965 research outputs found
The effect of imperfect corrections of PSF anisotropy on cosmic shear measurements
Current measurements of the weak lensing signal induced by large scale
structure provide useful constraints on a range of cosmological parameters.
However, the ultimate succes of this technique depends on the accuracy with
which one can correct for the effect of the Point Spread Function (PSF). In
this paper we examine the accuracy of the PSF anisotropy correction using
images of fields with a large number of stars. The ellipticity correlation
function of the residuals is studied to quantify the effect of imperfect
corrections for PSF anisotropy on cosmic shear studies. These imperfections
occur on the chip scale and consequently the systematic signal decreases
rapidly with increasing angular scale. Separation of the signal into ``E''
(curl-free) and ``B'' (curl) components can help to identify the presence of
residual systematics, but in general, the amplitude of the ``B''-mode is
different from that of the ``E''-mode. The study of fields with many stars can
be beneficial in finding a proper description of the variation of PSF
anisotropy, and consequently help to significantly improve the accuracy with
which the cosmic shear signal can be measured. We show that with such an
approach it is feasible that the accuracy of future cosmic shear studies is
limited by the statistical noise introduced by the intrinsic shapes of the
sources. In particular, the prospects for accurate measurements of the cosmic
shear signal on scales larger than ~10 arcminutes are excellent.Comment: submitted to MNRAS, 8 page
Non-Gaussian Foreground Residuals of the WMAP First Year Maps
We investigate the effect of foreground residuals in the WMAP data (Bennet et
al. 2004) by adding foreground contamination to Gaussian ensembles of CMB
signal and noise maps. We evaluate a set of non-Gaussian estimators on the
contaminated ensembles to determine with what accuracy any residual in the data
can be constrained using higher order statistics. We apply the estimators to
the raw and cleaned Q, V, and W band first year maps. The foreground
subtraction method applied to clean the data in Bennet et al. (2004a) appears
to have induced a correlation between the power spectra and normalized
bispectra of the maps which is absent in Gaussian simulations. It also appears
to increase the correlation between the dl=1 inter-l bispectrum of the cleaned
maps and the foreground templates. In a number of cases the significance of the
effect is above the 98% confidence level.Comment: 9 pages, 4 figure
Gravitational Instability of de Sitter Compactifications
We consider warped compactifications in (4+d)-dimensional theories, with four
dimensional de Sitter dS_4 vacua (with Hubble parameter H) and with a compact
internal space. After introducing a gauge-invariant formalism for the generic
metric perturbations of these backgrounds, we focus on modes which are scalar
with respect to dS_4. The physical eigenmasses of these modes acquire a large
universal tachyonic contribution -12d/(d+2) H^2, independently of the
stabilization mechanism for the compact space, in addition to the usual KK
masses, which instead encode the effects of the stabilization. General
arguments, as well as specific examples, lead us to conjecture that, for
sufficiently large dS curvature, the compactified geometry becomes
gravitationally unstable due to the tachyonic growth of the scalar
perturbations. This mean that for any stabilization mechanism the curvature of
the dS geometry cannot exceed some critical value. We relate this effect to the
anisotropy of the bulk geometry and suggest the end points of the instability.
Of relevance for inflationary cosmology, the perturbations of the bulk metric
inevitably induce a new modulus field, which describes the conformal
fluctuations of the 4 dimensional metric. If this mode is light during
inflation, the induced conformal fluctuations will be amplified with a scale
free spectrum and with an amplitude which is disentangled from the standard
result of slow-roll inflation. The conformal 4d metric fluctuations give rise
to a very generic realization of the mechanism of modulated cosmological
fluctuations, related to spatial variation of couplings during (p)reheating
after inflation.Comment: 12 p, 2 fig
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