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Title: Cosmic evolution and large-scale structure
Author: Berian James, J.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2010
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In recent decades, astronomical observations have led to a broad, if incomplete, consensus description of the Universe: its evolution is determined by the relative proportions of a handful of substances ('usual' matter like atoms, stars and galaxies; a hidden, but gravitationally attractive dark matter; the poorly understood and unfortunately named dark energy) all of which are changing in concert under the influence of physical laws. In the region of the Universe visible to astronomers, great cosmological structures are observed - dense clusters in which the majority of galaxies form vast, and apparently empty, voids; and a tangle of filaments that has been coined 'the cosmic web'. The complicated and evolving relationship between dark matter and galaxies - critical in determining the formation of cosmological structures as well as the formation and evolution of galaxies - is the subject of this work. In the first of the two major parts, the clustering of the galaxy distribution and its determination by the underlying dark matter field is explored by the means of correlation statistics. The two-point angular auto-correlation function w(θ) for galaxies in the Cosmic Evolution Survey is measured and compared to lightcones from the Millennium Simulation, with the luminosity evolution of this quantity being studied in particular. In this case, simulation is inadequate to fully explain observations and the reasons for this are explored. The second science topic in this part is the measurement of the projected cluster-galaxy cross-correlation function wp(rp), addressing the manner in which galaxies populate the dark matter haloes that host them (for which the clusters are a proxy). This measurement, the first of its kind with independent galaxy and halo catalogues, is decomposed by cluster mass and by cluster redshift; evolution in the cross-correlation function at different scales is observed depending on which parameter is varied. In order to quantify the measurement within the context of structure formation models, these measurements are subject to halo-based modelling, describing the bias of clusters as a function of their mass and redshift (on large scale) and the change in the number density profile of haloes (on small scales). In parallel with the science outcomes of both measurements, a number of methodological innovations are introduced: a method to improve the quality of masking and star-galaxy separation of objects in the COSMOS field; an efficient computational routine for the calculation of correlation estimators when more than ~ 10⁴ objects are present; improvements to the technique for calculating the projection of the redshift-space correlation function ξ{rp,π); and most importantly the halo model computation of the cluster-galaxy cross-correlation. The second major part of this work address two further topics. First, cosmological structures are investigated by means of topological statistics. In the case of galaxy redshift surveys, this amounts to the computation of the genus numbers of a sequence of surfaces of constant density through the smoothed galaxy distribution. Results are presented for the calculation of this quantity for the 2dF Galaxy Redshift Survey; the WiggleZ survey; the Cosmic Evolution Survey; and for independent simulations of the dark matter, halo and galaxy distributions. These results demonstrate the evolution of this statistic and its sensitivity to important physical process: in particular, galaxy bias, non-linear gravitational evolution and primordial non-Gaussianity. In order to explore these effects in detail, an important new transformation of the genus curve is constructed, allowing decomposition of density modes as a function of scale. This decomposition is performed for the measurements described above and the implications of the results for structure formation models are discussed. The other topic in this part is a review of an emerging field of inquiry into the foundation of relativistic cosmology, in which a combination of numerical simulation, analytical calculation and physical reasoning are used to address questions about the nature of expanding spacetimes. It is demonstrated that the interpretation of expanding space - and in particular, what it means to be in a comoving frame - depends on the cosmology under investigation, but that the notion of whether space is a dynamic entity undergoing expansion or a fixed background through which galaxies recede is a frame-dependent phenomenon: the observation of cosmological redshift can be interpreted in different ways depending on which frame is chosen, but that in a particular frame, one notion can properly be deemed more valid.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available