198 research outputs found
Developing a unified pipeline for large-scale structure data analysis with angular power spectra -- I. The importance of redshift-space distortions for galaxy number counts
We develop a cosmological parameter estimation code for (tomographic) angular
power spectra analyses of galaxy number counts, for which we include, for the
first time, redshift-space distortions (RSD) in the Limber approximation. This
allows for a speed-up in computation time, and we emphasise that only angular
scales where the Limber approximation is valid are included in our analysis.
Our main result shows that a correct modelling of RSD is crucial not to bias
cosmological parameter estimation. This happens not only for
spectroscopy-detected galaxies, but even in the case of galaxy surveys with
photometric redshift estimates. Moreover, a correct implementation of RSD is
especially valuable in alleviating the degeneracy between the amplitude of the
underlying matter power spectrum and the galaxy bias. We argue that our
findings are particularly relevant for present and planned observational
campaigns, such as the Euclid satellite or the Square Kilometre Array, which
aim at studying the cosmic large-scale structure and trace its growth over a
wide range of redshifts and scales.Comment: 18 pages, 11 figures, 4 tables. New expression for RSDs in Limber
approximation (Eq. 9), much easier to implement in numerical codes. Results
on "conservative scenario" slightly change
Magnification bias as a novel probe for primordial magnetic fields
In this paper we investigate magnetic fields generated in the early Universe.
These fields are important candidates at explaining the origin of astrophysical
magnetism observed in galaxies and galaxy clusters, whose genesis is still by
and large unclear. Compared to the standard inflationary power spectrum,
intermediate to small scales would experience further substantial matter
clustering, were a cosmological magnetic field present prior to recombination.
As a consequence, the bias and redshift distribution of galaxies would also be
modified. Hitherto, primordial magnetic fields (PMFs) have been tested and
constrained with a number of cosmological observables, e.g. the cosmic
microwave background radiation, galaxy clustering and, more recently, weak
gravitational lensing. Here, we explore the constraining potential of the
density fluctuation bias induced by gravitational lensing magnification onto
the galaxy-galaxy angular power spectrum. Such an effect is known as
magnification bias. Compared to the usual galaxy clustering approach,
magnification bias helps in lifting the pathological degeneracy present amongst
power spectrum normalisation and galaxy bias. This is because magnification
bias cross-correlates galaxy number density fluctuations of nearby objects with
weak lensing distortions of high-redshift sources. Thus, it takes advantage of
the gravitational deflection of light, which is insensitive to galaxy bias but
powerful in constraining the density fluctuation amplitude. To scrutinise the
potentiality of this method, we adopt a deep and wide-field spectroscopic
galaxy survey. We show that magnification bias does contain important
information on primordial magnetism, which will be useful in combination with
galaxy clustering and shear. We find we shall be able to rule out at 95.4% CL
amplitudes of PMFs larger than 0.0005 nG for values of the PMF power spectral
index ~0.Comment: 21 pages, 9 figures; published on JCA
Inclusive Constraints on Unified Dark Matter Models from Future Large-Scale Surveys
In the very last years, cosmological models where the properties of the dark
components of the Universe - dark matter and dark energy - are accounted for by
a single "dark fluid" have drawn increasing attention and interest. Amongst
many proposals, Unified Dark Matter (UDM) cosmologies are promising candidates
as effective theories. In these models, a scalar field with a non-canonical
kinetic term in its Lagrangian mimics both the accelerated expansion of the
Universe at late times and the clustering properties of the large-scale
structure of the cosmos. However, UDM models also present peculiar behaviours,
the most interesting one being the fact that the perturbations in the
dark-matter component of the scalar field do have a non-negligible speed of
sound. This gives rise to an effective Jeans scale for the Newtonian potential,
below which the dark fluid does not cluster any more. This implies a growth of
structures fairly different from that of the concordance LCDM model. In this
paper, we demonstrate that forthcoming large-scale surveys will be able to
discriminate between viable UDM models and LCDM to a good degree of accuracy.
To this purpose, the planned Euclid satellite will be a powerful tool, since it
will provide very accurate data on galaxy clustering and the weak lensing
effect of cosmic shear. Finally, we also exploit the constraining power of the
ongoing CMB Planck experiment. Although our approach is the most conservative,
with the inclusion of only well-understood, linear dynamics, in the end we also
show what could be done if some amount of non-linear information were included.Comment: 22 pages, 4 figures, 2 table
Does Quartessence Ease Tensions?
Tensions between cosmic microwave background observations and the growth of
the large-scale structure inferred from late-time probes pose a serious
challenge to the concordance CDM cosmological model. State-of-the-art
data from the Planck satellite predicts a higher rate of structure growth than
what preferred by low-redshift observables. Such tension has hitherto eluded
conclusive explanations in terms of straightforward modifications to
CDM, e.g. the inclusion of massive neutrinos or a dynamical dark
energy component. Here, we investigate models of 'quartessence' -- a single
dark component mimicking both dark matter and dark energy -- whose
non-vanishing sound speed inhibits structure growth at late times on scales
smaller than its corresponding Jeans' length. In principle, this could
reconcile high- and low-redshift observations. We put this hypothesis to test
against temperature and polarisation spectra from the latest Planck release,
SDSS DR12 measurements of baryon acoustic oscillations and redshift-space
distortions, and cosmic shear correlation functions from KiDS. This the first
time that any specific model of quartessence is applied to actual data. We show
that, if we naively apply CDM nonlinear prescription to quartessence,
the combined data sets allow for tight constraints on the model parameters.
