Science › Big Bang & cosmos
Galaxies and the cosmic web
1924· Discovery / experiment
ProvenTested many times; part of working physics.
The universe holds hundreds of billions of galaxies, arranged in a vast web of filaments and walls surrounding huge empty voids.
See it on the Science map →How it works
A galaxy is a gravity-bound city of billions of stars; ours is the Milky Way. Galaxies are not scattered evenly. They gather in groups and clusters, strung along filaments hundreds of millions of light-years long, with voids in between where almost nothing lives. Gravity built this pattern over billions of years, pulling matter from slightly thinner regions into slightly denser ones.
Why it matters
It is the large-scale floor plan of the universe, and its exact pattern is a precise test of dark matter, inflation and the Big Bang.
How we know
Hubble proved other galaxies exist (1924-25). Redshift surveys, starting with a 1986 Harvard map and expanded by the Sloan Digital Sky Survey, revealed the web. Computer simulations with dark matter reproduce it.
Still a mystery
Small details, such as the number of tiny satellite galaxies and how emptied-out voids are, do not always match simulations exactly.
Connections (4)
▸Edwin Hubble showed other galaxies exist, revealing Galaxies and the cosmic webProven
The relationship
Hubble found stars in the Andromeda nebula whose pulses reveal their true brightness, and so their distance. Andromeda was far outside the Milky Way: a separate galaxy. Suddenly the countless faint 'nebulae' were galaxies too. That opened the question of how galaxies are arranged, which later surveys answered with the cosmic web.
How the link was found
Edwin Hubble, using the 100-inch Hooker telescope at Mount Wilson; first Cepheid found in 1923, announced in January 1925. It settled the 1920 'Great Debate' between Harlow Shapley and Heber Curtis.
Why it's proven
His distance to Andromeda, about 900,000 light-years, was too short (today: 2.5 million), but far enough to prove the point.
Open questions and mysteries
The web itself was not mapped until the 1980s. Why its largest structures are as big as they are is still studied.
▸Cosmic inflation seeded Galaxies and the cosmic webWell supported
The relationship
During inflation, tiny quantum jitters in the driving field were stretched to astronomical size. They left regions very slightly denser than others, by about 1 part in 100,000. Over billions of years, gravity pulled matter from thinner regions into denser ones, growing those ripples into galaxies, clusters, filaments and voids.
How the link was found
Viatcheslav Mukhanov and Gennady Chibisov in 1981; Hawking, Starobinsky, Guth and others at a 1982 Cambridge workshop.
Is it proven?
The ripple pattern measured in the microwave background, fed into computer simulations, reproduces the cosmic web seen in galaxy surveys. That the ripples came from inflation is well supported but unconfirmed.
Open questions and mysteries
Whether the seeds were exactly as inflation predicts, or show small departures hinting at different physics, is being tested now.
▸Dark matter is the hidden scaffolding of Galaxies and the cosmic webWell supported
The relationship
Dark matter does not feel light's pressure, so in the early universe it began clumping first, while ordinary gas was still being pushed around by radiation. Its clumps became gravitational wells. Ordinary gas later fell in, forming galaxies at the densest points. The filaments of the cosmic web trace the underlying dark matter skeleton.
How the link was found
Developed in the 1980s; the 1985 'cold dark matter' simulations of Davis, Efstathiou, Frenk and White showed web-like structure. The 2005 Millennium Simulation made it vivid.
Is it proven?
Simulations with dark matter match galaxy surveys; without it, galaxies would not have formed in time. Gravitational-lensing maps show dark matter along filaments.
Open questions and mysteries
On small scales, simulations predict more tiny satellite galaxies and denser galaxy centers than are seen. Is this dark matter's nature or messy gas physics?
▸The first galaxies (seen by JWST) grow into Galaxies and the cosmic webProven
The relationship
Early galaxies sat at the densest knots of the dark-matter web. Over billions of years they kept pulling in gas along filaments and merging with neighbors, growing into today's large galaxies and clusters. The Milky Way assembled this way: it has absorbed many smaller galaxies, including a sizable one roughly 10 billion years ago.
How the link was found
Growth by mergers comes from White and Rees (1978); feeding by cold gas streams along filaments from Avishai Dekel and colleagues in the 2000s. The Milky Way's big merger was found in 2018 Gaia data by Amina Helmi's and Vasily Belokurov's teams.
Why it's proven
Deep images show galaxies growing in size and mass over cosmic time. Gaia's map of star motions reveals the debris of past mergers in our own galaxy.
Open questions and mysteries
JWST finds galaxies that had already stopped forming stars within the first billion years or so. How they shut down so early is debated.