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Title: The detection and measurement of cosmic shear
Author: Bacon, D.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2002
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This thesis presents the detection of weak gravitational lensing by large-scale structure, also known as 'cosmic shear'. This effect, in which the images of faint background galaxies are coherently distorted by the gravitational potential of intervening foreground structures, is poised to become the most direct measure of the mass fluctuations in the universe. It is sensitive to all components of mass present along a line of sight, whether visible matter or dark matter, and therefore does not suffer from poorly-justified assumptions in relating the two. In order to make a pioneering detection of cosmic shear, a strategy for observing the effect is developed here in the context of deep-field observations with the 4.2m William Herschel Telescope (WHT). The criteria for detecting the cosmic shear signal, and the resulting survey, are described in detail. The procedure for image data reduction is then explained. As the distortions expected from cosmic shear are very small, we require a method for measuring the shapes of galaxies to great precision, and a painstaking treatment of systematic effects. Various approaches to shape measurement are discussed, and the adopted approach using the Kaiser, Squires and Broadhurst (KSB) method is described. This is followed by a study of this algorithm's performance upon artificial sheared images, produced using detailed realisations of a galaxy ensemble drawn from the Hubble Space Telescope Groth strip, and including realistic systematics reproducing observational conditions for the WHT. It is demonstrated that, despite low-level biases in KSB, the method is capable of measuring cosmic shear signals. The secure detection of cosmic shear on the observed WHT fields is then described. The analysis and removal of systematic effects due to telescope optics and tracking are discussed, and the shear estimators to be used are considered in detail. The significance of detection is calculated, and the cosmological implications of this detection are discussed. It is found that our measurements are consistent with the predictions of cluster-normalised Cold Dark Matter (CDM) models, but a COBE-normalised Standard CDM model is ruled out at high significance. For the currently favoured cosmological constant-dominated ΛCDM model, our measurement provides a constraint on the normalisation of the mass power spectrum fully consistent with that derived from cluster abundances. The potential effect of intrinsic alignments between galaxies is considered. Following the detection of cosmic shear, precision measurements of the effect become possible, together with initial measurements of the amplitude of the mass power spectrum. For this purpose, a large survey with the 10m Keck II telescope is described, along with an extended WHT data set. The use of correlation functions is explained, and current constraints on the fluctuations of matter and the density of the Universe from these weak lensing surveys are given.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available