The cosmic web

A galaxy is a whole island of stars, and there are more galaxies than anyone could count. You might expect them to be sprinkled evenly through space, like dust in a sunbeam. They are not.

Galaxies keep company. Ours, the Milky Way, lives in a small gathering of more than 50 called the Local Group. Bigger gatherings, called clusters, hold hundreds or thousands of galaxies each.

Step back further and a pattern appears. The clusters are joined by long threads of galaxies, and between the threads are enormous empty spaces called voids. The whole thing looks like a sponge, or the walls of soap bubbles. Astronomers call it the cosmic web.

Gravity made it. Wherever there was slightly more matter to begin with, more was pulled in, and the places that started slightly emptier were drained. Over billions of years the difference grew into knots, threads and holes.

In the 3D view each point of the web is a real galaxy that has been measured, more than 43,000 of them. Nothing is filled in. Where you see a gap, the survey found nothing bright enough to record.

The cosmic web is the largest pattern there is. This level goes up through its parts in order of size, then looks at how a map of it is made and how far that map can be trusted.

The parts.

Structure	What it is	Example
Group	100 galaxies or fewer, held together by gravity	The Local Group: the Milky Way, Andromeda and more than 50 others
Cluster	Hundreds to thousands of galaxies, held together by gravity	The Coma Cluster: over 1,000 galaxies, more than 20 million light years across
Supercluster	A large collection of clusters and groups, mostly not bound together	Laniakea, which the Local Group sits on the edge of
Wall	A sheet of superclusters, hundreds of millions of light years long and about 20 million deep	The Coma Wall, about 500 million light years long
Void	The nearly empty space the rest is wrapped round

A cluster is more than its galaxies. It also holds a great deal of gas at tens of millions of degrees, and a large amount of dark matter.

How the web formed. Gravity pulls matter towards wherever there is already more of it. Over time that draws larger and larger things together, leaving voids between, with clusters and walls where the concentrations cross. The oldest light in the universe shows how even things were to start with: the warm and cool patches differ by about one part in 100,000. Everything in the web grew from those faint differences.

How the map is made. A galaxy’s direction in the sky is easy to measure. Its distance is not. The map in the 3D view uses the expansion of the universe as its ruler. Space is stretching, so distant galaxies move away from us, and the further away a galaxy is, the faster it goes:

speed = Hubble constant × distance

The speed can be read from the galaxy’s light, which is shifted towards red by an amount that depends on it. The 3D view takes the Hubble constant as 70 km a second for every megaparsec, so a galaxy moving away at 7,000 km/s is placed 100 megaparsecs away, about 330 million light years. The survey drawn here measured that shift for more than 43,000 galaxies over 91% of the sky. The missing strip, called the Zone of Avoidance, lies behind the disc of our own galaxy, which is why the web has an empty wedge in it. The flat chart in the atlas reaches out to 310 megaparsecs, about a billion light years.

The clusters the view names. These distances are worked out that way from each cluster’s speed.

Cluster	Distance in million light years
Virgo	50
Fornax	65
Centaurus	160
Hydra	180
Norma	230
Perseus	250
Coma	320
Shapley Supercluster	680

How far to trust it. Three cautions.

• The ruler is only roughly known. Different ways of measuring the Hubble constant give 67 and 73, so every distance in the table could be several per cent out.
• A galaxy’s speed is not all expansion. Galaxies also fall towards each other, and for nearby ones that matters. Virgo comes out at about 50 million light years from its speed, but a direct measurement gives 16.5 megaparsecs, about 54 million.
• The survey only records galaxies above a set brightness. Far away it catches only the brightest, so the web looks thinner with distance. That thinning is the survey running out, not the universe.

Where we are in it. The Milky Way belongs to the Local Group, which lies on the outer edge of Laniakea, a gathering of about 100,000 galaxies some 520 million light years across. Laniakea was mapped in 2014 by working out which way galaxies are being pulled.