The Hercules-Corona Borealis Great Wall is a huge, flat superstructure of galaxies more than 10 billion light years across. It has long been difficult to explain in models of the Universe because of its incredibly large size. Now, based on observations of gamma-ray bursts, it has been established that this structure is even larger, and some of its parts are closer to Earth than previously thought.
Image Credit: NASA Hubble Space Telescope / Unsplash
In fact, the superstructure occupies a much larger area than the constellations Hercules and Corona Borealis. It is located in the northern hemisphere, and its central part is in the region of the constellations Draco and Hercules. The largest structure in the observable part of the Universe was discovered in 2013 thanks to many years of gamma-ray observations conducted by scientists from the National University of Public Service in Budapest and the College of Charleston in South Carolina, including data from the Swift space telescope.
Interestingly, the scientists who discovered the structure did not give it an official name. It was given the name Hercules–Corona Borealis Great Wall by a Filipino teenager named Johndric Valdez, who dreamed of becoming an astronomer. However, this name does not quite accurately reflect the essence of the structure, since it is not entirely flat and covers the area of the celestial sphere from the constellation Bootes to Gemini.
«”While it is difficult to determine the exact extent of the Hercules-Corona Borealis Great Wall, the most interesting discovery is that its closest parts are closer to us than previously thought,” said Jon Hakkila of the University of Alabama in Huntsville, commenting on the recent study.
Our home galaxy, the Milky Way, is part of another superstructure called Laniakea, which is 520 million light-years across. While it is clear that the Hercules-Corona Borealis Great Wall is much larger, scientists note that its true scale is currently uncertain. “Our sample of gamma-ray bursts is not large enough to set more precise upper limits on the maximum size of the Hercules-Corona Borealis Great Wall than we already have. But it probably extends further than the 10 billion light-years we previously determined. It is larger than most objects with which it can be compared,” said John Hakkila.
Gamma rays played a key role in the discovery of the Hercules-Corona Borealis Great Wall in 2013, as well as subsequent deeper studies of this giant cosmic structure. Scientists note that gamma rays are associated with dying stars or collisions of two dead stars. Since stars are located in galaxies, gamma-ray bursts can serve as an indicator of the location of galaxies. Due to the brightness of such bursts, it is possible to detect the presence of a galaxy even where it is impossible to see it with existing astronomical instruments.
The reason structures like the Hercules-Corona Borealis Great Wall baffle scientists is partly due to the cosmological principle that underlies most models of the cosmos. It posits that the universe is homogeneous and isotropic on large scales, meaning it should look the same in all directions. However, tracking the distribution of matter using gamma-ray bursts suggests that this may not be the case. “What’s surprising is that the clustering of gamma-ray bursts is much stronger in the northern part of the galactic sky than in the southern part,” Hakkila explains.
In his new paper, he and his colleagues argue that the cosmological principle predicts that any cosmic structure larger than 1.2 billion light-years in diameter would not have had enough time to form in the 13.8 billion years that the universe has existed if matter is distributed uniformly and isotropically. So the giant Hercules-Corona Borealis Great Wall superstructure, which is more than 10 billion light-years in diameter, clearly defies the cosmological principle. “Some theoretical cosmological models can explain such large structures, while others cannot,” Hakkila said, adding that scientists are still divided on what this all means.
Gamma rays are a useful measurement tool in cosmology, but there are a number of limitations. The main one is that a huge number of gamma-ray bursts need to be recorded to draw statistically significant conclusions about their distribution. In addition, distortions associated with incorrectly determined positions of gamma-ray sources in space must be eliminated to obtain accurate data on the structure of the Universe. This means that it may be a long time before scientists get a more complete picture of the size of the Hercules-Corona Borealis Great Wall. “To collect such a large sample, years of observations will be required, mainly using Fermi and Swift data, which have already played a major role in creating this unprecedented data set. It took more than 20 years of observations to collect this amount of information, and we do not expect significant additions in the near future,” Hakkila said.