arXiv subjects

Lung-Yih Chiang

Publications and source records attributed to Lung-Yih Chiang.

29 records · Page 2Linked to original sources

arXiv2004

Spawning and merging of Fourier modes and phase coupling in cosmological density bispectrum

In the standard picture of cosmological structure formation, initially random-phase fluctuations are amplified by non-linear gravitational instability to produce a final distribution of mass which is highly non-Gaussian and has highly-coupled Fourier phases. We use the Zel'dovich approximation in one dimension to elucidate the onset of non-linearity including mode spawning, merging and coupling. We show that as gravitational clustering proceeds, Fourier modes are spawned from parent ones, with their phases following a harmonic relationship with the wavenumbers. Spawned modes could also merge leading to modulation of the amplitudes and phases which consequently breaks such harmonic relation. We also use simple toy models to demonstrate that bispectrum, Fourier transform of connected three-point correlation functions, measures phase coupling at most at the second-order only when the special wavenumber-phase harmonic relation holds. Phase information is therefore partly registered in bispectrum and it takes a complete hierarchy of polyspectra to fully characterize gravitational non-linearity.

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arXiv2003

Phase analysis of the 1-year WMAP data and its application for the CMB foreground separation

We present a new method based on phase analysis for the Galaxy and foreground component separation from the cosmic microwave background (CMB) signal. This method is based on a prevailing assumption that the phases of the underlying CMB signal should have no or little correlation with those of the foregrounds. This method takes into consideration all the phases of each multipole mode (l <= 50, -l <= m <=l) from the whole sky without galactic cut, masks or any dissection of the whole sky into disjoint regions. We use cross correlation of the phases to illustrate that significant correlations of the foregrounds manifest themselves in the phases of the WMAP 5 frequency bands, which are used for separation of the CMB from the signals. Our final phase-cleaned CMB map has the angular power spectrum in agreement with both the WMAP result and that from Tegmark, de Oliveira-Costa and Hamilton (TOH), the phases of our derived CMB signal, however, are slightly different from those of the WMAP Internal Linear Combination map and the TOH map.

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arXiv2003

On the linear filters for point source extraction of the Planck mission

The manifestation of point sources in the upcoming PLANCK maps is a direct reflection of the properties of the pixelized antenna beam shape for each frequency, which is related to the scan strategy, pointing accuracy, noise properties and map-making algorithm. In this paper we firstly compare analytically two filters for the PLANCK point source extraction, namely, the adaptive top-hat filter (ATHF), the theoretically-optimal filter (TOF). Our analyses are based on the premise that the experiment parameters the TOF assume and require are already known: the CMB and noise power spectrum and a circular Gaussian beam shape and size, whereas the ATHF does not need any a priori knowledge. The analyses show that the TOF is optimal in terms of the gain after the parameter inputs. We simulate the PLANCK HFI 100 GHz channel with elliptical beam in rotation to test the efficiency of the TOF and the ATHF. We also apply the ATHF on the WMAP Q-band map and the derived map (the foreground-cleaned map by Tegmark, de Oliveira Costa & Hamilton) from the WMAP 1-year data. The uncertainties on the angular power spectrum will hamper the efficiency of the TOF. To tackle the real situations for the PLANCK point source extraction, most importantly, the elliptical beam shape with slow precession and change of the ellipticity ratio due to possible mirror degradation effect, the ATHF is computationally efficient and well suited for the construction of the PLANCK Early Release Compact Source Catalogue.

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arXiv2003

Late reionizations of the Universe and their manifestation in the CMB data

We investigate two sets of two-epoched reionization models and their manifestation in the CMB anisotropy and polarization data of the recent WMAP project and make some predictions for the future PLANCK missions. In the first set of models, the universe was reionized twice, first at z~15 by population III stars and then at z~6 by stars in large galaxies. In the second set of models, the extra peak-like reionization at high redshifts z > 100 is induced by the decay of unstable particles, followed by the standard picture of reionization at z~6. We examine the general properties of these two-epoched reionization models and their implication in the WMAP CMB temperature anisotropy-polarization cross-correlation. We have shown that these models have comparable likelihood values for the WMAP data and distinct characters which can be tested with the Planck high sensitivities.

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arXiv2003

Non-Gaussianity of the derived maps from the first-year WMAP data

We present non-Gaussianity testing on recently-released derived maps from the first-year WMAP data by Tegmark, de Oliveria-Costa and Hamilton. Our test is based on a phase mapping technique which has the advantage of testing non-Gaussianity at separate multipole bands. We show that their foreground-cleaned map is against the random-phase hypothesis at all 4 multipole bands centered around l = 150, 290, 400 and 500. Their Wiener-filtered map, on the other hand, is Gaussian for l < 250, and marginally Gaussian for 224 < l < 350. However, we see the evidence of non-Gaussianity for l > 350 as we detect certain degrees of phase coupling, hence against the random-phase hypothesis. Our phase mapping technique is particularly useful for testing the accuracy of component separation methods.

