Abstract

The two ultra-large large-scale structure (uLSS) discoveries of the Giant Arc (GA) and the Big Ring (BR) are presented in this thesis. The GA (∼ 1 Gpc) and BR (Rh ∼ 370 Mpc), and add to an accumulating set of large-scale structure (LSS) discoveries that potentially challenge the Cosmological Principle (CP), on which the standard model of cosmology (ΛCDM) is founded. In cosmological terms, the GA and BR are very close; they are separated by only ∼ 12◦ on the sky and are both in the same redshift slice at z ∼ 0.8.

The observational properties of the GA and BR are presented, and they are assessed statistically using methods based on: (i) Single-Linkage Hierarchical Clustering; (ii) the Convex Hull of Member Spheres — GA: 4.53σ, and BR: (3.65±1.13)σ; (iii) the Minimal Spanning Tree — BR: (4.10±0.45)σ; (iv) the Cuzick-Edwards test — GA: 3.00σ, and BR: 2.0σ; (v) power spectrum analysis — GA: 4.8σ; (vi) and the FilFinder algorithm. Each of these methods has distinctive attributes and powers, and it is advised to consider the evidence from the ensemble. The approaches to mitigating any post-hoc aspects of analysing significance after discovery is discussed. The CHMS-overdensity and the MST-overdensity of the GA is δρ/ρ ≃ 0.9 ± 0.6 and δρ/ρ ≃ 1.3 ± 0.3, respectively. The alpha-hull overdensity of the BR(-all) is δρ/ρ ≃ 0.75.

The discoveries were made using the novel method of mapping intervening Mg II absorbers in the spectra of background quasars. This method allows for the detection of low-luminosity, distant matter which might otherwise go unseen with standard methods. The quasar spectra are obtained from the Sloan Digital Sky Survey (SDSS) and the Mg II catalogues are provided by independent authors.

Currently, the huge sizes and intriguing, circular-type morphologies of the GA and BR are not easily explained within ΛCDM. In addition, there have been hints of a second arc to the north of the GA and BR that appears to be an extension of the GA. If this is indeed the case, the GA might be extending into a Giant Ring (GR) which would then encompass the BR. The implications that these structures might have for cosmology could be important; but what they are telling us is not yet clear. Perhaps these uLSS discoveries may even be hinting at a direction of new physics within the standard model, or alternative theories in cosmology such as cosmic strings, pre-inflationary particles, or Conformal Cyclic Cosmology.