A Big Ring on the Sky

We present the discovery of `A Big Ring on the Sky' (BR), the second ultra-large large-scale structure (uLSS) found in MgII-absorber catalogues, following the previously reported Giant Arc (GA). In cosmological terms the BR is close to the GA - at the same redshift $z \sim 0.8$ and with a separation on the sky of only $\sim 12^\circ$. 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 $\sim 400$ Mpc (proper size, present epoch). The method of discovery is as described in the GA paper, but here using the new MgII-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 $5.2\sigma$. We present the discovery of the BR, assess its significance using the CHMS, Minimal Spanning Tree (MST), FilFinder and Cuzick & Edwards (CE) methods, show 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.