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On the Structure, Shapes, and Alignments of Voids

Beeftink, Timon (2019) On the Structure, Shapes, and Alignments of Voids. Bachelor's Thesis, Astronomy.


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On its largest scales, the Universe exhibits a particular structure, known as the cosmic web, in which clusters of galaxies are separated by tenuous voids. Due to their tenuousness, these voids contain valuable information on galaxy formation, the reionisation transition in the early Universe, the nature of dark matter and energy, and the spatial organisation of the cosmic web. In order to model voids, and to subsequently extract cosmological parameters governing processes in the Universe on its largest scales, an Alcock-Paczynski (AP) test is sometimes used, in which - under the assumption that they are not preferentially aligned - voids are identified and bundled in a large volume survey, such that a spherical total void is obtained. Several studies, however, have shown that voids in the large scale structure show alignments originating from tidal forces induced by the inhomogeneous cosmic matter distribution, questioning the validity of the application of the AP test on these scales. This study uses the Watershed Void Finder (WVF) to identify and map voids for an N-body ΛCDM-simulation, both in two and three dimensions, together with their density profiles, such that the shapes, possible alignments, and sphericity of voids can be investigated. Voids are identified and fitted by ellipses (two dimensions) and ellipsoids (three dimensions), together with their neighbouring voids, in order to find correlations in orientation, volume, and shape of neighbouring voids with respect to the central void. It is found that (three-dimensional) voids are prolate ellipsoids, only differing in the significance of their prolateness, that the principal semi-axes of voids are not oriented at random, i.e. exhibit a significant alignment, and that more voluminous voids tend to be aligned more substantially. These results imply that it is dubitably to assume that, on the largest scales, voids do not show any preferential alignments, i.e. to use the AP test - on a bundle of voids in a large volume survey - to describe voids properly, since they exhibit a significant (volume-dependent) alignment and tend to be prolate spheroids. This study asks for redefining the assumptions on which most studies in the field of voids are based on and shows that it is probably unjustified to simply use the AP test and spherical approximation of voids within the cosmic web.

Item Type: Thesis (Bachelor's Thesis)
Supervisor nameSupervisor E mail
Weijgaert, M.A.M. van
Degree programme: Astronomy
Thesis type: Bachelor's Thesis
Language: English
Date Deposited: 11 Jul 2019
Last Modified: 12 Jul 2019 11:35

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