Great Walls and Filaments: The Cosmic Web

What is a Great Wall?

Definition and composition of a Great Wall

The observable universe, that is, the part of the universe that we can potentially observe, is extremely vast. However, the universe as a whole could be much vaster still, and its exact extent beyond the observable universe is unknown.

Cosmologists have given names to the different structures in this vast expanse. The interior of the universe is composed of galaxy clusters, superclusters of galaxies, supercluster complexes, cosmic filaments, Great Walls, and cosmic voids.

In cosmology, the term "Great Wall" refers to several large-scale structures in the universe. These structures, several hundred million light-years across, are immense conglomerations of superclusters of galaxies, groups and filaments of galaxies, dark matter, and gas interconnected by gravitational force. It should be noted that a Great Wall is not necessarily a gravitationally bound structure as a whole: some, like the Sloan Great Wall, are rather alignments of matter that will not maintain themselves indefinitely under their own gravity.

The main Great Walls of the Universe

The largest Great Walls are the Hercules-Corona Borealis Great Wall (about 10 billion light-years, or even up to 15 billion according to a recent reassessment), the Huge-LQG (about 4 billion light-years), the Sloan Great Wall (about 1.37 billion light-years), and the CfA2 Great Wall (about 500 million light-years).

The study of such structures helps cosmologists better understand the formation and evolution of the universe on large scales, as well as the distribution of matter in "space-time". The question of whether the universe as a whole is infinite or finite remains an open question in cosmology.

N.B.:
Our Galaxy does not belong to a "Great Wall", it is part of a local group of galaxies (Milky Way, Andromeda galaxy, Triangulum galaxy, and a few dwarf galaxies), it is a small collection of galaxies. These galaxies are gravitationally bound to each other in a supercluster called the Virgo Supercluster. The Virgo Supercluster belongs to an even larger structure known as Laniakea, a supercluster of superclusters.

Comparison of the main Great Walls

2024-2025: new structures revive the debate

The Big Ring, a circle of 1.3 billion light-years

In January 2024, astrophysicist Alexia Lopez, then a doctoral student at the University of Central Lancashire, and her colleagues announced the discovery of a structure nicknamed the Big Ring. Spotted in the direction of the constellation Boötes, about 9.2 billion light-years from Earth, this nearly perfect ring-shaped structure measures about 1.3 billion light-years in diameter, with a circumference of more than 4 billion light-years. It was identified in the same data as the Giant Arc, another structure spotted by the same team in 2021 and located in the same region of the sky, at the same distance.

The Big Ring far exceeds the maximum size generally accepted for a gravitationally bound cosmic structure, on the order of 1.2 billion light-years according to the standard cosmological model. Its shape, its alignment with the Giant Arc, and its origin remain unexplained; some researchers evoke the still-speculative track of cosmic strings, hypothetical topological defects in the fabric of space-time.

Quipu, the largest known concentration of mass

In February 2025, a team led by astrophysicist Hans Böhringer (Max Planck Institute for Extraterrestrial Physics) announced the discovery of Quipu, a filamentary superstructure of the nearby Universe, named after the knotted cords used by Andean civilizations to record information. Identified thanks to the X CLASSIX survey, which maps galaxy clusters by their X-ray emission, Quipu extends over about 1.3 billion light-years (more than 400 megaparsecs) and concentrates an estimated mass of about 200 million billion (2×10¹⁷) solar masses, or about 200,000 times the mass of the Milky Way.

With four other neighboring superstructures, Quipu alone would group together nearly 45 % of galaxy clusters and about 25 % of the matter in the local volume studied, while it occupies only about 13 % of the corresponding space. This extreme concentration of mass can locally affect measurements of the Hubble constant and leave a measurable imprint on the cosmic microwave background, via the integrated Sachs-Wolfe effect.

The cosmological principle put to the test

The cosmological principle postulates that on very large scales, the Universe is homogeneous and isotropic: matter should be distributed there relatively uniformly, without structure exceeding a size limit of about 1.2 billion light-years. The Hercules-Corona Borealis Great Wall, the Big Ring, and Quipu are among a growing list of structures that appear to exceed this theoretical limit. Some cosmologists believe that these objects are not physically bound structures and will disappear as the Universe expands; others see in them a signal that the standard ΛCDM model should be refined, or even revised. The debate remains open and fuels an active part of research in observational cosmology.

What is a Filament?

Definition of a galactic filament

In cosmology, filaments are among the largest structures in the Universe.
A galactic filament is a thread-shaped structure (on the scale of the Universe), composed of galaxies and/or galaxy clusters, gas, and dark matter.
We know several filaments of several tens of megaparsecs (30 to 300 million light-years), the Coma Berenices filament, the Perseus-Pegasus filament, the Ursa Major filament, the Lynx-Ursa Major filament, the Laniakea filament, the Centaurus filament, the Virgo filament…

The known cosmic filaments

Laniakea, our supercluster of galaxies which integrates our Local Group and therefore the Milky Way, is not specifically associated with a single major cosmic filament. It is delimited by important filamentary structures and is connected to other superclusters by these cosmic filaments.

N.B.:
A megaparsec (Mpc) is equivalent to one million parsecs. A parsec is approximately 3.26 light-years, and a megaparsec is therefore equivalent to about 3,260,000 light-years.

What is a supercluster complex?

Definition of a supercluster complex

A supercluster complex is a cosmic structure even vaster than superclusters of galaxies. Superclusters of galaxies are massive concentrations of gravitationally bound galaxies, and supercluster complexes are even larger assemblages of these large-scale structures.

Laniakea, our supercluster complex

As far as we are concerned, the Pisces-Cetus supercluster complex is a vast cosmic structure that has been identified through large-scale sky observations. It is also known as Laniakea, a Hawaiian term meaning "immense open sky". This supercluster complex is of particular importance because it encompasses our own supercluster of galaxies, the Local Group, in which the Milky Way is located.

Laniakea is gigantic, extending over about 500 million light-years. It groups together several superclusters of galaxies (Virgo Supercluster, Boötes Supercluster, Serpens Supercluster, Hydra-Centaurus Supercluster, etc.).

Key takeaways

A structure shaped by gravitation

The formation of supercluster complexes, cosmic filaments, and Great Walls is intimately linked to the processes of structural growth of the universe.
Initially, at the "beginning" of the Universe, small fluctuations in matter density were amplified over time by gravitation, leading to the formation of these different large-scale structures.

The key role of dark matter

The presence of dark matter plays a crucial role in the formation of these structures, as it acts as a kind of "gravitational glue" that attracts and holds together ordinary matter, such as gas and galaxies.

Discoveries that continue to shake the standard model

The recent discoveries of the Big Ring and Quipu, as well as the upward reassessment of the size of the Hercules-Corona Borealis Great Wall, show that the mapping of the Universe on very large scales remains an active field of research. Future missions, combining optical surveys, X-rays, and gamma-ray bursts, should make it possible to confirm or refute the physical existence of these structures and to clarify the limits of the cosmological principle.