12,126 research outputs found
Measuring dark energy properties with 3D cosmic shear
We present parameter estimation forecasts for present and future 3D cosmic
shear surveys. We demonstrate that, in conjunction with results from cosmic
microwave background (CMB) experiments, the properties of dark energy can be
estimated with very high precision with large-scale, fully 3D weak lensing
surveys. In particular, a 5-band, 10,000 square degree ground-based survey to a
median redshift of zm=0.7 could achieve 1- marginal statistical errors,
in combination with the constraints expected from the CMB Planck Surveyor, of
w0=0.108 and wa=0.099 where we parameterize w by
w(a)=w0+wa(1-a) where a is the scale factor. Such a survey is achievable with a
wide-field camera on a 4 metre class telescope. The error on the value of w at
an intermediate pivot redshift of z=0.368 is constrained to
w(z=0.368)=0.0175. We compare and combine the 3D weak lensing
constraints with the cosmological and dark energy parameters measured from
planned Baryon Acoustic Oscillation (BAO) and supernova Type Ia experiments,
and find that 3D weak lensing significantly improves the marginalized errors. A
combination of 3D weak lensing, CMB and BAO experiments could achieve
w0=0.037 and wa=0.099. Fully 3D weak shear analysis avoids the
loss of information inherent in tomographic binning, and we show that the
sensitivity to systematic errors is much less. In conjunction with the fact
that the physics of lensing is very soundly based, this analysis demonstrates
that deep, wide-angle 3D weak lensing surveys are extremely promising for
measuring dark energy properties.Comment: 18 pages, 16 figures. Accepted to MNRAS. Figures now in grayscale.
Further discussions on non-Gaussianity and photometric redshift errors. Some
references adde
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
Cosmology with the largest galaxy cluster surveys: Going beyond Fisher matrix forecasts
We make the first detailed MCMC likelihood study of cosmological constraints
that are expected from some of the largest, ongoing and proposed, cluster
surveys in different wave-bands and compare the estimates to the prevalent
Fisher matrix forecasts. Mock catalogs of cluster counts expected from the
surveys -- eROSITA, WFXT, RCS2, DES and Planck, along with a mock dataset of
follow-up mass calibrations are analyzed for this purpose. A fair agreement
between MCMC and Fisher results is found only in the case of minimal models.
However, for many cases, the marginalized constraints obtained from Fisher and
MCMC methods can differ by factors of 30-100%. The discrepancy can be
alarmingly large for a time dependent dark energy equation of state, w(a); the
Fisher methods are seen to under-estimate the constraints by as much as a
factor of 4--5. Typically, Fisher estimates become more and more inappropriate
as we move away from LCDM, to a constant-w dark energy to varying-w dark energy
cosmologies. Fisher analysis, also, predicts incorrect parameter degeneracies.
From the point of mass-calibration uncertainties, a high value of unknown
scatter about the mean mass-observable relation, and its redshift dependence,
is seen to have large degeneracies with the cosmological parameters sigma_8 and
w(a) and can degrade the cosmological constraints considerably. We find that
the addition of mass-calibrated cluster datasets can improve dark energy and
sigma_8 constraints by factors of 2--3 from what can be obtained compared to
CMB+SNe+BAO only. Since, details of future cluster surveys are still being
planned, we emphasize that optimal survey design must be done using MCMC
analysis rather than Fisher forecasting. [abridged]Comment: 26 pages, 13 figures, 7 tables, accepted for publication in JCA
Simultaneous measurement of multiple parameters of a subwavelength structure based on the weak value formalism
A mathematical extension of the weak value formalism to the simultaneous
measurement of multiple parameters is presented in the context of an optical
focused vector beam scatterometry experiment. In this example, preselection and
postselection are achieved via spatially-varying polarization control, which
can be tailored to optimize the sensitivity to parameter variations. Initial
experiments for the two-parameter case demonstrate that this method can be used
to measure physical parameters with resolutions at least 1000 times smaller
than the wavelength of illumination
Spectral Templates from Multicolor Redshift Surveys
Understanding how the physical properties of galaxies (e.g. their spectral
type or age) evolve as a function of redshift relies on having an accurate
representation of galaxy spectral energy distributions. While it has been known
for some time that galaxy spectra can be reconstructed from a handful of
orthogonal basis templates, the underlying basis is poorly constrained. The
limiting factor has been the lack of large samples of galaxies (covering a wide
range in spectral type) with high signal-to-noise spectrophotometric
observations. To alleviate this problem we introduce here a new technique for
reconstructing galaxy spectral energy distributions directly from samples of
galaxies with broadband photometric data and spectroscopic redshifts.
Exploiting the statistical approach of the Karhunen-Loeve expansion, our
iterative training procedure increasingly improves the eigenbasis, so that it
provides better agreement with the photometry. We demonstrate the utility of
this approach by applying these improved spectral energy distributions to the
estimation of photometric redshifts for the HDF sample of galaxies. We find
that in a small number of iterations the dispersion in the photometric
redshifts estimator (a comparison between predicted and measured redshifts) can
decrease by up to a factor of 2.Comment: 25 pages, 9 figures, LaTeX AASTeX, accepted for publication in A
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