How big is Hercules–Corona Borealis Great Wall?

The Hercules–Corona Borealis Great Wall is a giant structure in the distant universe, proposed in 2014, though other researchers doubt that it exists. Its discoverers estimate it at 2,000–3,000 megaparsecs across, or about 6.5–9.8 billion light-years. A light-year is the distance light travels in one year, and a megaparsec is about 3.26 million light-years. The size shown above, about 9.5 billion light-years, is near the top of that range.

Size

The structure was found in the pattern of gamma-ray bursts, the most energetic explosions known. They come from the deaths of massive stars in distant galaxies, so they mark where galaxies and ordinary matter are. Astronomers find a burst's distance from its redshift, a measure of how much the expansion of the universe has stretched its light.

In 2014, István Horváth, Jon Hakkila and Zsolt Bagoly sorted 283 bursts with known redshifts into nine groups by distance. In the group at redshifts 1.6–2.1, 14 of the 31 bursts were packed into about one-eighth of the sky. A 2015 study with 361 bursts found the same cluster. It covers all of the constellations Hercules and Corona Borealis, which gave the structure its popular name.

The size is the structure's width across the sky, where 2,000–3,000 megaparsecs is 6.5–9.8 billion light-years. Its depth along the line of sight is poorly known. Light from this region set out about 10 billion years ago, when the universe was about 3–4 billion years old.

Size comparisons

If the Milky Way, about 100,000 light-years across, were shrunk to the size of a basketball, the Great Wall would be about 16–23 kilometers (10–15 miles) across.
About 65,000–98,000 Milky Way disks would fit across it side by side.
It would be about 7–10 times as long as the Pisces-Cetus Supercluster Complex, the band of galaxy clusters that includes the Milky Way, which is about 1 billion light-years long.
It would be 5–7 times as long as the Sloan Great Wall, a band of galaxies 1.37 billion light-years long that was called the largest observed structure in 2005.
The observable universe, about 93 billion light-years across, is 10–14 times as wide as the Great Wall.

Doubts about its existence

The claim is disputed. A 2016 study of bursts from three space telescopes found little evidence that bursts cluster by redshift. Telescopes also watch some parts of the sky more than others, which can make bursts look clustered when they are not.

A 2020 study ran the original tests on many simulated skies with no structure in them, and often found clusters as strong as the real one. With newer data, the cluster looked even less significant. The study concluded that the Great Wall's existence "must be treated as doubtful at best".

The discoverers still argue that the clustering is real. In 2020 they wrote that it may be the largest structure in the universe, but that a future mission such as the THESEUS satellite is needed to decide.

Why its size matters

Cosmologists expect the universe to look smooth on the largest scales, with no structures bigger than a few hundred megaparsecs. The 2015 paper noted that the Great Wall would be far larger than the roughly 100 megaparsecs where structure had been thought to end. The 2020 critique puts the largest expected structures at about 260 megaparsecs.

The discoverers also noted that a few very large structures can arise by chance. They added that the Great Wall might be a group of smaller structures lined up along the same direction.