Cosmological applications of weak gravitational flexion
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Date
09/2008Author
Rowe, Barnaby Thomas Peter
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Abstract
Modern cosmology has reached an important juncture, at which the ability to make measurements of unprecedented
accuracy has led to conclusions that are a fundamental challenge to natural science. The
discovery that, in our current best model, the dynamics of the Universe are completely dominated by unseen
dark matter and dark energy can do little but completely alter the shape of physics research in the 21st
Century. Unfortunately,much of our insight into these phenomenamust come from observations of visible
matter alone; this raises serious problems, as the tracing of dark matter by visible matter is as yet poorly
understood.
Gravitational lensing offers strong prospects for probing the interwoven history of dark and visible matter,
as mass in any form may be detected where it exists untraced by baryons. In this Thesis I describe
advances made in the field of weak gravitational lensing, which constrains the properties of the matter
distribution on cosmological scales using a statistical analysis of the coherent gravitational distortions of
distant galaxy images. I summarize the development of gravitational flexion, a higher order extension to
traditional weak lensing, and describe my work done to bring the study of flexion to a stage where it may be
employed to make accurate cosmological measurements. I show how flexion is sensitive to matter structure
on smaller physical scales than existing lensing techniques and, therefore, promises to shed new light upon
key untested predictions of cosmological models if it can be measured to sufficient accuracy. I discuss the
success of my efforts in this direction, and describe the issues to be encountered in the careful analysis of
this subtle gravitational signal.
This research has involved advances in many areas: the calculation of theoretical flexion predictions, the
refinement of image analysis methods for accurate galaxy shape estimation, and the practical application
of these new flexion techniques to extragalactic imaging data. The culmination of these efforts is a new
maximum likelihood analysis of the galaxy-galaxy lensing signal in the Hubble Space Telescope Galaxy
Evolution from Morphology and SEDs (GEMS) Survey, incorporating improvements and modifications
necessary for the combination of flexion with traditional weak lensing measurements. The results of this
work, and particularly the extent to which measurements of flexion provide extra cosmological insight, are
discussed in detail.
The conclusion is a summary of all that has been learned about the use of flexion as an accurate probe
of cosmology, and a discussion of its prospects for answering some of the many questions that remain
about dark matter. Within the next few year wide-area survey telescopes will begin imaging huge volumes
of deep space, with the measurement of the gravitational lensing signal being given high priority in the
analysis of these data. Within this context, the primary inquiry of this Thesis is the extent to which the
application of flexion measurement techniques will help shed new light upon the unseen, and currently
poorly understood, components of the Universe.