Abstract
Abstract
We present the discovery of `A Big Ring on the Sky' (BR), the
second ultra-large large-scale structure (uLSS) found in Mg II-absorber catalogues, following the previously reported Giant Arc
(GA). In cosmological terms the BR is close to the GA — at the same
redshift z ∼ 0.8 and with a separation on the sky of only ∼
12°. Two extraordinary uLSSs in such close configuration raises the
possibility that together they form an even more extraordinary cosmological
system. The BR is a striking circular, annulus-like, structure of diameter
∼ 400 Mpc (proper size, present epoch). The method of discovery is as
described in the GA paper, but here using the new Mg II-absorber
catalogues restricted to DR16Q quasars. Using the Convex Hull of Member
Spheres (CHMS) algorithm, we estimate that the annulus and inner absorbers
of the BR have departures from random expectations, at the density of the
control field, of up to 5.2σ. We present the discovery of the BR,
assess its significance using the CHMS, Minimal Spanning Tree (MST),
FilFinder and Cuzick & Edwards (CE) methods, discuss it in the context of the
GA+BR system, and suggest some implications for the origins of uLSS and for
our understanding of cosmology. For example, it may be that unusual
geometric patterns, such as these uLSSs, have an origin in cosmic strings.