Astronomers have discovered a ring-shaped cosmic megastructure, the relationships of which challenge existing theories of the universe.
The so-called Great Ring has a diameter of about 1.3 billion light years, making it one of the largest structures ever observed. At more than 9 billion light-years from Earth, it is too faint to see directly, but its diameter in the night sky would be equivalent to 15 full moons.
The sightings, which took place on Thursday at the 243rd meeting of the American Astronomical Society in New Orleans, is significant because the size of the Great Ring seems to defy a fundamental assumption in cosmology called the cosmological principle. It states that above a certain spatial scale, the universe is homogeneous and looks identical in every direction.
“From current cosmological theories, we didn’t think structures on this scale were possible,” said Alexia Lopez, a PhD student at the University of Central Lancashire, who led the analysis. “We might expect one unusually large structure in our entire observable universe.”
Zooming out on the universe should, in theory, reveal a vast, featureless expanse. Yet the Great Ring is one of a growing list of unexpectedly large structures. Others include the Giant Archwhich appears just off the Great Ring and was also discovered by Lopez in 2021. Cosmologists calculate the current theoretical size limit of structures to be 1.2 billion light-years, but the Great Ring and the Giant Arc, which span an estimated 3.3 billion light-years. year, violates this limit.
Interestingly, the two structures are at the same distance from Earth, near the constellations of Boötes the Shepherd, raising the possibility that they are part of a connected cosmological system.
“These oddities continue to be swept under the rug, but the more we find, the more we will have to face the fact that our standard model may need to be reconsidered,” Lopez said. “At a minimum, it is incomplete. As a maximum, we need an entirely new statement of cosmology.”
The Big Ring was discovered by analyzing data from the Sloan Digital Sky Survey (SDSS), a catalog of distant quasars. These objects are so bright that they can be seen from billions of light-years away and act like giant, distant lamps, illuminating intervening galaxies that pass their light on their way and that would otherwise be unseen.
Lopez and colleagues used several different statistical algorithms to identify potential large-scale structures and the Big Ring emerged. The structure appears as an almost perfect ring on the sky, but further analysis revealed that it has more of a coil shape, like a corkscrew, aligned face-up with the Earth.
Cosmologists are unsure what mechanism could have given rise to the structure. One possibility is a type of acoustic wave in the early universe, known as baryonic acoustic oscillations, which could give rise to spherical shells in the arrangement of galaxies today. Another explanation is the existence of cosmic strings, hypothetical “defects” in the fabric of the universe that can cause matter to clump together along large-scale fault lines.
Dr. Jenny Wagner, a cosmologist at the Bahamas Advanced Study Institute &
Conferences, described the discovery as significant. “This does not appear to be a mere coincidental alignment,” she said.
Wagner said it is possible to accommodate the Great Ring within the cosmological principle, depending on how its boundaries are defined, but that the more of these outlier, large-scale structures that are discovered, the less statistically plausible this view becomes. “This is why the search for further giant structures is so valuable,” she said. “Personally, I would not be surprised if we
cosmological principle to future discoveries.”