The observable universe contains a wide variety of different bodies, from asteroids to galaxies, including planets and stars. Most of these bodies form gravitational arrangements, configurations in which these bodies are related to each other. Although galaxies are considered the largest objects in the universe, they can form structures whose size defies imagination.
Galaxies can first arrange themselves in groups of galaxies, that is to say structures composed of less than a hundred galaxies. Past the threshold of 100 galaxies, the structure formed becomes a cluster of galaxies. If several clusters are linked together, then we speak of superclusters.
This is the case, for example, of our galaxy, the Milky Way, which belongs to the Local Group, itself belonging to the Virgo cluster located in the Virgo supercluster. The latter contains about 10,000 galaxies with a diameter of 200 million light-years and a mass of 2×10 46 kg (i.e. 10 15 solar masses). The Virgo supercluster is in turn contained within the Laniakea supercluster having a diameter of 500 million light-years .
But the universe didn't stop there. Superclusters can gravitate together to form the largest known structures in the universe:galactic filaments. These huge arrangements of galactic superclusters are bordered by cosmic voids, i.e. areas of space containing very few or no galaxies.
In the Standard Model of cosmology, galactic filaments form along the dark matter network. The first filaments were discovered in 1987 as "galactic walls". Indeed, galactic filaments can adopt various configurations including galactic walls and "galactic sheets".
At the end of 2013, a team of astrophysicists led by I. Horvath, J. Hakkila and Zs. Bagoly analyzed data from the SWIFT and GLAST telescopes concerning the detection of 283 gamma-ray bursts. They divide and distribute these 283 gamma-ray bursts into 9 groups of 31 gamma-ray bursts each. By studying the areas of appearance of these bursts, the astronomers discover that, in the fourth group, 19 out of the 31 bursts took place in an area covering the second, third and fourth galactic quadrants; these three quadrants span 125° in the sky.
Gamma-ray bursts come from the explosion of very massive stars. However, these stars can only form in areas of the universe rich in matter. The occurrence of a gamma-ray burst is therefore a globally rare event.
Such a frequency concentrated in such a delimited area then appears very unlikely to researchers, unless it is a huge galactic filament:the Great Wall with Gamma Bursts or the Great Wall of Hercules-Corona Borealis (GMHCB ). According to the data collected by the team, this filament would extend over more than 15 billion light-years (more than 20 constellations) with a width of 7.2 billion light-years and a mass of 2× 10 19 solar masses. This is about 11% of the diameter of the observable universe. However, it is extremely difficult to give the exact dimensions of such a structure because of the observation biases imposed by the size of the filament in the sky.
With these dimensions, the Great Wall of Hercules-Corona Borealis is currently the largest known structure in the observable universe. It is also so large that its existence violates the cosmological principle establishing the homogeneity and isotropy of the universe. To be valid, this principle imposes a theoretical limit on the length that the structures of the universe can possess; this limit being between 4×10 8 and 6×10 8 light-years.
However, with its ~15 billion light-years, the Great Wall of Hercules-Corona Borealis lies well beyond this limit. In this regard, Professor J. Akkila states that “the GMHCB is much larger than the theoretical upper limit that a structure can possess. Such an object should not exist, and yet it does.
So much so that today the mechanism of formation of such a structure remains unknown. Although the Standard Model of cosmology does not prohibit the existence of these structures, an unknown remains about their origin. Several hypotheses have been put forward in recent years to explain their formation. One of them involves very intense density fluctuations during the Big Bang, which would have led to significant aggregations of matter coupled with the primordial gravitational action of dark matter.
