A map on the largest scale
On the scale of a galaxy, matter looks clumpy and local. Zoom out by a factor of a thousand and a pattern appears: galaxies line up along filaments and sheets, which meet in dense knots and wrap around huge empty regions. Astronomers call this pattern the cosmic web.
Its parts have names:
- Nodes: galaxy clusters, the densest places, where filaments intersect.
- Filaments: long bridges of galaxies, gas and dark matter connecting the nodes.
- Walls (sheets): flattened structures between filaments.
- Voids: nearly empty regions that take up most of the volume of the universe.
How it formed
The early universe was almost perfectly smooth, with density ripples of about one part in a hundred thousand, the same ripples seen in the cosmic microwave background. Gravity amplified them. Slightly denser regions pulled in matter, emptier regions emptied further.
In 1970 Yakov Zeldovich showed that collapse under gravity happens first along one axis, producing flattened "pancakes", then filaments, then knots. That sequence, sheets to filaments to clusters, is still how the web is described. Dark matter does most of the pulling because there is about five times more of it than ordinary matter; gas and galaxies settle into the dark-matter scaffolding.
Large computer simulations such as the Millennium Simulation (Springel et al., Nature, 2005) reproduce the web from these initial ripples very closely. The match between simulated and observed structure is one of the strongest supports for the standard cosmological model.
How it is mapped
The first clear sign came from redshift surveys in the 1980s. In 1989 Margaret Geller and John Huchra found the "Great Wall", a sheet of galaxies hundreds of millions of light-years long. Later surveys, above all the Sloan Digital Sky Survey, mapped millions of galaxies and turned the web into a detailed three-dimensional atlas.
In 2014 Brent Tully and colleagues used galaxy motions to define our home supercluster, Laniakea, about 500 million light-years across, with the Milky Way on its outskirts (Nature, 2014).
Seeing the gas in filaments directly is harder because it is thin. Since 2019 deep observations have imaged glowing gas along individual filaments, and in 2020 fast radio bursts, millisecond flashes whose signal is slowed by free electrons along the way, were used to count the diffuse ordinary matter between galaxies (Macquart et al., Nature, 2020). The result filled much of the long-standing gap in the census of ordinary matter.
The brain comparison
Pictures of the cosmic web and of neural tissue look alike, and in 2020 Franco Vazza and Alberto Feletti checked whether the resemblance survives measurement. For several statistics, such as how density fluctuations distribute across scales and how many connections each node has, the numbers came out close, despite a difference in size of about 27 orders of magnitude.
The careful reading: two very different processes arrive at comparable network statistics, probably because both solve a similar problem, connecting many nodes with little material. The question of what that shared problem is, and whether the universe processes information like a network, is taken up on the page Is the universe a neural network?.
Open questions
- How galaxies inherit their spin and shape from the filaments they sit in.
- How much of ordinary matter sits in the hottest, thinnest gas of filaments.
- Whether voids are as empty and as numerous as the standard model predicts; void statistics are becoming a test of dark energy and gravity.
Bottom line
The cosmic web is the skeleton of the universe: filaments, walls and clusters around vast voids, grown by gravity from tiny early ripples and dominated by dark matter. It is mapped in three dimensions by galaxy surveys and, increasingly, seen directly in the gas of its filaments.