Gamma-ray bursts reveal largest structure in the universe is bigger and closer to Earth than we knew: 'The jury is still out on what it all means.'
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The universe's largest structure, the Hercules-Corona Borealis Great Wall, was already a challenge to explain with models of the universe due to its incredibly vast size — and now, using the most powerful blasts of energy in the universe, Gamma-Ray Bursts (GRBs), astronomers have discovered this structure is even bigger than they realized. Plus, the team even found that parts of the Hercules-Corona Borealis Great Wall are actually closer to Earth than previously suspected.
The Hercules-Corona Borealis Great Wall is a so-called "supercluster" of galaxies; it's a filament of the cosmic web around which the first galaxies in the universe gathered and grew. Its name was coined by Johndric Valdez, a Filipino teenager who aspires to be an astronomer. That name isn't very literal, however. This is because the round-shaped Great Wall spans not just the constellations Hercules and Corona Borealis but also the region of the celestial sphere from the constellations Boötes to Gemini.
The Hercules-Corona Borealis Great Wall was first discovered in 2014 by a team led by István Horváth, Jon Hakkila and Zsolt Bagoly, who also led the team that has now determined the size of this structure more accurately than ever before. In particular, the team found that it extends over a larger radial range than previously calculated. Before this research, scientists didn't recognize that some nearby gamma-ray bursts are also part of this massive structure.
The finding is extraordinary because the Hercules-Corona Borealis Great Wall was already known to cover an area that's 10 billion light-years wide by 7.2 billion light-years and be almost 1 billion light-years thick! For context, that is large enough to fit over 94,000 Milky Way galaxies placed side by side along the Great Wall's longest side, which stretches out for around 10% of the total width of the entire observable universe.
"Since the most distant extent of the Hercules-Corona Borealis Great Wall is hard to verify, the most interesting finding is that the closest parts of it lie closer to us than had previously been identified," Jon Hakkila of the University of Alabama in Huntsville told Space.com.
The Milky Way, our home galaxy, is part of a different supercluster called Laniakea, which, at 500 million light-years wide, is dwarfed by the Hercules–Corona Borealis Great Wall. In fact, the team says the true extent of the latter structure is currently undetermined.
"Our gamma-ray burst sample is not large enough to place better upper limits on the maximum size of the Hercules-Corona Borealis Great Wall than we already have," Hakkila said. "But it probably extends farther than the 10 billion light-years we had previously identified. It is larger than the size of most anything to which it might be compared."
GRBs were key to the discovery of the Hercules-Corona Borealis Great Wall in 2014, and indeed to the recent, deeper investigation of this vast cosmic structure. Considered the most luminous and most energetic explosions in the universe, two different types of GRBs are thought to originate from two mechanisms of stellar-mass black hole formation, Hakkila explained.
Long-duration GRBs, which are blasts of high-energy gamma rays that last over two seconds, come from the core collapse of massive stars that leads to a supernova explosion. Short-duration GRBs, on the other hand, are thought to originate from the collision and merger of two ultradense stellar remnants called neutron stars in double-star systems.
"In both cases, the tremendous energies produced from the collapse of the star system are ejected in the form of relativistic particle jets. Far from a jet's nozzle, the particles react to produce gamma-rays and X-rays," Hakkila said. "Gamma-ray bursts can be seen at incredibly large distances because they are so luminous."
Hakkila says that because gamma-ray bursts are related to dying stars or the collision of two dead stars, and since stars are found in galaxies, gamma-ray bursts can act as measures of where galaxies are, too. Because of how bright they are, GRBs can indicate the presence of a galaxy even when that galaxy itself is too faint to be seen.
"The tremendous brightness of gamma-ray bursts allows them to be markers of where matter can be found in the universe," Hakkila said.
Part of the reason structures like the Hercules–Corona Borealis Great Wall are so perplexing to scientists has to do with the cosmological principle, which most models of the cosmos are founded upon.
The cosmological principle suggests the universe is homogeneous and isotropic on large scales, meaning it should look the same in all directions. Tracing the location of matter with GRBs, however, shows that this is not the case.
"It is surprising that gamma-ray bursts clustering is so much more pronounced in the northern galactic sky than in the southern galactic sky," Hakkila explained.
In their new paper, Hakkila and colleagues assert that, according to the cosmological principle, any cosmic structure larger than 1.2 billion light-years long shouldn't have had sufficient time in the 13.8 billion-year-old universe to form if the spread of matter is homogeneous and isotropic.
So, as a vast 10 billion light-year structure of galaxies located around 10 billion light-years away (as indicated by dense GRBs clustering toward the northwestern region of the celestial sphere over Earth_ the Hercules–Corona Borealis Great Wall definitely challenges the cosmological principle.
"Some theoretical cosmological models can account for structures this large, while others cannot," Hakkila added. "The jury is still out on what it all means."
The team reached their new hint at the size of the Hercules–Corona Borealis Great Wall using a database of 542 GRBs resulting from observations collected up until 2018, predominantly by NASA's Fermi Gamma-ray Space Telescope and the Neil Gehrels Swift Observatory.
Gamma-ray bursts are useful measurement tools in cosmology — with a few caveats. The main one is that it takes the observation of a tremendously large number of GRBs to draw meaningful conclusions about their distribution.
Additionally, if scientists want to draw accurate conclusions about the structure of the universe, misidentification of GRB origin positions in space must be eliminated. Thus, it may take a long time before scientists can use GRBs to gather a better picture of the Hercules–Corona Borealis Great Wall.
"It has taken years of observation to collect a sample this large, using data mainly from Fermi and Swift, which have been instrumental in building this unprecedented dataset," Hakkila said. "Assembling a sample of this size took more than 20 years of observations, and we do not anticipate significant additions in the near future."
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Moving forward, the team intends to continue analyzing the properties of the GRBs in the sample used for this research.
"We probably need to study it more carefully and in greater detail than has been done before," Hakkila explained. "Looking ahead, new missions will be essential to overcome current limitations. We are actively contributing to the development of THESEUS, a proposed ESA mission designed to revolutionize GRB studies."
With its unparalleled sensitivity and sky coverage, Hakkila said that THESEUS, or the "Transient High Energy Sources and Early Universe Surveyor," is expected to dramatically increase the number of known GRBs, particularly at great cosmic distances or high redshifts.
"This could finally provide the observational leverage needed to map the Hercules–Corona Borealis Great Wall in its full extent, offering a breakthrough in understanding large-scale structure formation and the cosmic web," Hakkila said.
A pre-peer-reviewed version of the team's research appears on the paper repository site arXiv.
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