Investigating ultra-large large-scale structures: potential implications for cosmology.

Large-scale structure (LSS) studies in cosmology map and analyse matter in the Universe on the largest scales. Understanding LSSs can provide observational support for the cosmological principle (CP) and the standard cosmological model ([Formula: see text]CDM). In recent years, many discoveries have been made of LSSs that are so large that they become difficult to understand within [Formula: see text]CDM. Reasons for this are that they potentially challenge the CP (i.e. the scale of homogeneity) and their formation and origin are not fully understood. In this article, we review two recent LSS discoveries: the Giant Arc (GA, approx. 1 Gpc) and the Big Ring (BR, approx. 400 Mpc). Both structures are in the same cosmological neighbourhood-at the same redshift ([Formula: see text]) and with a separation on the sky of approximately only [Formula: see text]. Both structures exceed the often-cited scale of homogeneity (Yadav+ 2010), so individually and together, these two intriguing structures raise more questions for the validity of the CP and potentially hint at new physics beyond the standard model. The GA and BR were discovered using a novel method of mapping faint matter at intermediate redshifts, interpreted from the Mg II absorption doublets seen in the spectra of background quasars.This article is part of the discussion meeting issue 'Challenging the standard cosmological model'.