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

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

Halogen bonding enhances nonlinear optical response in poled supramolecular polymers

We demonstrate that halogen bonding strongly enhances the nonlinear optical response of poled supramolecular polymer systems. We compare three nonlinear optical chromophores with similar electronic structures but different bond-donating units, and show that both the type and the strength of the noncovalent interaction between the chromophores and the polymer matrix play their own distinctive roles in the optical nonlinearity of the systems

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

Detection and discrimination of cosmological non-Gaussian signatures by multi-scale methods

Recent Cosmic Microwave Background (CMB) observations indicate that the temperature anisotropies arise from quantum fluctuations in the inflationary scenario. In the simplest inflationary models, the distribution of CMB temperature fluctuations should be Gaussian. However, non-Gaussian signatures can be present. They might have different origins and thus different statistical and morphological characteristics. In this context and motivated by recent and future CMB experiments, we search for, and discriminate between, different non-Gaussian signatures. We analyse simulated maps of three cosmological sources of temperature anisotropies: Gaussian distributed CMB anisotropies from inflation, temperature fluctuations from cosmic strings and anisotropies due to the kinetic Sunyaev-Zel'dovich (SZ) effect both showing a non-Gaussian character. We use different multi-scale methods, namely, wavelet, ridgelet and curvelet transforms. The sensitivity and the discriminating power of the methods is evaluated using simulated data sets. We find that the bi-orthogonal wavelet transform is the most powerful for the detection of non-Gaussian signatures and that the curvelet and ridgelet transforms characterise quite precisely and exclusively the cosmic strings. They allow us thus to detect them in a mixture of CMB + SZ + cosmic strings. We show that not one method only should be applied to understand non-Gaussianity but rather a set of different robust and complementary methods should be used.Comment: Accepted for publication in A&A. Paper with high resolution figures can be found at http://jstarck.free.fr/cmb03.pd

Bismuth-Based Coordination Polymers with Efficient Aggregation-Induced Phosphorescence and Reversible Mechanochromic Luminescence

Two bismuth coordination polymers (CPs), (TBA)[BiBr4(bp4mo)] (TBA=tetrabutylammonium) and [BiBr3(bp4mo)2], which are based on the rarely used simple ditopic ligand N-oxide-4,4′-bipyridine (bp4mo), show mechanochromic luminescence (MCL). High solid-state phosphorescence quantum yields of up to 85 % were determined for (TBA)[BiBr4(bp4mo)] (λem=540 nm). Thorough investigations of the luminescence properties combined with DFT and TDDFT calculations revealed that the emission is due to aggregation-induced phosphorescence (AIP). Upon grinding, both samples became amorphous, and their luminescence changed from yellow to orange and red, respectively. Heating or exposure to water vapor led to the recovery of the initial luminescence. These materials are the first examples of mechanochromic phosphors based on bismuth(III)

Aggregation induced phosphorescent N-oxyde-2,2′-bipyridine bismuth complexes and polymorphism-dependent emission

Unprecedented bismuth complexes, based on the rarely used ditopic ligand N-oxide-2,2′-bipyridine (bp2mo), crystallizing as three polymorphs, α- (1), β- (2) and γ-[BiBr3(bp2mo)2] (3), exhibit phosphorescence with a quantum yield up to 17% for the crystal phase (1), while the complex displays a weak fluorescence in solution. A study of the luminescence properties combined with DFT/TDDFT calculations reveals that the lighting phenomenon originated by aggregation induced phosphorescence correlated with the weak intermolecular interactions present in the different crystal phases