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arXiv2002

Return mapping of phases and the analysis of the gravitational clustering hierarchy

In the standard paradigm for cosmological structure formation, clustering develops from initially random-phase (Gaussian) density fluctuations in the early Universe by a process of gravitational instability. The later, non-linear stages of this process involve Fourier mode-mode interactions that result in a complex pattern of non-random phases. We present a novel mapping technique that reveals mode coupling induced by this form of nonlinear interaction and allows it to be quantified statistically. The phase mapping technique circumvents the difficulty of the circular characteristic of phi_k and illustrates the statistical significance of phase difference at the same time. This generalized method on phases allows us to detect weak coupling of phases on any Delta k scales.

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arXiv2002

Phase Mapping as a Powerful Diagnostic of Primordial Non-Gaussianity

The identification and extraction of non-Gaussian signals is one of the main cosmological challenges facing future experimental measurements of the cosmic microwave background temperature pattern. We present a generalized statistical measure based on a novel technique representation of Fourier phases using the return map. We show that this method is both robust and powerful in comparison, for example, with morphological measures.

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arXiv2001

The Importance of Fourier Phases for the Morphology of Gravitational Clustering

The phases of the Fourier modes appearing in a plane-wave expansion of cosmological density fields play a vital role in determining the morphology of gravitationally-developed clustering. We demonstrate this qualitatively and quantitatively using simulations. In particular, we use cross-correlation and rank-correlation techniques to quantify the agreement between a simulated distribution and phase-only reconstructions. The phase-only reconstructions exhibit a high degree of correlation with the original distributions, showing how meaningful spatial reconstruction of cosmological density fields depends more on phase accuracy than on amplitudes.

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arXiv2000

Non-linearity and Non-Gaussianity through Phase Information

In the standard picture of structure formation, initially random-phase fluctuations are amplified by non-linear gravitational instability to produce a final distribution of mass which is highly non-Gaussian and has highly coupled Fourier phases. Second-order statistics, such as the power spectrum, are blind to this kind of phase association. We discuss the information contained in the phases of cosmological density fluctuations and their possible use in statistical analysis tools. In particular, we show how the bispectrum measures a particular form of phase association called quadratic phase coupling, show how to visualise phase association using colour models. These techniques offer the prospect of more complete tests of initial non-Gaussianity than those available at present.

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arXiv2000

Characterizing the non-linear growth of large-scale structure in the Universe

The local Universe displays a rich hierarchical pattern of galaxy clusters and superclusters. The early Universe, however, was almost smooth, with only slight 'ripples' seen in the cosmic microwave background radiation. Models of the evolution of structure link these observations through the effect of gravity, because the small initially overdense fluctuations attract additional mass as the Universe expands. During the early stages, the ripples evolve independently, like linear waves on the surface of deep water. As the structures grow in mass, they interact with other in non-linear ways, more like waves breaking in shallow water. We have recently shown how cosmic structure can be characterized by phase correlations associated with these non-linear interactions, but hitherto there was no way to use that information to reach quantitative insights into the growth of structures. Here we report a method of revealing phase information, and quantify how this relates to the formation of a filaments, sheets and clusters of galaxies by non-linear collapse. We use a new statistic based on information entropy to separate linear from non-linear effects and thereby are able to disentangle those aspects of galaxy clustering that arise from initial conditions (the ripples) from the subsequent dynamical evolution.

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arXiv1999

Phase Information and the Evolution of Cosmological Density Perturbations

The Fourier transform of cosmological density perturbations can be represented in terms of amplitudes and phases for each Fourier mode. We investigate the phase evolution of these modes using a mixture of analytical and numerical techniques. Using a toy model of one-dimensional perturbations evolving under the Zel'dovich approximation as an initial motivation, we develop a statistic that quantifies the information content of the distribution of phases. Using numerical simulations beginning with more realistic Gaussian random-phase initial conditions, we show that the information content of the phases grows from zero in the initial conditions, first slowly and then rapidly when structures become non-linear. This growth of phase information can be expressed in terms of an effective entropy: Gaussian initial conditions are a maximum entropy realisation of the initial power spectrum, gravitational evolution decreases the phase entropy. We show that our definition of phase entropy results in a statistic that explicitly quantifies the information stored in the phases of density perturbations (rather than their amplitudes) and that this statistic displays interesting scaling behaviour for self-similar initial conditions.

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