17 research outputs found
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
Observational constraints in scalar tensor theory with tachyonic potential
We study the dynamics of the scalar tensor cosmological model in the presence
of tachyon field. In an alternative approach, in two exponential and power law
form of the scalar field functions in the model, field equations are solved by
simultaneously best fitting the model parameters with the most recent
observational data. This approach gives us an observationally verified
interpretation of the dynamics of the universe. We then discuss the best fitted
of equation of state parameter, the statefinder parameters and the
reconstructed scalar field in the model.Comment: 16 pages, 19 figures, Will be published in JCA
Stability analysis and Observational Measurement in Chameleonic Generalised Brans--Dicke Cosmology
We investigate the dynamics of the chameleonic Generalised Brans--Dicke model
in flat FRW cosmology. In a new approach, a framework to study stability and
attractor solutions in the phase space is developed for the model by
simultaneously best fitting the stability and model parameters with the
observational data. The results show that for an accelerating universe the
phantom crossing does not occur in the past and near future.Comment: 15 pages, 18 figure
A Calibration of NICMOS Camera 2 for Low Count-Rates
NICMOS 2 observations are crucial for constraining distances to most of the
existing sample of z > 1 SNe Ia. Unlike the conventional calibration programs,
these observations involve long exposure times and low count rates. Reciprocity
failure is known to exist in HgCdTe devices and a correction for this effect
has already been implemented for high and medium count-rates. However
observations at faint count-rates rely on extrapolations. Here instead, we
provide a new zeropoint calibration directly applicable to faint sources. This
is obtained via inter-calibration of NIC2 F110W/F160W with WFC3 in the low
count-rate regime using z ~ 1 elliptical galaxies as tertiary calibrators.
These objects have relatively simple near-IR SEDs, uniform colors, and their
extended nature gives superior signal-to-noise at the same count rate than
would stars. The use of extended objects also allows greater tolerances on PSF
profiles. We find ST magnitude zeropoints (after the installation of the NICMOS
cooling system, NCS) of 25.296 +- 0.022 for F110W and 25.803 +- 0.023 for
F160W, both in agreement with the calibration extrapolated from count-rates
1,000 times larger (25.262 and 25.799). Before the installation of the NCS, we
find 24.843 +- 0.025 for F110W and 25.498 +- 0.021 for F160W, also in agreement
with the high-count-rate calibration (24.815 and 25.470). We also check the
standard bandpasses of WFC3 and NICMOS 2 using a range of stars and galaxies at
different colors and find mild tension for WFC3, limiting the accuracy of the
zeropoints. To avoid human bias, our cross-calibration was "blinded" in that
the fitted zeropoint differences were hidden until the analysis was finalized.Comment: Accepted for Publication in the Astronomical Journal. New version
contains added referenc
Bulk scalar field in brane-worlds with induced gravity inspired by the term
We obtain the effective field equations in a brane-world scenario within the
framework of a DGP model where the action on the brane is an arbitrary function
of the Ricci scalar, , and the bulk action includes a scalar field
in the matter Lagrangian. We obtain the Friedmann equations and acceleration
conditions in the presence of the bulk scalar field for the term in
four-dimensional gravity.Comment: 9 pages, to appear in JCA
Dynamics of Void and its Shape in Redshift Space
We investigate the dynamics of a single spherical void embedded in a
Friedmann-Lema\^itre universe, and analyze the void shape in the redshift
space. We find that the void in the redshift space appears as an ellipse shape
elongated in the direction of the line of sight (i.e., an opposite deformation
to the Kaiser effect). Applying this result to observed void candidates at the
redshift z~1-2, it may provide us with a new method to evaluate the
cosmological parameters, in particular the value of a cosmological constant.Comment: 19 pages, 11 figure
Tension between SN and BAO: current status and future forecasts
Using real and synthetic Type Ia SNe (SNeIa) and baryon acoustic oscillations
(BAO) data representing current observations forecasts, this paper investigates
the tension between those probes in the dark energy equation of state (EoS)
reconstruction considering the well known CPL model and Wang's low correlation
reformulation. In particular, here we present simulations of BAO data from both
the the radial and transverse directions. We also explore the influence of
priors on Omega_m and Omega_b on the tension issue, by considering 1-sigma
deviations in either one or both of them. Our results indicate that for some
priors there is no tension between a single dataset (either SNeIa or BAO) and
their combination (SNeIa+BAO). Our criterion to discern the existence of
tension (sigma-distance) is also useful to establish which is the dataset with
most constraining power; in this respect SNeIa and BAO data switch roles when
current and future data are considered, as forecasts predict and spectacular
quality improvement on BAO data. We also find that the results on the tension
are blind to the way the CPL model is addressed: there is a perfect match
between the original formulation and that by the low correlation optimized, but
the errors on the parameters are much narrower in all cases of our exhaustive
exploration, thus serving the purpose of stressing the convenience of this
reparametrization.Comment: 21 pages, under review in JCA
Casimir Energies for 6D Supergravities Compactified on T_2/Z_N with Wilson Lines
We compute (as functions of the shape and Wilson-line moduli) the one-loop
Casimir energy induced by higher-dimensional supergravities compactified from
6D to 4D on 2-tori, and on some of their Z_N orbifolds. Detailed calculations
are given for a 6D scalar field having an arbitrary 6D mass m, and we show how
to extend these results to higher-spin fields for supersymmetric 6D theories.
