A massive cosmic ring may challenge a key assumption about the universe

PHOENIX — An abnormally huge cosmic structure has put a ring on it — and that bling might threaten a fundamental assumption of cosmology.

A ring of dense matter spans more than 3.3 billion light-years, cosmologist Alexia Lopez reported January 6 at a meeting of the American Astronomical Society. If real, the structure could pose a problem for the cosmological principle, which holds that the universe looks the same in all directions on large scales.

That principle is “the second most fundamental assumption in the field,” after Einstein’s theory of general relativity, says mathematical physicist Eoin ÓColgáin of Atlantic Technological University in Ireland, who studies challenges to the cosmological principle but was not involved in the new work. Every theoretical model of the universe assumes that matter is evenly distributed when you look at large enough volumes of space. Without that assumption, ÓColgáin says, “all hell would break loose.”

The giant ring joins a growing list of huge structures that shouldn’t exist if that assumption holds. It is apparently an extension of a previously reported “giant arc,” and encircles a smaller — but still huge — “big ring” of material.

“They appear to present more of a challenge to the cosmological principle together now,” says Lopez, of the University of Central Lancashire in Preston, England. “Can we explain something like a ring and an arc together?”

Lopez and her colleagues spotted the structures in light from distant quasars — glowing disks of matter surrounding supermassive black holes — captured by the Sloan Digital Sky Survey in New Mexico. As the quasars’ light traverses the universe, some of it can be absorbed and changed by atoms in and around galaxies in the intervening space. Studying changes in the quasars’ light lets astronomers map out that matter.

Lopez first noticed the “giant arc” of galaxies in 2021. All the galaxies in the arc seemed to be at the same cosmic distance, sending their signals from when the universe was half its current age, or nearly 7 billion years ago. In 2024, she added the “big ring,” an apparent circle of galaxies at the same distance hovering above the arc like a cyclops eye over a smile.

Then Lopez noticed a thin filament arcing above the big ring. “It almost looked like it could be a continuation of the giant arc,” she says. Or it could have been a coincidence, her eye picking up shapes that weren’t really there. To rule out that possibility, she ran statistical tests and found that the ring was unlikely to have formed by accident. Because a homogeneous universe shouldn’t have such immense structures in it, they challenge current models of cosmology, she says.

Other researchers disagree. ÓColgáin thinks large structures on their own are not enough to unseat the cosmological principle, although the assumption faces other challenges. And some say the current model of the universe has no trouble forming such immense features. In a paper posted on arXiv.org in February 2025, theoretical astrophysicist Till Sawala of the University of Helsinki and colleagues reported computer simulations of universes that include both the cosmological principle and structures like the giant arc (the giant ring was not included because it hadn’t been presented yet). Lopez counters that the analogue arcs that group found were different than hers, so the studies aren’t comparable.

“One can argue about that forever,” says astrophysicist Subir Sarkar of the University of Oxford. “But I think the rings change the game altogether. That is not something one would expect to find by chance.… That does sound pretty extraordinary.”

Sarkar notes that Lopez’s work has not been published in a peer-reviewed journal yet, so he can’t check her math. But larger sky surveys, such as those with the Dark Energy Spectroscopic Instrument in Arizona or the Vera C. Rubin Observatory in Chile, should find more large-scale structures — if they’re really there.

“We don’t need to keep arguing until the end of time about whether this structure is real or accidental or whatever,” Sarkar says. “We should just get more data, and more things should show up.”