2,282 research outputs found
Measuring dark energy with the shear triplet statistics
The shear triplet statistics is a geometric method to measure cosmological
parameters with observations in the weak gravitational lensing regime towards
massive haloes. Here, this proposal is considered to probe the dark energy
equation of state and its time derivative in view of future wide-field galaxy
surveys. A survey with a median redshift of nearly 0.7 and a total area of
nearly 10000 square degrees would be pretty effective in determining the dark
matter cosmological density and in putting useful constraints on the dark
energy properties.Comment: 5 pages, 3 figures, accepted for publication in MNRA
Weak field limit of Reissner-Nordstrom black hole lensing
We study gravitational lensing by a Reissner-Nordstrom (RN) black hole in the
weak field limit. We obtain the basic equations for the deflection angle and
time delay and find analytical expressions for the positions and amplifications
of the primary and secondary images. Due to a net positive charge, the
separation between images increases, but no change in the total magnification
occurs.Comment: 8 pages, to appear in Phys. Rev.
Kinematic effect in gravitational lensing by clusters of galaxies
Gravitational lensing provides an efficient tool for the investigation of
matter structures, independent of the dynamical or hydrostatic equilibrium
properties of the deflecting system. However, it depends on the kinematic
status. In fact, either a translational motion or a coherent rotation of the
mass distribution can affect the lensing properties. Here, light deflection by
galaxy clusters in motion is considered. Even if gravitational lensing mass
measurements of galaxy clusters are regarded as very reliable estimates, the
kinematic effect should be considered. A typical peculiar motion with respect
to the Hubble flow brings about a systematic error < 0.3%, independent of the
mass of the cluster. On the other hand, the effect of the spin increases with
the total mass. For cluster masses ~ 10^{15}M_{sun}, the effect of the
gravitomagnetic term is < 0.04% on strong lensing estimates and < 0.5% in the
weak lensing analyses. The total kinematic effect on the mass estimate is then
< 1%, which is negligible in current statistical studies. In the weak lensing
regime, the rotation imprints a typical angular modulation in the tangential
shear distortion. This would allow in principle a detection of the
gravitomagnetic field and a direct measurement of the angular velocity of the
cluster but the required background source densities are well beyond current
tecnological capabilities.Comment: 6 pages; accepted for publication in MNRA
Lensing in an interior Kottler solution
We derive the interior Kottler solution of the incompressible fluid and show
that the bending of light in this solution does depend on the cosmological
constant.Comment: The inner Kottler solution derived and used in this paper is not new.
Corresponding references to Stuchlik (2000) and Boehmer (2003) are added.
Also added: a numerical example and a figure. This is the version accepted by
Gen. Rel. Grav. However it includes a short passage that an anonymous referee
had me suppress
Spherical symmetry in a dark energy permeated space-time
The properties of a spherically symmetric static space-time permeated of dark
energy are worked out. Dark energy is viewed as the strain energy of an
elastically deformable four dimensional manifold. The metric is worked out in
the vacuum region around a central spherical mass/defect in the linear
approximation. We discuss analogies and differences with the analogue in the de
Sitter space time and how these competing scenarios could be differentiated on
an observational ground. The comparison with the tests at the solar system
scale puts upper limits to the parameters of the theory, consistent with the
values obtained applying the classical cosmological tests.Comment: 14 pages, 1 figure, in press on Classical and Quantum Gravit
Cosmological constraints for the Cosmic Defect theory
The Cosmic Defect theory has been confronted with four observational
constraints: primordial nuclear species abundances emerging from the big bang
nucleosynthesis; large scale structure formation in the universe; cosmic
microwave background acoustic scale; luminosity distances of type Ia
supernovae. The test has been based on a statistical analysis of the a
posteriori probabilities for three parameters of the theory. The result has
been quite satisfactory and such that the performance of the theory is not
distinguishable from the one of the Lambda-CDM theory. The use of the optimal
values of the parameters for the calculation of the Hubble constant and the age
of the universe confirms the compatibility of the Cosmic Defect approach with
observations.Comment: 13 pages, 1 figure, in press on IJMP
Cosmography with strong lensing of LISA gravitational wave sources
LISA might detect gravitational waves from mergers of massive black hole
