A HUGE RING IN THE SKY

MYSTERIES OF THE UNIVERSE

An astronomer has discovered a ring 1.3 billion light years in size that could turn a long-established principle on its head

Reported by David Crookes

The cosmological principle is based on the assumption that the universe is isotropic T and homogeneous. In other words, the universe looks the same in all directions regardless of where it is being observed from, allowing astronomers to extrapolate from what is visible. Abiding by the principle, astronomers would say that matter is evenly spread. No structure should be able to exceed 1.2 billion light years in diameter either, as when you take into account the timescales involved in forming large objects through the process of gravitational instability, there simply has not been sufficient time in the 13.8-billionyear history of the universe for anything larger. But as PhD student Alexia Lopez from the Jeremiah Horrocks Institute at the University of Central Lancashire (UCLan) has noted – not once, but twice – such assumptions can be firmly challenged.

In 2021, Lopez and her colleagues were using data from the Sloan Digital Sky Survey, which has been cataloguing distant quasars. The multispectral imaging and spectroscopic redshift survey makes use of a telescope at the Apache Point Observatory in the Sacramento Mountains of New Mexico, and it’s capable of imaging 1.5 square degrees of sky at a time. Using this, it’s been possible to observe more than 600 galaxies and quasars at once. It enabled Lopez to make her first discovery: a giant, near-symmetrical arc of galaxies more than 9.2 billion light years away. Lopez, with her advisor Roger Clowe, also from UCLan, and collaborator Gerard Williger from the University of Louisville, were understandably surprised. The observation spanned 3.3 billion light years, which was far and above the 1.2 billion threshold. Given the observable universe is 94 billion light years wide, it gives the mammoth discovery some perspective.

The Giant Arc, as it’s been called, was in the constellation of Boötes – a cluster of around 50 gas clouds. Its discovery followed mammoth finds by other astronomers elsewhere, including the Sloan Great Wall, detected in 2003 and measuring 1.4 billion light years, and the Huge Large Quasar Group, announced in 2013, at 4 billion light years across. The Hercules-Corona Borealis Great Wall observed in November 2013 is the biggest of them all, extending more than 10 billion light years, but that doesn’t make Lopez’s discovery any less significant. Combined, these large structures make the cosmological principle more and more difficult to justify. “The cosmological principle assumes that the part of the universe we can see is viewed as a ‘fair sample’ of what we expect the rest of the universe to be like,” Lopez says. “We expect matter to be evenly distributed everywhere in space when we view the universe on a large scale, so there should be no noticeable irregularities above a certain size.”