799 research outputs found
Gravity and Non-gravity Modes in the VIRMOS-DESCART Weak Lensing Survey
We analyze the weak lensing data of the VIRMOS imaging survey using
projections (called E and B-modes) of the two independents observed correlation
functions. The E-mode contains all the lensing signal, while noise and
systematics contribute equally to the E and B modes provided that intrinsic
alignment is negligible. The mode separation allows a measurement of the signal
with a \sqrt{2} smaller error bars, and a separate channel to test for
systematic errors. We apply various transformations, including a spherical
harmonic space power spectrum C^E_l and C^B_l, which provides a direct
measurement of the projected dark matter distribution for 500<l<10^4.Comment: accepted version, minor changes, 18 pages including 6 figure
Cross-Correlating Probes of Primordial Gravitational Waves
One of the most promising ways of detecting primordial gravitational waves
generated during inflation is to observe B-modes of polarization, generated by
Thomson scattering after reionization, in the cosmic microwave background
(CMB). Large scale foregrounds though are expected to be a major systematic
issue, so -- in the event of a tentative detection -- an independent
confirmation of large scale gravitational waves would be most welcome. Previous
authors have suggested searching for the analogous mode of cosmic shear in weak
lensing surveys but have shown that the signal to noise of this mode is
marginal at best. This argument is reconsidered here, accounting for the
cross-correlations of the polarization and lensing B-modes. A lensing survey
can potentially strengthen the argument for a detection of primordial
gravitational waves, although it is unlikely to help constrain the amplitude of
the signal.Comment: 13 pages, 8 figure
Breaking the Degeneracy: Optimal Use of Three-point Weak Lensing Statistics
We study the optimal use of third order statistics in the analysis of weak
lensing by large-scale structure. These higher order statistics have long been
advocated as a powerful tool to break measured degeneracies between
cosmological parameters. Using ray-tracing simulations, incorporating important
survey features such as a realistic depth-dependent redshift distribution, we
find that a joint two- and three-point correlation function analysis is a much
stronger probe of cosmology than the skewness statistic. We compare different
observing strategies, showing that for a limited survey time there is an
optimal depth for the measurement of third-order statistics, which balances
statistical noise and cosmic variance against signal amplitude. We find that
the chosen CFHTLS observing strategy was optimal and forecast that a joint two-
and three-point analysis of the completed CFHTLS-Wide will constrain the
amplitude of the matter power spectrum to 10% and the matter density
parameter to 17%, a factor of ~2.5 improvement on the two-point
analysis alone. Our error analysis includes all non-Gaussian terms, finding
that the coupling between cosmic variance and shot noise is a non-negligible
contribution which should be included in any future analytical error
calculations.Comment: 27 pages, 13 figures, 3 table
Measuring the Deviation from the Linear and Deterministic Bias through Cosmic Gravitational Lensing Effects
Since gravitational lensing effects directly probe inhomogeneities of dark
matter, lensing-galaxy cross-correlations can provide us important information
on the relation between dark matter and galaxy distributions, i.e., the bias.
In this paper, we propose a method to measure the stochasticity/nonlinearity of
the galaxy bias through correlation studies of the cosmic shear and galaxy
number fluctuations. Specifically, we employ the aperture mass statistics
to describe the cosmic shear. We divide the foreground galaxy redshift
into several bins, where is the redshift of the source
galaxies, and calculate the quantity for
each redshift bin. Then the ratio of the summation of over the bins to gives a measure of the
nonlinear/stochastic bias. Here is the projected surface number
density fluctuation of foreground galaxies at redshift , and is
the aperture mass from the cosmic-shear analysis. We estimate that for a
moderately deep weak-lensing survey with , source galaxy surface number
density and a survey area of , the effective -parameter that represents the deviation from the
linear and deterministic bias is detectable in the angular range of 1'-10' if
|r-1|\gsim 10%. For shallow, wide surveys such as the Sloan Digital Sky
Survey with , , and a survey area
of , a 10% detection of is possible over the angular
range .Comment: ApJ in pres
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