter as in previous work on closely
related topics. The resulting Hamilton equations are obtained; interestingly, a
static solution exists, that reduces to Schwarzschild geometry in the limit of
constant G, describing a Newton parameter ruled by a nonlinear differential
equation in the radial variable r. A remarkable limiting case is the one for
which the Newton parameter obeys an almost linear growth law at large r. An
exact solution for G as a function of r is also obtained in the case of
vanishing cosmological constant. Some observational implications of these
solutions are obtained and briefly discussed.Comment: 16 pages, 2 figures. The presentation has been improved in all
section
The Accelerated expansion of the Universe as a crossover phenomenon
We show that the accelerated expansion of the Universe can be viewed as a
crossover phenomenon where the Newton constant and the Cosmological constant
are actually scaling operators, dynamically evolving in the attraction basin of
a non-Gaussian infrared fixed point, whose existence has been recently
discussed. By linearization of the renormalized flow it is possible to evaluate
the critical exponents, and it turns out that the approach to the fixed point
is ruled by a marginal and a relevant direction. A smooth transition between
the standard Friedmann--Lemaitre--Robertson--Walker (FLRW) cosmology and the
observed accelerated expansion is then obtained, so that at late times.Comment: 12 pages, latex, use bibtex. In the final version, the presentation
has been improved, and new references have been adde
Noether symmetry approach in pure gravity with variable G and Lambda
We find exact cosmological solutions when the Newton parameter and the
cosmological term are dynamically evolving in a renormalization-group improved
Hamiltonian approach. In our derivation we use the Noether symmetry approach,
leading to an interesting variable transformation which yields exact and
general integration of the cosmological equations. The functional dependence of
Lambda on G is determined by the method itself, therefore generalizing previous
results on symmetry principles in cosmology. We find new functional relations
between Lambda and G, jointly with power-law inflation for pure gravity.Comment: 18 pages, 2 figures. In the final version, physical interpretation
and presentation have been improved in all section
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