Particular attention is paid to regularization issues and to the identification
of the divergences of the potential, as well as the dependence of the result on
m, including limits for which m^2 A> 1 where A is the volume of
the internal 2 dimensions. Our calculation extends those in the literature to
very general boundary conditions for fields about the various cycles of these
geometries. The results have potential applications towards Supersymmetric
Large Extra Dimensions (SLED) as a theory of the Dark Energy. First, they
provide an explicit calculation within which to follow the dependence of the
result on the mass of the bulk states which travel within the loop, and for
heavy masses these results bear out the more general analysis of the
UV-sensitivity obtained using heat-kernel methods. Second, because the
potentials we find describe the dynamics of the classical flat directions of
these compactifications, within SLED they would describe the present-day
dynamics of the Dark Energy.Comment: 40 pages, 7 figure
Implications For The Hubble Constant from the First Seven Supernovae at z >= 0.35
The Supernova Cosmology Project has discovered over twenty-eight supernovae
(SNe) at 0.35 <z < 0.65 in an ongoing program that uses Type Ia SNe as
high-redshift distance indicators. Here we present measurements of the ratio
between the locally observed and global Hubble constants, H_0^L/H_0^G, based on
the first 7 SNe of this high-redshift data set compared with 18 SNe at z <= 0.1
from the Calan/Tololo survey. If Omega_M <= 1, then light-curve-width corrected
SN magnitudes yield H_0^L/H_0^G < 1.10 (95% confidence level) in both a
Lambda=0 and a flat universe. The analysis using the SNe Ia as standard candles
without a light-curve-width correction yields similar results. These results
rule out the hypothesis that the discrepant ages of the Universe derived from
globular clusters and recent measurements of the Hubble constant are
attributable to a locally underdense bubble. Using the
Cepheid-distance-calibrated absolute magnitudes for SNe Ia of Sandage (1996},
we can also measure the global Hubble constant, H_0^G. If Omega_M >= 0.2, we
find that H_0^G < 70 km/s/Mpc in a Lambda=0 universe and H_0^G < 78 km/s/Mpc in
a flat universe, correcting the distant and local SN apparent magnitudes for
light curve width. Lower results for H_0^G are obtained if the magnitudes are
not width corrected.Comment: 13 pages, 2 Postscript figures. Preprint also available at
http://www-supernova.lbl.gov . To appear in ApJ Letter
Timescale Stretch Parameterization of Type Ia Supernova B-band Light Curves
R-band intensity measurements along the light curve of Type Ia supernovae discovered by the Supernova Cosmology Project (SCP) are fitted in brightness to templates allowing a free parameter the time-axis width factor w = s(1+z). The data points are then individually aligned in the time-axis, normalized and K-corrected back to the rest frame, after which the nearly 1300 normalized intensity measurements are found to lie on a well-determined common rest-frame B-band curve which we call the ``composite curve''. The same procedure is applied to 18 low-redshift Calan/Tololo SNe with z < 0.11; these nearly 300 B-band photometry points are found to lie on the composite curve equally well. The SCP search technique produces several measurements before maximum light for each supernova. We demonstrate that the linear stretch factor, s, which parameterizes the light-curve timescale appears independent of z,and applies equally well to the declining and rising parts of the light curve. In fact, the B-band template that best fits this composite curve fits the individual supernova photometry data when stretched by a factor s with chi^2/DoF approx = 1, thus as well as any parameterization can, given the current data sets. The measurement of the date of explosion, however, is model dependent and not tightly constrained by the current data. We also demonstrate the 1+z light-curve time-axis broadening expected from cosmological expansion. This argues strongly against alternative explanations, such as tired light, for the redshift of distant objects