Giant arc and Big ring: Why do they conflict with standard model of cosmology, and what will studying them reveal?

According to the Standard Cosmological Model, the early Universe was expected to be fairly homogeneous. However, in 2021, astronomers discovered a large grouping of galaxies forming a giant arc spanning 3.3 billion light-years. Now, another large-scale formation has emerged — a massive ring of galaxies dubbed the "Great Ring." Together, the giant arc and the Great Ring pose a challenge to the Standard Model and may prompt cosmologists to reconsider their understanding of the early Universe.

"The Great Ring and the Giant Arc are at the same distance from us, near the constellation of Bootes, indicating that they existed at the same time when the Universe was only half its current age. They are also in the same region of the sky, within just 12 degrees. Detecting two extremely large structures so close to each other raises the possibility that together they form an even more remarkable cosmological system," says Alexia Lopez, who discovered both formations alongside her research supervisor Roger Clowes and colleague Gerard Williger.

The Great Ring and the Giant Arc consist of galaxies so faint and weak that they are usually impossible to see. However, distant quasars (active black holes at the centers of galaxies) allow light to pass through these faint galaxies, where some of the light is absorbed by matter.

In particular, Lopez and her colleagues were looking for evidence that faint galaxies block magnesium ions, Mg-II. "We found them in Sloan Digital Sky Survey data, providing us with the position and distance from invisible galaxies. This allowed us to create a three-dimensional map of galaxies and identify the Giant Arc and the Great Ring, located at a distance of 9.2 billion light-years," says Lopez.

"The Great Ring has a significance of 5.2 sigma. This exceeds the 5 sigma threshold, which is an astronomical level of significance required to confirm a discovery," explains Lopez, emphasizing the importance of this finding.

One possible explanation for such large structures is called Baryon Acoustic Oscillation (BAO). It is assumed that waves formed by gravitational interactions in the early Universe could create "bubbles" of matter on large scales. However, BAO typically generates spherical structures, while the Great Ring is two-dimensional.

At the annual meeting of the American Astronomical Society on January 10, 2024, Lopez mentioned two possible alternative explanations. The first is related to the existence of cosmic strings proposed in string theory in the 1970s. "Cosmic strings could have been created in the early Universe and left their mark on the structure of matter," says Lopez.

The Great Ring and the Giant Arc could also be explained by another model of cosmology, such as Conformal Cyclic Cosmology (CCC) proposed by physicist Roger Penrose. "In CCC, the Universe undergoes infinite cycles of the Big Bang. It expands infinitely, and all matter decays until the difference between the empty expanded Universe and the singularity of the Big Bang reduces only to a matter of scale," explains Lopez.

Further research into these structures and their significance in the context of the Universe's development poses a challenge for cosmology. These discoveries bring us closer to understanding the origin and evolution of the Universe and may lead to a reevaluation of existing theories.