Event Details

MARS seminar series. Speaker: Alexia Lopez (University of Lancashire) Title: Ultra-Large-Scale Structures in Cosmology

Abstract:

Background: Ultra-large-scale structures in cosmology are physically huge (exceeding the estimated scale of homogeneity), statistically-significant features in the distribution of matter in the Universe. The accumulating list of uLSSs raises questions about the assumption of homogeneity on large scales, which the Cosmological Principle --- the foundation of the standard cosmological model --- requires. I use a method of mapping intervening absorption features detected in the spectra of bright, background quasars, to trace the underlying matter distribution. In particular, singly-ionised magnesium (MgII) is well known to trace galaxies and galaxy clusters.

Results: I will present the discovery of `A Giant Ring on the Sky’, which is a ring-like uLSS that appears to extend from the previously-reported Giant Arc (Lopez et al. 2022). The Giant Ring (GR) is an almost contiguous, overdense filament of MgII absorbers which appears as a Giant Ring, approximately 1Gpc across, from our line of sight. Previously, the Giant Arc (GA) and the Big Ring (BR; Lopez et al 2024) were reported in the literature as uLSS discoveries that are very interesting for cosmology due to their huge sizes, their close cosmological proximity to each other, and their curious ring-like morphologies. Following this, there were hints that the GA could extend into a GR (Lopez et al. 2025), which was investigated further.

Conclusions: Substantial evidence now supports the reality of a GR: different observational methods affirm the presence of an overdense filament encompassed by underdense void regions, and multiple statistical tests confirm a >3 sigma detection of a GR. The Giant Ring was discovered in the same field, and at the same redshift as the two previously-reported uLSSs making this third discovery an even bigger curiosity for cosmology. Nested rings (such as the BR and GR) seem unlikely to occur in a FLRW homogeneous universe, so perhaps the explanation for these structures lies beyond standard cosmology; in either case, it could be productive to follow the hints that these structures are providing.