1,364 research outputs found

### Searching for non-gaussianity: Statistical tests

Non-gaussianity represents the statistical signature of physical processes
such as turbulence. It can also be used as a powerful tool to discriminate
between competing cosmological scenarios. A canonical analysis of
non-gaussianity is based on the study of the distribution of the signal in the
real (or direct) space (e.g. brightness, temperature).
This work presents an image processing method in which we propose statistical
tests to indicate and quantify the non-gaussian nature of a signal. Our method
is based on a wavelet analysis of a signal. Because the temperature or
brightness distribution is a rather weak discriminator, the search for the
statistical signature of non-gaussianity relies on the study of the coefficient
distribution of an image in the wavelet decomposition basis which is much more
sensitive.
We develop two statistical tests for non-gaussianity. In order to test their
reliability, we apply them to sets of test maps representing a combination of
gaussian and non-gaussian signals. We deliberately choose a signal with a weak
non-gaussian signature and we find that such a non-gaussian signature is easily
detected using our statistical discriminators. In a second paper, we apply the
tests in a cosmological context.Comment: 14 pages, 7 figures, in press in Astronomy & Astrophysics Supplement
Serie

### On the detectability of non-trivial topologies

We explore the main physical processes which potentially affect the
topological signal in the Cosmic Microwave Background (CMB) for a range of
toroidal universes. We consider specifically reionisation, the integrated
Sachs-Wolfe (ISW) effect, the size of the causal horizon, topological defects
and primordial gravitational waves. We use three estimators: the information
content, the S/N statistic and the Bayesian evidence. While reionisation has
nearly no effect on the estimators, we show that taking into account the ISW
strongly decreases our ability to detect the topological signal. We also study
the impact of varying the relevant cosmological parameters within the 2 sigma
ranges allowed by present data. We find that only Omega_Lambda, which
influences both ISW and the size of the causal horizon, significantly alters
the detection for all three estimators considered here.Comment: 11 pages, 9 figure

### The Maximum B-mode Polarization of the Cosmic Microwave Background from Inhomogeneous Reionization

We compute the B-mode polarization power spectrum of the CMB from an epoch of
inhomogeneous reionization, using a simple model in which HII regions are
represented by ionized spherical bubbles with a log normal distribution of
sizes whose clustering properties are determined by large-scale structure. Both
the global ionization fraction and the characteristic radius of HII regions are
allowed to be free functions of redshift. Models that would produce substantial
contamination to degree scale gravitational wave B-mode measurements have power
that is dominated by the shot noise of the bubbles. Rare bubbles of >100 Mpc at
z>20 can produce signals that in fact exceed the B-modes from gravitational
lensing and are comparable to the maximal allowed signal of gravitational waves
(~0.1uK) while still being consistent with global constraints on the total
optical depth. Even bubbles down to 20 Mpc at z~15, or 40 Mpc at z~10 can be
relevant (0.01uK) once the lensing signal is removed either statistically or
directly. However, currently favored theoretical models that have ionization
bubbles that only grow to such sizes at the very end of a fairly prompt and
late reionization produce signals which are at most at these levels.Comment: 14 pages, 11 figures; published in ApJ; corrected Fig. 4 and updated
reference

### Separating the kinetic SZ effect from primary CMB fluctuations

In the present work, we propose a new method aiming at extracting the kinetic
Sunyaev-Zel'dovich (KSZ) temperature fluctuations embedded in the primary
anisotropies of the cosmic microwave background (CMB). We base our study on
simulated maps without noise and we consider very simple and minimal
assumptions. Our method essentially takes benefit from the spatial correlation
between KSZ and the Compton parameter distribution associated with the thermal
Sunyaev-Zel'dovich (TSZ) effect of the galaxy clusters, the later can be
obtained by means of multi-frequency based component separation techniques. We
reconstruct the KSZ signal by interpolating the CMB fluctuations without making
any hypothesis besides the CMB fluctuations are Gaussian distributed. We
present two ways of estimating the KSZ fluctuations, after the interpolation
step. In the first way we use a blind technique based on canonical Principal
Component Analysis, while the second uses a minimisation criterion based on the
fact that KSZ dominates a small angular scales and that it follows a
non-Gaussian distribution. We show using the correlation between the input and
reconstructed KSZ map that the latter can be reconstructed in a very
satisfactory manner (average correlation coefficient between 0.62 and 0.90),
furthermore both the retrieved KSZ power spectrum and temperature fluctuation
distribution are in quite good agreement with the original signal. The ratio
between the input and reconstructed power spectrum is indeed very close to one
up to a multipole $\ell\sim 200$ in the best case. The method presented here
can be considered as a promising starting point to identify in CMB observations
the temperature fluctuation associated with the KSZ effect.Comment: 12 pages, 9 figure

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