Two of the largest reported structures in the distant universe do not just look strange on their own. They appear to sit in the same part of deep space, at the same cosmic distance, as if the universe placed two oversized patterns beside each other where standard cosmology expected matter to look much smoother.
The first is the Giant Arc, reported by Alexia M. Lopez, Roger G. Clowes and Gerard M. Williger after an analysis of quasar light. It stretches about 1 gigaparsec across, or roughly 3.3 billion light-years, at a redshift of about 0.8. The second is the Big Ring, reported by the same team, with a diameter of about 400 megaparsecs, or about 1.3 billion light-years, also at about the same redshift.
That redshift places both structures about 9.2 billion light-years from Earth, seen from an era when the universe was roughly half its present age. On the sky, they are separated by only about 12 degrees in the direction of Bootes. For large-scale cosmology, that makes them neighbours.
Lopez, now one of the astronomers most closely associated with the discoveries, has said that neither structure is easy to explain in current understanding. The harder question is what it means if both are real, both are that large, and both belong to the same broad cosmic neighbourhood.
Not a photograph of a ring
The Giant Arc and Big Ring were not found by taking a direct image of a glowing arc or a neat circle of galaxies. They were inferred from absorption lines in the spectra of distant quasars, using data from the Sloan Digital Sky Survey.
Quasars are extremely bright galactic nuclei, so distant and luminous that they can act like background lamps. If gas and galaxies sit between Earth and a quasar, atoms in that intervening material can absorb specific wavelengths of light. In these studies, the key tracer is Mg II, singly ionised magnesium, seen as a pair of absorption lines. By measuring the redshift of those absorption features across many quasar sightlines, astronomers can map otherwise faint matter lying in front of the quasars.
The Giant Arc emerged as an enormous, curved arrangement of these absorbers. The Big Ring emerged later in the same general field, using updated Mg II absorber catalogues from Sloan quasar data. In the published Big Ring paper, the authors describe it as an annulus-like structure whose departure from random expectations reached up to 5.2 sigma under one of their statistical methods.
That does not mean every cosmologist accepts the interpretation without reservation. Large-scale structure work is statistically difficult, and unusual-looking patterns can sometimes appear when many possible configurations are searched. The authors themselves use several methods and discuss the problem of post-hoc significance. But the reason the pair keeps attracting attention is that the structures are not merely large. They are large, geometrically striking and close to each other in cosmological terms.
The problem for smoothness
The standard model of cosmology, often called Lambda-CDM, does not require the universe to be perfectly smooth at every scale. Galaxies cluster. Clusters form filaments. Voids open between them. The cosmic web is real, and it is one of the model’s successes.
But Lambda-CDM also leans on the cosmological principle: at sufficiently large scales, the universe should look broadly the same in all directions and locations. It can have local clumps and empty regions, but those irregularities should average out when the scale becomes large enough.
That is why the Giant Arc and Big Ring are awkward. The often-cited scale of homogeneity is around 370 megaparsecs, or about 1.2 billion light-years, although the exact interpretation depends on the definition and the dataset. The Giant Arc is far larger than that. The Big Ring’s diameter is comparable to the scale, and its circumference is roughly 4 billion light-years.
In a 2025 review in Philosophical Transactions of the Royal Society A, Lopez, Clowes and Williger argued that both structures exceed the commonly cited homogeneity scale and, taken together, raise questions about whether the visible universe is as statistically typical as the cosmological principle assumes.
The claim is not that the standard model collapses because of two patterns. It is more precise than that. If the structures are confirmed as physical systems rather than chance alignments or statistical artefacts, then cosmologists need to explain how such large, organised features arose within a universe that should be increasingly uniform on the largest scales.
Why two is harder than one
A single extreme object can be treated as an outlier. The observable universe is enormous, and rare things happen somewhere. A very large structure might be surprising but not necessarily fatal to a model.
The pairing is what sharpens the puzzle. The Big Ring and Giant Arc appear at the same redshift, so their light has been travelling for about the same span of cosmic history. They are also close on the sky, only about 12 degrees apart. In the 2024 announcement of the Big Ring, Lopez described the possibility that the two together form an even larger cosmological system.
If that is right, the issue is no longer just a record-breaking arc or an oversized ring. It becomes a question about whether a region of the early universe contains a connected pattern much larger than expected. That would press on one of cosmology’s working assumptions: that no special region of the observable universe should dominate the statistics once the scale is large enough.
Several possible explanations have been discussed. Baryon acoustic oscillations, the imprint of early-universe sound waves, can produce preferred scales in the distribution of galaxies. But the Big Ring is too large and not spherical enough to fit the usual BAO picture cleanly. More speculative ideas, including cosmic strings or conformal cyclic cosmology, have also been mentioned, but none is established as the answer.
A challenge, not a verdict
There is also a healthy counterpressure in the field. Cosmology has seen many proposed anomalies. Some remain important. Some fade when larger surveys, better simulations or more rigorous statistics arrive. The Giant Arc and Big Ring sit in that active territory, where the data are provocative but the implications are still being tested.
Future surveys should help. Larger quasar catalogues, deeper galaxy maps and independent absorption-line analyses can test whether the structures persist, whether their membership changes, and whether similar features appear elsewhere. Simulations can also ask a sharper question: how often should Lambda-CDM produce patterns that look this large and this organised when analysed in the same way as the real data?
That last condition is crucial. A fair test must compare observations with simulations using the same selection effects, redshift ranges, absorber catalogues and pattern-finding algorithms. Otherwise, the universe can look more surprising, or less surprising, simply because the test is not measuring the same thing.
For now, the Giant Arc and Big Ring remain two of the most intriguing reported features in the large-scale universe. They are not visible monuments in a telescope image, but statistical outlines traced through the light of distant quasars. If they are confirmed as real physical structures, their closeness in space and time may be harder to ignore than either one alone.
The universe is allowed to be lumpy. The question raised by Lopez and colleagues is whether, in this one distant region, it may be lumpy on a scale that current cosmology did not expect to see twice.
Sources
Lopez, Clowes and Williger, A Giant Arc on the Sky, Monthly Notices of the Royal Astronomical Society, 2022
Lopez, Clowes and Williger, A Big Ring on the sky, Journal of Cosmology and Astroparticle Physics, 2024
Lopez, Clowes and Williger, Investigating ultra-large large-scale structures: potential implications for cosmology, Philosophical Transactions of the Royal Society A, 2025
University of Central Lancashire / Phys.org release on the Big Ring and Giant Arc
arXiv preprint: A Big Ring on the Sky
