216 research outputs found
Implications for the missing low-mass galaxies (satellites) problem from cosmic shear
The number of observed dwarf galaxies, with dark matter mass M in the Milky Way or the Andromeda galaxy does not agree
with predictions from the successful CDM paradigm. To alleviate this
problem a suppression of dark matter clustering power on very small scales has
been conjectured. However, the abundance of dark matter halos outside our
immediate neighbourhood (the Local Group) seem to agree with the
CDM--expected abundance. Here we connect these problems to
observations of weak lensing cosmic shear, pointing out that cosmic shear can
make significant statements about the missing satellites problem in a
statistical way. As an example and pedagogical application we use recent
constraints on small-scales power suppression from measurements of the CFHTLenS
data. We find that, on average, in a region of Gpc there is no
significant small-scale power suppression. This implies that suppression of
small-scale power is not a viable solution to the `missing satellites problem'
or, alternatively, that on average in this volume there is no `missing
satellites problem' for dark matter masses M.
Further analysis of current and future weak lensing surveys will probe much
smaller scales, Mpc corresponding roughly to masses .Comment: Matches published version in MNRAS Letters; no change
Non-parametric Cosmology with Cosmic Shear
We present a method to measure the growth of structure and the background
geometry of the Universe -- with no a priori assumption about the underlying
cosmological model. Using Canada-France-Hawaii Lensing Survey (CFHTLenS) shear
data we simultaneously reconstruct the lensing amplitude, the linear intrinsic
alignment amplitude, the redshift evolving matter power spectrum, P(k,z), and
the co-moving distance, r(z). We find that lensing predominately constrains a
single global power spectrum amplitude and several co-moving distance bins. Our
approach can localise precise scales and redshifts where Lambda-Cold Dark
Matter (LCDM) fails -- if any. We find that below z = 0.4, the measured
co-moving distance r (z) is higher than that expected from the Planck LCDM
cosmology by ~1.5 sigma, while at higher redshifts, our reconstruction is fully
consistent. To validate our reconstruction, we compare LCDM parameter
constraints from the standard cosmic shear likelihood analysis to those found
by fitting to the non-parametric information and we find good agreement.Comment: 13 pages. Matches PRD accepted versio
3D weak lensing with spin wavelets on the ball
We construct the spin flaglet transform, a wavelet transform to analyze spin
signals in three dimensions. Spin flaglets can probe signal content localized
simultaneously in space and frequency and, moreover, are separable so that
their angular and radial properties can be controlled independently. They are
particularly suited to analyzing of cosmological observations such as the weak
gravitational lensing of galaxies. Such observations have a unique 3D
geometrical setting since they are natively made on the sky, have spin angular
symmetries, and are extended in the radial direction by additional distance or
redshift information. Flaglets are constructed in the harmonic space defined by
the Fourier-Laguerre transform, previously defined for scalar functions and
extended here to signals with spin symmetries. Thanks to various sampling
theorems, both the Fourier-Laguerre and flaglet transforms are theoretically
exact when applied to bandlimited signals. In other words, in numerical
computations the only loss of information is due to the finite representation
of floating point numbers. We develop a 3D framework relating the weak lensing
power spectrum to covariances of flaglet coefficients. We suggest that the
resulting novel flaglet weak lensing estimator offers a powerful alternative to
common 2D and 3D approaches to accurately capture cosmological information.
While standard weak lensing analyses focus on either real or harmonic space
representations (i.e., correlation functions or Fourier-Bessel power spectra,
respectively), a wavelet approach inherits the advantages of both techniques,
where both complicated sky coverage and uncertainties associated with the
physical modeling of small scales can be handled effectively. Our codes to
compute the Fourier-Laguerre and flaglet transforms are made publicly
available.Comment: 24 pages, 4 figures, version accepted for publication in PR
Post-Limber Weak Lensing Bispectrum, Reduced Shear Correction, and Magnification Bias Correction
The significant increase in precision that will be achieved by Stage IV
cosmic shear surveys means that several currently used theoretical
approximations may cease to be valid. An additional layer of complexity arises
from the fact that many of these approximations are interdependent; the
procedure to correct for one involves making another. Two such approximations
that must be relaxed for upcoming experiments are the reduced shear
approximation and the effect of neglecting magnification bias. Accomplishing
this involves the calculation of the convergence bispectrum; typically subject
to the Limber approximation. In this work, we compute the post-Limber
convergence bispectrum, and the post-Limber reduced shear and magnification
bias corrections to the angular power spectrum for a Euclid-like survey. We
find that the Limber approximation significantly overestimates the bispectrum
when any side of the bispectrum triangle, . However, the resulting
changes in the reduced shear and magnification bias corrections are well below
the sample variance for . We also compute a worst-case scenario
for the additional biases on CDM cosmological parameters that result
from the difference between the post-Limber and Limber approximated forms of
the corrections. These further demonstrate that the reduced shear and
magnification bias corrections can safely be treated under the Limber
approximation for upcoming surveys.Comment: 12 pages, 4 figures. Accepted by Phys. Rev. D. Matches published
versio
Cosmological Parameter Biases from Doppler-Shifted Weak Lensing in Stage IV Experiments
The advent of Stage IV weak lensing surveys will open up a new era in
precision cosmology. These experiments will offer more than an
order-of-magnitude leap in precision over existing surveys, and we must ensure
that the accuracy of our theory matches this. Accordingly, it is necessary to
explicitly evaluate the impact of the theoretical assumptions made in current
analyses on upcoming surveys. One effect typically neglected in present
analyses is the Doppler-shift of the measured source comoving distances. Using
Fisher matrices, we calculate the biases on the cosmological parameter values
inferred from a Euclid-like survey, if the correction for this Doppler-shift is
omitted. We find that this Doppler-shift can be safely neglected for Stage IV
surveys. The code used in this investigation is made publicly available.Comment: 8 pages, 1 figure. Accepted to Phys. Rev. D. Matches published
version. Code available at https://github.com/desh1701/k-cut_reduced_shea
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