61 research outputs found
Scalar-field quintessence by cosmic shear: CFHT data analysis and forecasts for DUNE
A light scalar field, minimally or not-minimally coupled to the metric field,
is a well-defined candidate for the dark energy, overcoming the coincidence
problem intrinsic to the cosmological constant and avoiding the difficulties of
parameterizations. We present a general description of the weak gravitational
lensing valid for every metric theory of gravity, including vector and tensor
perturbations for a non-flat spatial metric. Based on this description, we
investigate two minimally-coupled scalar field quintessence models using
VIRMOS-Descart and CFHTLS cosmic shear data, and forecast the constraints for
the proposed space-borne wide-field imager DUNE.Comment: 7 pages, 4 figures. To appear in proceedings of IRGAC06 (Barcelona,
July 06
Examination of the astrophysical S-factors of the radiative proton capture on 2H, 6Li, 7Li, 12C and 13C
Astrophysical S-factors of radiative capture reactions on light nuclei have
been calculated in a two-cluster potential model, taking into account the
separation of orbital states by the use of Young schemes. The local two-body
potentials describing the interaction of the clusters were determined by
fitting scattering data and properties of bound states. The many-body character
of the problem is approximatively accounted for by Pauli forbidden states. An
important feature of the approach is the consideration of the dependence of the
interaction potential between the clusters on the orbital Young schemes, which
determine the permutation symmetry of the nucleon system. Proton capture on 2H,
6Li, 7Li, 12C, and 13C was analyzed in this approach. Experimental data at low
energies were described reasonably well when the phase shifts for
cluster-cluster scattering, extracted from precise data, were used. This shows
that decreasing the experimental error on differential elastic scattering cross
sections of light nuclei at astrophysical energies is very important also to
allow a more accurate phase shift analysis. A future increase in precision will
allow more definite conclusions regarding the reaction mechanisms and
astrophysical conditions of thermonuclear reactions.Comment: 40p., 9 fig., 83 ref. arXiv admin note: substantial text overlap with
arXiv:1005.1794, arXiv:1112.1760, arXiv:1005.198
Measuring the dark side (with weak lensing)
We introduce a convenient parametrization of dark energy models that is
general enough to include several modified gravity models and generalized forms
of dark energy. In particular we take into account the linear perturbation
growth factor, the anisotropic stress and the modified Poisson equation. We
discuss the sensitivity of large scale weak lensing surveys like the proposed
DUNE satellite to these parameters. We find that a large-scale weak-lensing
tomographic survey is able to easily distinguish the Dvali-Gabadadze-Porrati
model from LCDM and to determine the perturbation growth index to an absolute
error of 0.02-0.03.Comment: 19 pages, 11 figure
A sub-horizon framework for probing the relationship between the cosmological matter distribution and metric perturbations
The relationship between the metric and nonrelativistic matter distribution
depends on the theory of gravity and additional fields, providing a possible
way of distinguishing competing theories. With the assumption that the geometry
and kinematics of the homogeneous universe have been measured to sufficient
accuracy, we present a procedure for understanding and testing the relationship
between the cosmological matter distribution and metric perturbations (along
with their respective evolution) using the ratio of the physical size of the
perturbation to the size of the horizon as our small expansion parameter. We
expand around Newtonian gravity on linear, subhorizon scales with coefficient
functions in front of the expansion parameter. Our framework relies on an
ansatz which ensures that (i) the Poisson equation is recovered on small scales
(ii) the metric variables (and any additional fields) are generated and
supported by the nonrelativistic matter overdensity. The scales for which our
framework is intended are small enough so that cosmic variance does not
significantly limit the accuracy of the measurements and large enough to avoid
complications from nonlinear effects and baryon cooling. The coefficient
functions provide a general framework for contrasting the consequences of
Lambda CDM and its alternatives. We calculate the coefficient functions for
general relativity with a cosmological constant and dark matter, GR with dark
matter and quintessence, scalar-tensor theories, f(R) gravity and braneworld
models. We identify a possibly unique signature of braneworld models.
Constraining the coefficient functions provides a streamlined approach for
testing gravity in a scale dependent manner. We briefly discuss the
observations best suited for an application of our framework.Comment: Updated references and minor changes to match the published version
in MNRA
The VIMOS Public Extragalactic Redshift Survey - Searching for Cosmic Voids
The characterisation of cosmic voids gives unique information about the
large-scale distribution of galaxies, their evolution and the cosmological
model. We identify and characterise cosmic voids in the VIMOS Public
Extragalactic Redshift Survey (VIPERS) at redshift 0.55 < z < 0.9. A new void
search method is developed based upon the identification of empty spheres that
fit between galaxies. The method can be used to characterise the cosmic voids
despite the presence of complex survey boundaries and internal gaps. We
investigate the impact of systematic observational effects and validate the
method against mock catalogues. We measure the void size distribution and the
void-galaxy correlation function. We construct a catalogue of voids in VIPERS.
