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Optimal survey design for skewness measurements in weak gravitational lensing Vafaei, Sanaz

Abstract

The properties and structure of the Universe we live in has been the subject of many studies since human kind started to watch the sky. Today, with rapid advancements in technology, cosmology has become a science where theories are rigorously tested against the evidence of observation. Many attempts have been taken in order to find ways to explain the behavior and nature of the Universe as a whole. Already we know that only a small fraction of our Universe is made up of the baryonic matter and the rest comprises an unknown "dark matter" component and an unknown "dark energy" component that drives the accelerating expansion of’ the Universe. The exact fraction of each of these components drives the fate of the Universe. More importantly, in order to understand the nature of these components, we need to perform very accurate measurements before the theoretical models can be ruled out. Ultimately cosmologists believe that understanding dark matter and dark energy will help to understand some of the most fundamental questions in modern physics. We can infer the existence of dark matter by observing its gravitational effect on the distant galaxies that we see. Computational simulations confirmed by observations, suggest that dark matter collects into long filament structures, clusters and sheets. The best method to detect the dark matter is to investigate its gravitational effects on the light of distant galaxies traveling in the Universe along the line of sight, since this effect is totally independent of the nature of dark matter. Due to the strong gravitational field of such massive body, the light rays are deflected and typically travel through a zigzag path rather than a straight line. The resulting images of the distant galaxies are magnified, and distorted. By studying these sheared and distorted images we can reconstruct the distribution of the mass which caused it. Gravitational Lensing is the only method which probes the total mass distribution of the Universe including the dark matter directly. Weak Gravitational Lensing deals with small image distortions. In order to get more accurate results deeper and wider surveys are needed. The detection of weak gravitational lensing by large-scale structure enables us to set constraints on the cosmological parameters such as amplitude of the matter power spectrum and the matter density parameter. Measuring third-order statistics such as the skewness allows us to break the degeneracies that exist between these two cosmological parameters when they are determined from two-point statistics. In this thesis, we ask what type of survey should be performed for an optimal constraint of the third-order statistics of weak gravitational lensing. We use an extensive set of cosmological lensing simulations and determine the signal-to-noise ratio of different third-order statistics for ground and space-based surveys of various depths. We conclude with the prospects of measuring the skewness with existing and forthcoming surveys.

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