binaries strongly lensed by intervening galaxies (Sereno et al. 2010). The
detection of multiple gravitational lensing events would provide a new tool for
cosmography. Constraints on cosmological parameters could be placed by
exploiting either lensing statistics of strongly lensed sources or time delay
measurements of lensed gravitational wave signals. These lensing methods do not
need the measurement of the redshifts of the sources and the identification of
their electromagnetic counterparts. They would extend cosmological probes to
redshift z <= 10 and are then complementary to other lower or higher redshift
tests, such as type Ia supernovae or cosmic microwave background. The accuracy
of lensing tests strongly depends on the formation history of the merging
binaries, and the related number of total detectable multiple images. Lensing
amplification might also help to find the host galaxies. Any measurement of the
source redshifts would allow to exploit the distance-redshift test in
combination with lensing methods. Time-delay analyses might measure the Hubble
parameter H_0 with accuracy of >= 10 km s^{-1}Mpc^{-1}. With prior knowledge of
H_0, lensing statistics and time delays might constrain the dark matter density
(delta Omega_M >= 0.08, due to parameter degeneracy). Inclusion of our methods
with other available orthogonal techniques might significantly reduce the
uncertainty contours for Omega_M and the dark energy equation of state.Comment: 10 pages, 10 figures, in press on MNRA
Dark matter vs. modifications of the gravitational inverse-square law. Results from planetary motion in the solar system
Dark matter or modifications of the Newtonian inverse-square law in the
solar-system are studied with accurate planetary astrometric data. From
extra-perihelion precession and possible changes in the third Kepler's law, we
get an upper limit on the local dark matter density, rho_{DM} < 3*10^{-16}
kg/m^3 at the 2-sigma confidence level. Variations in the 1/r^2 behavior are
considered in the form of either a possible Yukawa-like interaction or a
modification of gravity of MOND type. Up to scales of 10^{11} m,
scale-dependent deviations in the gravitational acceleration are really small.
We examined the MOND interpolating function mu in the regime of strong gravity.
Gradually varying mu suggested by fits of rotation curves are excluded, whereas
the standard form mu(x)= x/(1+x^2)^{1/2} is still compatible with data. In
combination with constraints from galactic rotation curves and theoretical
considerations on the external field effect, the absence of any significant
deviation from inverse square attraction in the solar system makes the range of
acceptable interpolating functions significantly narrow. Future radio ranging
observations of outer planets with an accuracy of few tenths of a meter could
either give positive evidence of dark matter or disprove modifications of
gravity.Comment: 7 pages, 4 figures, accepted for publication in MNRA
Measuring dark energy with the shear triplet statistics
The shear triplet statistics is a geometric method to measure cosmological parameters with observations in the weak gravitational lensing regime towards massive haloes. Here, this proposal is considered to probe the dark energy equation of state and its time derivative in view of future wide-field galaxy surveys. A survey with a median redshift of ∼0.7 and a total area of ∼10 000 deg2 would be pretty effective in determining the dark matter cosmological density and in putting useful constraints on the dark energy propertie
Imprints of deviations from the gravitational inverse-square law on the power spectrum of mass fluctuations
Deviations from the gravitational inverse-square law would imprint
scale-dependent features on the power spectrum of mass density fluctuations. We
model such deviations as a Yukawa-like contribution to the gravitational
potential and discuss the growth function in a mixed dark matter model with
adiabatic initial conditions. Evolution of perturbations is considered in
general non-flat cosmological models with a cosmological constant, and an
analytical approximation for the growth function is provided. The coupling
between baryons and cold dark matter across recombination is negligibly
affected by modified gravity physics if the proper cutoff length of the
long-range Yukawa-like force is > 10 h^{-1} Mpc. Enhancement of gravity affects
the subsequent evolution, boosting large-scale power in a way that resembles
the effect of a lower matter density. This phenomenon is almost perfectly
degenerate in power-spectrum shape with the effect of a background of massive
neutrinos. Back-reaction on density growth from a modified cosmic expansion
rate should however also affect the normalization of the power spectrum, with a
shape distortion similar to the case of a non-modified background.Comment: 8 pages, 7 figures; submitted to MNRA
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