61 research outputs found
Removing the ISW-lensing bias from the local-form primordial non-Gaussianity estimation
The Integrated Sachs-Wolfe (ISW) effect produces a secondary temperature
anisotropy of CMB. The main contribution comes from z<2, where dark energy
leads to a decay of potentials. As the same photons are gravitationally lensed
by these decaying potentials, there exists a high degree of correlation between
the ISW effect and CMB lensing, leading to a non-zero three-point correlation
(bispectrum) of the observed temperature anisotropy. This ISW-lensing
bispectrum, whose shape resembles that of the so-called "local-form" primordial
bispectrum parametrized by fNL, is known to be the largest contamination of
fNL. In order to avoid a spurious detection of primordial non-Gaussianity, we
need to remove the ISW-lensing bias. In this work, we investigate three
debiasing methods: (I) subtraction of an expected, ensemble average of the
ISW-lensing bispectrum; (II) subtraction of a measured ISW-lensing bispectrum;
and (III) direct subtraction of an estimated ISW signal from an observed
temperature map. One may use an estimation of the ISW map from external non-CMB
data or that from the CMB data themselves. As the methods II and III are based
on fewer assumptions about the nature of dark energy, they are preferred over
the method I. While the methods I and II yield unbiased estimates of fNL with
comparable error bars, the method III yields a biased result when the
underlying primordial fNL is non-zero and the ISW map is estimated from a
lensing potential reconstructed from the observed temperature map. One of the
sources of the bias is a lensing reconstruction noise bias which is independent
of fNL and can be calculated precisely, but other fNL-dependent terms are
difficult to compute reliably. We thus conclude that the method II is the best,
model-independent way to remove the ISW-lensing bias of fNL, enabling us to
test the physics of inflation with smaller systematic errors.Comment: 17 pages, comments are welcomed v2: references added, v3: references
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Can we neglect relativistic temperature corrections in the Planck thermal SZ analysis?
Measurements of the thermal Sunyaev-Zel'dovich (tSZ) effect have long been
recognized as a powerful cosmological probe. Here we assess the importance of
relativistic temperature corrections to the tSZ signal on the power spectrum
analysis of the Planck Compton- map, developing a novel formalism to account
for the associated effects. The amplitude of the tSZ power spectrum is found to
be sensitive to the effective electron temperature, , of the cluster
sample. Omitting the corresponding modifications leads to an underestimation of
the -power spectrum amplitude. Relativistic corrections thus add to the
error budget of tSZ power spectrum observables such as . This could
help alleviate the tension between various cosmological probes, with the
correction scaling as for Planck. At the current level of
precision, this implies a systematic shift by , which can also
be interpreted as an overestimation of the hydrostatic mass bias by , bringing it into better
agreement with hydrodynamical simulations. It is thus time to consider
relativistic temperature corrections in the processing of current and future
tSZ data.Comment: 6 pages, 4 figures, minor changes, updated to match version accepted
by MNRA
A novel approach to reconstructing signals of isotropy violation from a masked CMB sky
Statistical isotropy (SI) is one of the fundamental assumptions made in
cosmological model building. This assumption is now being rigorously tested
using the almost full sky measurements of the CMB anisotropies. A major hurdle
in any such analysis is to handle the large biases induced due to the process
of masking. We have developed a new method of analysis, using the bipolar
spherical harmonic basis functions, in which we semi-analytically evaluate the
modifications to SI violation induced by the mask. The method developed here is
generic and can be potentially used to search for any arbitrary form of SI
violation. We specifically demonstrate the working of this method by recovering
the Doppler boost signal from a set of simulated, masked CMB skies.Comment: 8 pages, 3 figure
Statistical isotropy violation in WMAP CMB maps resulting from non-circular beams
Statistical isotropy (SI) of Cosmic Microwave Background (CMB) fluctuations
is a key observational test to validate the cosmological principle underlying
the standard model of cosmology. While a detection of SI violation would have
immense cosmological ramification, it is important to recognise their possible
origin in systematic effects of observations. WMAP seven year (WMAP-7) release
claimed significant deviation from SI in the bipolar spherical harmonic
(BipoSH) coefficients and . Here we present the
first explicit reproduction of the measurements reported in WMAP-7, confirming
that beam systematics alone can completely account for the measured SI
violation. The possibility of such a systematic origin was alluded to in WMAP-7
paper itself and other authors but not as explicitly so as to account for it
accurately. We simulate CMB maps using the actual WMAP non-circular beams and
scanning strategy. Our estimated BipoSH spectra from these maps match the
WMAP-7 results very well. It is also evident that only a very careful and
adequately detailed modelling, as carried out here, can conclusively establish
that the entire signal arises from non-circular beam effect. This is important
since cosmic SI violation signals are expected to be subtle and dismissing a
large SI violation signal as observational artefact based on simplistic
plausibility arguments run the serious risk of "throwing the baby out with the
bathwater".Comment: 4 pages, 3 figures. Published version. Includes major revision in the
text and one important figure. No change in the result
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