1,233 research outputs found
CMB anisotropies from pre-big bang cosmology
We present an alternative scenario for cosmic structure formation where
initial fluctuations are due to Kalb-Ramond axions produced during a pre-big
bang phase of inflation. We investigate whether this scenario, where the
fluctuations are induced by seeds and therefore are of isocurvature nature, can
be brought in agreement with present observations by a suitable choice of
cosmological parameters. We also discuss several observational signatures which
can distinguish axion seeds from standard inflationary models. We finally
discuss the gravitational wave background induced in this model and we show
that it may be well within the range of future observations.Comment: 33 pages, 18 figures, corrected some typo
From Cavendish to PLANCK: Constraining Newton's Gravitational Constant with CMB Temperature and Polarization Anisotropy
We present new constraints on cosmic variations of Newton's gravitational
constant by making use of the latest CMB data from WMAP, BOOMERANG, CBI and
ACBAR experiments and independent constraints coming from Big Bang
Nucleosynthesis. We found that current CMB data provide constraints at the 10%
level, that can be improved to 3% by including BBN data. We show that future
data expected from the Planck satellite could constrain G at the 1.5% level
while an ultimate, cosmic variance limited, CMB experiment could reach a
precision of about 0.4%, competitive with current laboratory measurements.Comment: 6 pages, 8 figures, corrected typos, added reference
A Multi-Parameter Investigation of Gravitational Slip
A detailed analysis of gravitational slip, a new post-general relativity
cosmological parameter characterizing the degree of departure of the laws of
gravitation from general relativity on cosmological scales, is presented. This
phenomenological approach assumes that cosmic acceleration is due to new
gravitational effects; the amount of spacetime curvature produced per unit mass
is changed in such a way that a universe containing only matter and radiation
begins to accelerate as if under the influence of a cosmological constant.
Changes in the law of gravitation are further manifest in the behavior of the
inhomogeneous gravitational field, as reflected in the cosmic microwave
background, weak lensing, and evolution of large-scale structure. The new
parameter, , is naively expected to be of order unity. However, a
multiparameter analysis, allowing for variation of all the standard
cosmological parameters, finds that
where corresponds to a CDM universe under general
relativity. Future probes of the cosmic microwave background (Planck) and
large-scale structure (Euclid) may improve the limits by a factor of four.Comment: 7 pages, 9 figures, colo
Cosmic Microwave Background Temperature at Galaxy Clusters
We have deduced the cosmic microwave background (CMB) temperature in the Coma
cluster (A1656, ), and in A2163 () from spectral
measurements of the Sunyaev-Zel'dovich (SZ) effect over four passbands at radio
and microwave frequencies. The resulting temperatures at these redshifts are
K and K, respectively. These values confirm the expected
relation , where K is the value
measured by the COBE/FIRAS experiment. Alternative scaling relations that are
conjectured in non-standard cosmologies can be constrained by the data; for
example, if or , then
and (at 95% confidence). We
briefly discuss future prospects for more precise SZ measurements of at
higher redshifts.Comment: 13 pages, 1 figure, ApJL accepted for publicatio
Cosmic Microwave Background anisotropies and extra dimensions in String Cosmology
A recently proposed mechanism for large-scale structure in string cosmology --based on massless axionic seeds-- is further analyzed and extended to the acoustic-peak region. Existence, structure, and normalization of the peaks turn out to depend crucially on the overall evolution of extra dimensions during the pre-big bang phase: conversely, precise cosmic microwave background anisotropy data in the acoustic-peak region will provide, within the next decade, a window on string-theory's extra dimensions before their eventual compactification
The power spectrum of systematics in cosmic shear tomography and the bias on cosmological parameters
Cosmic shear tomography has emerged as one of the most promising tools to
both investigate the nature of dark energy and discriminate between General
Relativity and modified gravity theories. In order to successfully achieve
these goals, systematics in shear measurements have to be taken into account;
their impact on the weak lensing power spectrum has to be carefully
investigated in order to estimate the bias induced on the inferred cosmological
parameters. To this end, we develop here an efficient tool to compute the power
spectrum of systematics by propagating, in a realistic way, shear measurement,
source properties and survey setup uncertainties. Starting from analytical
results for unweighted moments and general assumptions on the relation between
measured and actual shear, we derive analytical expressions for the
multiplicative and additive bias, showing how these terms depend not only on
the shape measurement errors, but also on the properties of the source galaxies
(namely, size, magnitude and spectral energy distribution). We are then able to
compute the amplitude of the systematics power spectrum and its scaling with
redshift, while we propose a multigaussian expansion to model in a
non-parametric way its angular scale dependence. Our method allows to
self-consistently propagate the systematics uncertainties to the finally
observed shear power spectrum, thus allowing us to quantify the departures from
the actual spectrum. We show that even a modest level of systematics can induce
non-negligible deviations, thus leading to a significant bias on the recovered
cosmological parameters.Comment: 19 pages, 5 tables, 4 figure
Constraining Dark Matter annihilation with the Cosmic Microwave Background
I review one of the numerous physical processes that might change the standard model of recombination, i.e. the annihilation of Dark Matter particles. The high precision of current and future CMB data may allow the detection of these processes, that leave recognizable imprints on the angular power spectra. I review some of the results obtained in constraining this phenomenon using current WMAP5
data and forecasted data for future experiments such as the Planck satellite mission
Laue Lens Development for Hard X-rays (>60 keV)
Results of reflectivity measurements of mosaic crystal samples of Cu (111)
are reported. These tests were performed in the context of a feasibility study
of a hard X-ray focusing telescope for space astronomy with energy passband
from 60 to 600 keV. The technique envisaged is that of using mosaic crystals in
transmission configuration that diffract X-rays for Bragg diffraction (Laue
lens). The Laue lens assumed has a spherical shape with focal length . It is
made of flat mosaic crystal tiles suitably positioned in the lens. The samples
were grown and worked for this project at the Institute Laue-Langevin (ILL) in
Grenoble (France), while the reflectivity tests were performed at the X-ray
facility of the Physics Department of the University of Ferrara.Comment: 6 pages, 12 figures, accepted for publication in IEEE Transactions on
Nuclear Scienc
Triple Experiment Spectrum of the Sunyaev-Zeldovich Effect in the Coma Cluster: H_0
The Sunyaev-Zeldovich (SZ) effect was previously measured in the Coma cluster
by the Owens Valley Radio Observatory and Millimeter and IR Testa Grigia
Observatory experiments and recently also with the Wilkinson Microwave
Anisotropy Probe satellite. We assess the consistency of these results and
their implications on the feasibility of high-frequency SZ work with
ground-based telescopes. The unique data set from the combined measurements at
six frequency bands is jointly analyzed, resulting in a best-fit value for the
Thomson optical depth at the cluster center, tau_{0}=(5.35 \pm 0.67) 10^{-3}.
The combined X-ray and SZ determined properties of the gas are used to
determine the Hubble constant. For isothermal gas with a \beta density profile
we derive H_0 = 84 \pm 26 km/(s\cdot Mpc); the (1\sigma) error includes only
observational SZ and X-ray uncertainties.Comment: 11 pages, 1 figur
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