Hercules–Corona Borealis Great Wall
|Hercules–Corona Borealis Great Wall|
|Observation data (Epoch J2000)|
|Constellation(s)||Hercules and Corona Borealis|
|Right ascension||17h 50m|
|Major axis||3 Gpc (10 Gly)|
|Minor axis||2.2 Gpc (7 Gly) h−1
|Redshift||1.6 to 2.1|
|9.612 to 10.538 billion light-years (light travel distance)
15.049 to 17.675 billion light-years
(present comoving distance)
|Binding mass||1.5×1019 M☉|
|See also: Galaxy groups, Galaxy clusters, List of superclusters|
The Hercules–Corona Borealis Great Wall (Her–CrB GW) is an immense superstructure of galaxies that measures more than 10 billion light-years across. It is the largest and the most massive structure known in the observable universe.
This huge structure was discovered in November 2013 by a mapping of gamma-ray bursts that occur in the distant universe. The astronomers used data from the Swift Gamma-Ray Burst Mission and the Fermi Gamma-ray Space Telescope.
The Hercules–Corona Borealis Great Wall was also the first structure other than large quasar groups that held the title as largest known structure in the universe, since 1991.
The structure is a galaxy filament, or a huge group of galaxies assembled by gravity. It is about 10 billion light-years (3 Gpc) at its longest dimension, which is approximately 1/9 (10.7%) of the diameter of the observable universe, 7.2 billion light-years (2.2 Gpc; 150,000 km/s in redshift space) wide, but only 900 million light-years (300 Mpc) thick, and is the largest known structure in the universe. It is at redshift 1.6–2.1, corresponding to a distance of approximately 10 billion light-years away, and is located in the sky in the direction of the constellations Hercules and Corona Borealis.
Gamma-ray bursts (GRBs) are some of the most powerful events in the known universe. They are very luminous flashes of gamma rays heralding the death of distant, massive stars in cataclysmic explosions. Gamma-ray bursts are rare; only one happens in an average galaxy like the Milky Way every few million years. Since it is currently theorized that the stars causing these events are the massive, luminous ones, such stars form in regions with more matter in general. Therefore, gamma-ray bursts can be indicators of galaxies to track down traces of matter decoupling in such a region of the universe.
Using the observed data from 1997 to 2012 by Istvan Horvath, Jon Hakkila, and Zsolt Bagoly, the sky was subdivided into 9 parts with 31 GRBs each. In the data for one of the subdivisions, using the two-dimensional Kolmogorov–Smirnov test, 14 out of the 31 GRBs are concentrated in a radial area that is 45° wide, with redshifts from 1.6 to 2.1; if many GRBs occur in a region, it must be a decoupling of thousands—or possibly millions—of galaxies.
The team also found out that the probability to find a similar clustering was less than 0.00055%, rendering its existence due to quantum fluctations in the early universe very unlikely to almost impossible in the standard Gaussian statistics. So the structure's existence under the accepted cosmological models was not only doubtful, but very impossible.
According to the cosmological principle, any visible random fluctations of the quantities of distribution of matter and energy within the universe will be considered sufficiently small in extremely large scales. This means that the universe will be properly homogenized and isotropized due to the basic assumption that the universe is governed by the same laws of physics regardless of location, space and time.
Prior to the discovery of the Hercules–Corona Borealis Great Wall, the largest scale at which the universe showed evidence of hierarchical structure was on the scale of superclusters and filaments. At larger scales, around 250–300 million light-years, no more fractal structuring is apparent; this was called the "End of Greatness". The homogeneity exhibited at this scale and the apparent normal density of the universe (as determined by the cosmic microwave background) implied an upper homogeneity scale about 4 times as large (1 to 1.2 billion light years; 307 to 370 Mpc). Yadav et al suggested that the tips of the scales might be as well to 260/h Mpc based on the fractal dimension of the universe, consistently smaller than the homogeneity scale above. Some scientists say that the maximum sizes of structures was somewhere around 70-130/h Mpc based on the measure of the homogeneity scale. No structures are expected to be larger than the scale since, in accordance to the homogeneous and isotropic distribution of matter in the universe, do not expect objects to be larger than the said maximum scale. However, in spite of this, some structures are discovered that exceed the scale consistently, such as:
The Clowes–Campusano LQG, discovered in 1991, is 630 Mpc across, and is marginally larger than the scale.
U1.11, another large quasar group discovered in 2011, is 780 Mpc across, and is two times larger than the scale.
The Huge-LQG (Huge Large Quasar Group), discovered in 2012, has a length of 1.24 Gpc, and is three times larger than the upper limit of the homogeneity scale. However, the scale of the individual quasars of this structure does not have a chance correlation to each other, providing the evidence of the impossibility of this structure.
The Hercules–Corona Borealis Great Wall is more than eight times larger than the scale, and so greatly exceeds the homogeneity scale. In accordance with this, the structure would still be heterogeneous as compared to the other parts of the universe even at the scale of the "End of Greatness", thereby putting the cosmological principle into further doubt.
The structure also poses a problem to the current models of the universe's evolution. In the standard model of the evolution of the universe, such structures as galaxy filaments form along and follow web-like strings of dark matter. It is thought that this dark matter dictates the structure of the Universe on the grandest of scales. Dark matter gravitationally attracts baryonic matter, and it is this "normal" matter that astronomers see forming long, thin walls of super-galactic clusters. However, the structure is so big, so complex, and so massive to exist even under the standard models of the evolution of the universe. A light travel distance of 10 billion light years means that we see the structure as it was 10 billion years ago, or roughly 3.8 billion years after the Big Bang. Current models of the universe's evolution, however, do not allow the said structure to form in just a mere 3 billion years, since this is a very short time for dark matter to attract sufficient baryonic matter to create the giant structure. The structure is itself too big, and too complex, to exist so early in the universe. There is currently no existing model to explain the existence of the structure.
Evidence of a cosmic web
Such large structures like the Hercules–Corona Borealis Great Wall may form part of the vast intergalactic cosmic web, an endless continuous sheet of galaxies and dark matter. Although this web was never directly observed, the relatively large sizes of structures in the nearby universe provides the possibility of the existence of this web. Such gigaparsec-scale structures, including the Hercules–Corona Borealis Great Wall, may be the intersections of smaller subfilaments within this vast structure, where there are overdensities of galaxies connecting other filaments within this vast web. If verified, the Hercules–Corona Borealis Great Wall will be one of the first evidences of the existence of this web.
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- Horvath I., Hakkila J., and Bagoly Z. (2013). "The largest structure of the Universe, defined by Gamma-Ray Bursts". 7th Huntsville Gamma-Ray Burst Symposium, GRB 2013: paper 33 in eConf Proceedings C1304143. arXiv:1311.1104. Bibcode:2013arXiv1311.1104H.
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