A possible wall of galaxies billions of light-years long, traced only by the flashes of dying stars
DisputedArtist's impression · drag to rotate
Estimated size2 000 to 3 000 megaparsecsabout 10 billion light-years, in Wikipedia's conversion[2]
Found bygamma-ray bursts14 of 31 bursts crowded into about an eighth of the sky[2]
Distance (original)redshift 1.6 to 2.1light that left it 9.6 to 10.5 billion years ago[1]
Distance (2025 study)possibly redshift 0.33 to 2.43[5]
Bursts in the sample283 → 487 → 5422014, 2020 and 2024 studies[4]
Expected largest structureabout 260 megaparsecsthe scale above which the universe should look smooth[6]
Compared withSloan Great Wall, more than 400 megaparsecs[2]
Independent check (2020)"doubtful at best"Christian, MNRAS[6]
About the image. Drawn as a long, faint sheet of galaxies with a few gamma-ray bursts flaring inside it. No galaxy belonging to the structure has ever been mapped: the only data are the sky positions and distances of a few dozen bursts. Its galaxies, shape, edges and colours are all imagined.
The Hercules–Corona Borealis Great Wall is the largest structure anyone has claimed to find in the universe. If it is real, it is a vast region of galaxies 2 000 to 3 000 megaparsecs across, about 10 billion light-years. Nobody has seen a single one of its galaxies. It exists as a pattern in the brief flashes of dying stars.
Seen only in flashes
Gamma-ray bursts are the brightest explosions known, set off when some very massive stars collapse. They can be seen from billions of light-years away, which makes them useful markers of where galaxies are, even when the galaxies themselves are far too faint to see. In 2013 István Horváth, Jon Hakkila and Zsolt Bagoly sorted the 283 bursts whose distances had been measured into nine groups by distance. One group, at redshift 1.6 to 2.1, stood out: 14 of its 31 bursts fell within about an eighth of the sky, towards the constellations Hercules and Corona Borealis. Light from that region left it 9.6 to 10.5 billion years ago.
Too big for the rules
Standard cosmology says that on the largest scales the universe should look smooth. Above about 260 megaparsecs, lumps should average out. This one is estimated at roughly ten times that size, and that is why it is controversial. In 2020 S. Christian applied the same statistical tests to computer-made skies with no structure at all and found that random chance often produced clusters like this one. His conclusion was that its existence “must be treated as doubtful at best”. An earlier study by Ukwatta and Woźniak had also found little sign of such clustering.
Growing on paper
The discoverers have kept testing it as more bursts are measured. With 487 bursts in 2020, they still found the cluster, while writing that some observational bias could not yet be ruled out. With 542 bursts in 2024, the range shifted to redshift 0.9 to 2.1. A 2025 study suggested its members could span redshift 0.33 to 2.43. News reports turned that into “15 billion light-years”, a figure the paper itself does not give.
The weak point is the sample. Only about two in five bursts have a measured distance, and which ones do depends on where telescopes could look. A proposed space mission, THESEUS, which would catch far more bursts, is the test the discoverers themselves call for. It is still only a candidate mission at the European Space Agency.
A name from Wikipedia
The astronomers did not name it. The name “Hercules–Corona Borealis Great Wall” first appeared on Wikipedia in November 2013 and was later used by the team itself. It is not much of a wall: the region the bursts outline is roughly round, and far larger than the two constellations it is named after.
Gallery
A long, thin, faint band of countless tiny golden galaxies crossing black space diagonally, with five small blue-white flashes along it, an impression of the Hercules–Corona Borealis Great Wall traced by gamma-ray bursts. AI-generated painting.A broad, grainy band of countless faint points of light running diagonally across black space, with a few small blue-white flashes scattered along it, an impression of the Hercules–Corona Borealis Great Wall. AI-generated painting.Hercules–Corona Borealis Great Wall, 3D artist's impression, rendered from the interactive model.Hercules–Corona Borealis Great Wall, 3D artist's impression, from another angle, rendered from the interactive model.Hercules–Corona Borealis Great Wall, 3D artist's impression, close-up, rendered from the interactive model.
Timeline
2013Horváth, Hakkila and Bagoly report a cluster of gamma-ray bursts at redshift 1.6 to 2.1; the name appears on Wikipedia in November.
2014The discovery paper estimates its size at 2 000 to 3 000 megaparsecs.
2020Christian finds random skies often produce such clusters and calls it "doubtful at best"; the discoverers confirm it with 487 bursts.
2024With 542 bursts, the range shifts to redshift 0.9 to 2.1.
2025A new study suggests it could span redshift 0.33 to 2.43.
Questions people ask
What is the largest structure in the universe?
The largest claimed is the Hercules–Corona Borealis Great Wall, estimated at 2 000 to 3 000 megaparsecs, about 10 billion light-years. It is traced only by gamma-ray bursts, and whether it is real is disputed.
Is the Hercules–Corona Borealis Great Wall real?
Nobody knows yet. Its discoverers find it again in every larger sample of bursts, but a 2020 study found that random skies often produce similar clusters and called it doubtful at best.
How was the Hercules–Corona Borealis Great Wall discovered?
By mapping gamma-ray bursts with measured distances. In the 2014 study, 14 of the 31 bursts at redshift 1.6 to 2.1 were crowded into about an eighth of the sky.