316 research outputs found
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
The Cosmic Microwave Background and Inflation Parameters
We review the currrent cosmic parameter determinations of relevance to
inflation using the WMAP-1year, Boomerang, CBI, Acbar and other CMB data. The
basic steps in the pipelines which determine the bandpowers from the raw data
from which these estimations are made are summarized. We forecast how the
precision is likely to improve with more years of WMAP in combination with
future ground-based experiments and with Planck. We address whether the current
data indicates strong breaking from uniform acceleration through the relatively
small region of the inflaton potential that the CMB probes, manifest in the
much-discussed running spectral index or in even more radical braking/breaking
scenarios. Although some weak ``anomalies'' appear in the current data, the
statistical case is not there. However increased precision, at the high
multipole end and with polarization measurements, will significantly curtail
current freedom.Comment: 24 pages, 10 figures, 2 tables, Int. J. Theor. Phys. 2004, ed. E.
Verdaguer, "Peyresq Physics 8", "The Early Universe: Confronting theory with
observations" (June 21-27, 2003
Application of XFaster power spectrum and likelihood estimator to Planck
We develop the XFaster Cosmic Microwave Background (CMB) temperature and
polarization anisotropy power spectrum and likelihood technique for the Planck
CMB satellite mission. We give an overview of this estimator and its current
implementation and present the results of applying this algorithm to simulated
Planck data. We show that it can accurately extract the power spectrum of
Planck data for the high-l multipoles range. We compare the XFaster
approximation for the likelihood to other high-l likelihood approximations such
as Gaussian and Offset Lognormal and a low-l pixel-based likelihood. We show
that the XFaster likelihood is not only accurate at high-l, but also performs
well at moderately low multipoles. We also present results for cosmological
parameter Markov Chain Monte Carlo estimation with the XFaster likelihood. As
long as the low-l polarization and temperature power are properly accounted
for, e.g., by adding an adequate low-l likelihood ingredient, the input
parameters are recovered to a high level of accuracy.Comment: 25 pages, 20 figures, updated to reflect published version: slightly
extended account of XFaster technique, added improved plots and minor
corrections. Accepted for publication in MNRA
A Bayesian estimate of the skewness of the Cosmic Microwave Background
We propose a formalism for estimating the skewness and angular power spectrum
of a general Cosmic Microwave Background data set. We use the Edgeworth
Expansion to define a non-Gaussian likelihood function that takes into account
the anisotropic nature of the noise and the incompleteness of the sky coverage.
The formalism is then applied to estimate the skewness of the publicly
available 4 year Cosmic Background Explorer (COBE) Differential Microwave
Radiometer data. We find that the data is consistent with a Gaussian skewness,
and with isotropy. Inclusion of non Gaussian degrees of freedom has essentially
no effect on estimates of the power spectrum, if each is regarded as a
separate parameter or if the angular power spectrum is parametrized in terms of
an amplitude (Q) and spectral index (n). Fixing the value of the angular power
spectrum at its maxiumum likelihood estimate, the best fit skewness is
S=6.5\pm6.0\times10^4(\muK)^3; marginalizing over Q the estimate of the
skewness is S=6.5\pm8.4\times10^4(\muK)^3 and marginalizing over n one has
S=6.5\pm8.5\times10^4(\muK)^3.Comment: submitted to Astrophysical Journal Letter
On the evolution of tachyonic perturbations at super-Hubble scales
In the slow-roll inflationary scenario, the amplitude of the curvature
perturbations approaches a constant value soon after the modes leave the Hubble
radius. However, relatively recently, it was shown that the amplitude of the
curvature perturbations induced by the canonical scalar field can grow at
super-Hubble scales if there is either a transition to fast roll inflation or
if inflation is interrupted for some period of time. In this work, we extend
the earlier analysis to the case of a non-canonical scalar field described by
the Dirac-Born-Infeld action. With the help of a specific example, we show that
the amplitude of the tachyonic perturbations can be enhanced or suppressed at
super-Hubble scales if there is a transition from slow roll to fast roll
inflation. We also illustrate as to how the growth of the entropy perturbations
during the fast roll regime proves to be responsible for the change in the
amplitude of the curvature perturbations at super-Hubble scales. Furthermore,
following the earlier analysis for the canonical scalar field, we show that the
power spectrum evaluated in the long wavelength approximation matches the exact
power spectrum obtained numerically very well. Finally, we briefly comment on
an application of this phenomenon.Comment: v1: 15 pages, 4 figures; v2: 16 pages, 5 figures, power spectrum
included, discussion in section 5 enlarged, references added; v3: 17 pages, 5
figures, enhancement AS WELL AS suppression of modes at super-Hubble scales
pointed out, title changed, discussions enlarged, references added, to appear
in JCA
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