Astronomers Find "Big Ring" 1.3 Billion Light-Years Across

Close to the handle of the Big Dipper, there’s a huge ring on the sky that shouldn’t be there. The circular structure — an apparent overdensity of distant galaxies — has a circumference of 4.1 billion light-years. Standard cosmology models cannot easily explain such humongous structures in the mass distribution of the universe. According to PhD student Alexia Lopez (University of Central Lancashire, UK), the discovery “leads to the ultimate question: do we need a new standard model?”

Lopez first presented the result at the 243rd meeting of the American Astronomical Society in New Orleans in early January. Her research paper, coauthored with Roger Clowes (also at University of Central Lancashire) and Gerard Williger (University of Louisville), has now been posted to the arXiv astronomy preprint server (see preprint here).

Two years ago, the same team presented the discovery of another ultra-large-scale structure (uLSS): a giant arc, at a similar distance of 9.2 billion light-years, and more or less in the same part of the sky. “Two extraordinary uLSSs in such close configuration raises the possibility that together they form an even more extraordinary cosmological system,” they write.

Both the Giant Arc and the newly discovered Big Ring show up indirectly, via the absorption lines seen in the spectra of many thousands of distant quasars — active galaxies powered by supermassive black holes. Matter along the line of sight to a quasar absorbs light at specific wavelengths. In particular, the team is looking for the absorption of ionized magnesium atoms (MgII) — both in galaxies and in the gas between them. Due to the expansion of the universe, the wavelength of MgII absorption shifts to the red side of the spectrum (longer wavelengths) when the absorber is farther away – a phenomenon known as redshift.

By measuring the precise redshifts of many absorption lines, the team can calculate the distance to matter lying between the quasar and us, enabling the discovery of 3D patterns.

On the basis of a comprehensive statistical analysis of the data, Lopez and her coauthors conclude that the Big Ring they've found is “real and statistically significant,” adding it to a growing list of large-scale structure candidates that are in tension with the cosmological principle, which holds that matter in the universe is similarly distributed everywhere when you zoom out to large enough scales.

Indeed, according to the ΛCDM model of cosmology, in which the evolution of the universe is dominated by dark energy (Λ) and cold dark matter (CDM), there hasn’t been enough time since the Big Bang to form gravitationally bound structures larger than 1.2 billion light-years or so. (Note that large-scale structures such as the Giant Arc and the Big Ring grow as the universe expands, so dimensions are given for the present epoch.)

Jim Peebles (Princeton University), who won the 2019 Nobel Prize in Physics for pioneering the cosmological standard model, says the ultra-large-scale structures found by Lopez, Clowes and Williger “might be real and significant.” According to Peebles, it is very likely that the ΛCDM model is only an approximation. “I am hoping for anomalies that could prove to be empirical hints to a better theory,” he says. However, he warns that the team might be “finding apparent structure in pure noise.”

“Patterns like these are seen in sufficiently large cosmological simulations,” adds Carlos Frenk (Durham University, UK), who was not involved in the study. “However, they are not bound structures.”

So are there any problems for the ΛCDM model? “None of note,” answers Frenk.

Cosmologist Rien van de Weijgaert (University of Groningen, The Netherlands), agrees. “It’s a very interesting result,” he says, “but I don’t think that these structures in the distribution of galaxies imply an equally large structure in the underlying mass distribution in the universe.” In other words: Ultra large-scale structures do not necessarily pose a problem for cosmology.

According to van de Weijgaert, a bias is introduced because Lopez and her colleagues only find galaxies that produce magnesium absorption lines in the light of quasars.

“Where there isn’t a quasar, we can’t detect intervening matter,” admits Lopez. “And it’s not that I’m saying that the standard [cosmological] model is wrong. Like everyone else, I also have skepticism. I’d love to have the answers.” These answers may have to await a thorough exploration of the full database of quasar spectra obtained by the Sloan Digital Sky Survey, since the findings so far are based on a relatively small subset.

As the authors write in their paper: “The Big Ring and Giant Arc together, given their sizes and morphologies, are presumably telling us something intriguing, and quite possibly important, about the universe, but at the moment we can only speculate what that might be.”