The distribution of voids is found to agree well with the distribution of voids
found in mock catalogues. The void-galaxy correlation function shows
indications of outflow velocity from the voids
Ray-tracing through the Millennium Simulation: Born corrections and lens-lens coupling in cosmic shear and galaxy-galaxy lensing
(abridged) We study the accuracy of various approximations to cosmic shear
and weak galaxy-galaxy lensing and investigate effects of Born corrections and
lens-lens coupling. We use ray-tracing through the Millennium Simulation to
calculate various cosmic-shear and galaxy-galaxy-lensing statistics. We compare
the results from ray-tracing to semi-analytic predictions. We find: (i) The
linear approximation provides an excellent fit to cosmic-shear power spectra as
long as the actual matter power spectrum is used as input. Common fitting
formulae, however, strongly underestimate the cosmic-shear power spectra. Halo
models provide a better fit to cosmic shear-power spectra, but there are still
noticeable deviations. (ii) Cosmic-shear B-modes induced by Born corrections
and lens-lens coupling are at least three orders of magnitude smaller than
cosmic-shear E-modes. Semi-analytic extensions to the linear approximation
predict the right order of magnitude for the B-mode. Compared to the
ray-tracing results, however, the semi-analytic predictions may differ by a
factor two on small scales and also show a different scale dependence. (iii)
The linear approximation may under- or overestimate the galaxy-galaxy-lensing
shear signal by several percent due to the neglect of magnification bias, which
may lead to a correlation between the shear and the observed number density of
lenses. We conclude: (i) Current semi-analytic models need to be improved in
order to match the degree of statistical accuracy expected for future
weak-lensing surveys. (ii) Shear B-modes induced by corrections to the linear
approximation are not important for future cosmic-shear surveys. (iii)
Magnification bias can be important for galaxy-galaxy-lensing surveys.Comment: version taking comments into accoun
Weak lensing, dark matter and dark energy
Weak gravitational lensing is rapidly becoming one of the principal probes of
dark matter and dark energy in the universe. In this brief review we outline
how weak lensing helps determine the structure of dark matter halos, measure
the expansion rate of the universe, and distinguish between modified gravity
and dark energy explanations for the acceleration of the universe. We also
discuss requirements on the control of systematic errors so that the
systematics do not appreciably degrade the power of weak lensing as a
cosmological probe.Comment: Invited review article for the GRG special issue on gravitational
lensing (P. Jetzer, Y. Mellier and V. Perlick Eds.). V3: subsection on
three-point function and some references added. Matches the published versio
Constraints on Quintessence From Using Cosmological Data
Recent data, including the three--year WMAP data, the full 2dF galaxy power
spectrum and the first--year data of the Supernova Legacy Survey, are used to
constrain model parameters in quintessence cosmologies. In particular, we
discuss the inverse power--law (RP) and SUGRA potentials and compare parameter
constraints with those for LCDM. Both potentials fit current observations with
a goodness of fit comparable or better than LCDM. The constraints on the energy
scale Lambda_DE appearing in both potential expressions are however different.
For RP, only energy scales around the cosmological constant limit are allowed,
making the allowed models quite similar to LCDM. For SUGRA, Lambda_DE values
approximately up to Electroweak energy scale are still allowed, while other
parameter intervals are slightly but significantly displaced. In particular a
value of the primeval spectral index n_s = 1 is still allowed at the 95% c.l.,
and this can have an impact on constraints on possible inflationary potentials.Comment: 12 pages, 4 figures, submitted to JCA
Evidence for the accelerated expansion of the Universe from weak lensing tomography with COSMOS
We present a tomographic cosmological weak lensing analysis of the HST COSMOS
Survey. Applying our lensing-optimized data reduction, principal component
interpolation for the ACS PSF, and improved modelling of charge-transfer
inefficiency, we measure a lensing signal which is consistent with pure
gravitational modes and no significant shape systematics. We carefully estimate
the statistical uncertainty from simulated COSMOS-like fields obtained from
ray-tracing through the Millennium Simulation. We test our pipeline on
simulated space-based data, recalibrate non-linear power spectrum corrections
using the ray-tracing, employ photometric redshifts to reduce potential
contamination by intrinsic galaxy alignments, and marginalize over systematic
uncertainties. We find that the lensing signal scales with redshift as expected
from General Relativity for a concordance LCDM cosmology, including the full
cross-correlations between different redshift bins. For a flat LCDM cosmology,
we measure sigma_8(Omega_m/0.3)^0.51=0.75+-0.08 from lensing, in perfect
agreement with WMAP-5, yielding joint constraints Omega_m=0.266+0.025-0.023,
sigma_8=0.802+0.028-0.029 (all 68% conf.). Dropping the assumption of flatness
and using HST Key Project and BBN priors only, we find a negative deceleration
parameter q_0 at 94.3% conf. from the tomographic lensing analysis, providing
independent evidence for the accelerated expansion of the Universe. For a flat
wCDM cosmology and prior w in [-2,0], we obtain w<-0.41 (90% conf.). Our dark
energy constraints are still relatively weak solely due to the limited area of
COSMOS. However, they provide an important demonstration for the usefulness of
tomographic weak lensing measurements from space. (abridged)Comment: 26 pages, 25 figures, matches version accepted for publication by
Astronomy and Astrophysic
- …