Apparently, quartessence alleviates the tension between the total matter
fraction and late-time structure clustering, although in fact the tension is
transferred from the latter to the quartessence sound speed parameter. However,
we found that this strongly depends upon information from nonlinear scales.
Indeed, if we relax this assumption, quartessence models appear still viable.
For this reason, we argue that the nonlinear behaviour of quartessence deserves
further investigation and may lead to a deeper understanding of the physics of
the dark Universe.Comment: 8 pages, 6 figures, 1 table; matching published versio
An updated analysis of two classes of f(R) theories of gravity
The observed accelerated cosmic expansion can be a signature of
fourth\,-\,order gravity theories, where the acceleration of the Universe is a
consequence of departures from Einstein General Relativity, rather than the
sign of the existence of a fluid with negative pressure. In the
fourth\,-\,order gravity theories, the gravity Lagrangian is described by an
analytic function of the scalar curvature subject to the demanding
conditions that no detectable deviations from standard GR is observed on the
Solar System scale. Here we consider two classes of theories able to
pass Solar System tests and investigate their viability on cosmological scales.
To this end, we fit the theories to a large dataset including the combined
Hubble diagram of Type Ia Supernovae and Gamma Ray Bursts, the Hubble parameter
data from passively evolving red galaxies, Baryon Acoustic Oscillations
extracted from the seventh data release of the Sloan Digital Sky Survey (SDSS)
and the distance priors from the Wilkinson Microwave Anisotropy Probe seven
years (WMAP7) data. We find that both classes of fit very well this
large dataset with the present\,-\,day values of the matter density, Hubble
constant and deceleration parameter in agreement with previous estimates;
however, the strong degeneracy among the parameters prevents us from
strongly constraining their values. We also derive the growth factor , with the matter density
perturbation, and show that it can still be well approximated by . We finally constrain (on some representative
scales) and investigate its redshift dependence to see whether future data can
discriminate between these classes of theories and standard dark energy
models.Comment: 27 pages, 5 figures, 1 table, accepted for publication on JCAP. Note
that this paper updates and supersedes preprint arXiv:0907.468
SKA Weak Lensing II: Simulated Performance and Survey Design Considerations
We construct a pipeline for simulating weak lensing cosmology surveys with
the Square Kilometre Array (SKA), taking as inputs telescope sensitivity
curves; correlated source flux, size and redshift distributions; a simple
ionospheric model; source redshift and ellipticity measurement errors. We then
use this simulation pipeline to optimise a 2-year weak lensing survey performed
with the first deployment of the SKA (SKA1). Our assessments are based on the
total signal-to-noise of the recovered shear power spectra, a metric that we
find to correlate very well with a standard dark energy figure of merit. We
first consider the choice of frequency band, trading off increases in number
counts at lower frequencies against poorer resolution; our analysis strongly
prefers the higher frequency Band 2 (950-1760 MHz) channel of the SKA-MID
telescope to the lower frequency Band 1 (350-1050 MHz). Best results would be
obtained by allowing the centre of Band 2 to shift towards lower frequency,
around 1.1 GHz. We then move on to consider survey size, finding that an area
of 5,000 square degrees is optimal for most SKA1 instrumental configurations.
Finally, we forecast the performance of a weak lensing survey with the second
deployment of the SKA. The increased survey size (3\,steradian) and
sensitivity improves both the signal-to-noise and the dark energy metrics by
two orders of magnitude.Comment: 15 pages, 11 figures, 1 table. Comments welcome. Updated to match
published versio
SKA Weak Lensing III: Added Value of Multi-Wavelength Synergies for the Mitigation of Systematics
In this third paper of a series on radio weak lensing for cosmology with the
Square Kilometre Array, we scrutinise synergies between cosmic shear
measurements in the radio and optical/near-IR bands for mitigating systematic
effects. We focus on three main classes of systematics: (i) experimental
systematic errors in the observed shear; (ii) signal contamination by intrinsic
alignments; and (iii) systematic effects due to an incorrect modelling of
non-linear scales. First, we show that a comprehensive, multi-wavelength
analysis provides a self-calibration method for experimental systematic
effects, only implying <50% increment on the errors on cosmological parameters.
We also illustrate how the cross-correlation between radio and optical/near-IR
surveys alone is able to remove residual systematics with variance as large as
0.00001, i.e. the same order of magnitude of the cosmological signal. This also
opens the possibility of using such a cross-correlation as a means to detect
unknown experimental systematics. Secondly, we demonstrate that, thanks to
polarisation information, radio weak lensing surveys will be able to mitigate
contamination by intrinsic alignments, in a way similar but fully complementary
to available self-calibration methods based on position-shear correlations.
Lastly, we illustrate how radio weak lensing experiments, reaching higher
redshifts than those accessible to optical surveys, will probe dark energy and
the growth of cosmic structures in regimes less contaminated by non-linearities
in the matter perturbations. For instance, the higher-redshift bins of radio
catalogues peak at z~0.8-1, whereas their optical/near-IR counterparts are
limited to z<0.5-0.7. This translates into having a cosmological signal 2 to 5
times less contaminated by non-linear perturbations.Comment: 16 pages, 10 figures, 2 tables; improved discussion of experimental
systematics in Sec. 2; updated to match published versio
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