Galaxy Formation
Galaxy formation is the process by which galaxies formed and evolved from tiny density fluctuations present in the early universe — driven primarily by dark matter's gravitational collapse into filamentary structures called "haloes," which then attracted ordinary matter that cooled and condensed into the stars and gas making up the visible galaxies observed today.
Curious
The cosmic microwave background (see the Codex's cosmic microwave background entry) reveals that the early universe was almost, but not perfectly, uniform — tiny density fluctuations of roughly 1 part in 100,000 were already present at the time of recombination. In regions that were very slightly denser than average, gravity's attractive pull was correspondingly very slightly stronger, gradually drawing in more matter over cosmic time — a slow, self-reinforcing process that eventually amplified these initially tiny differences into the dramatic structure of galaxies and galaxy clusters observed billions of years later.
Dark matter played the leading role in this process: because it doesn't interact electromagnetically (and so wasn't affected by the pressure of the hot, radiation-dominated early universe the way ordinary matter was), dark matter could begin collapsing gravitationally earlier and more efficiently, forming the initial gravitational "scaffolding" (dark matter haloes) into which ordinary matter later fell, cooled, and condensed to form the visible galaxies we observe.
Exploring
| Stage | Approximate cosmic time |
|---|---|
| Recombination / density fluctuations set | ~380,000 years |
| First stars and protogalaxies | ~100–400 million years |
| Major galaxy assembly | ~1–5 billion years |
| Milky Way's current structure | ~13.6 billion years (ongoing) |
Deep Dive
Simon White and Martin Rees's influential 1978 paper established the basic "two-stage" theoretical framework still underlying modern galaxy formation theory: dark matter haloes collapse first under gravity, and ordinary (baryonic) gas subsequently falls into these haloes, where it can cool, condense, and form stars — a framework later substantially refined and made quantitatively precise through large-scale cosmological computer simulations that model the gravitational and gas dynamics involved across cosmic time.
Cosmological simulations and the cosmic web: Modern galaxy formation research relies heavily on large-scale computer simulations (such as the Illustris, EAGLE, and IllustrisTNG projects) that model dark matter gravitational collapse alongside gas dynamics, star formation, and feedback processes (such as supernova explosions and supermassive black hole activity, which can heat or expel gas and regulate further star formation) across cosmic time. These simulations successfully reproduce the observed large-scale "cosmic web" structure — a vast, filamentary network of galaxies and galaxy clusters connected by thinner filaments of gas and dark matter, with large, relatively empty voids in between — providing strong, independent support for the underlying dark-matter-driven hierarchical formation picture.
Significant open questions remain in galaxy formation theory, including precisely how supermassive black holes and their host galaxies co-evolve (see the Codex's supermassive black holes entry), how galaxies regulate their own star formation over billions of years (avoiding either fully depleting their gas too quickly or fully suppressing star formation), and how to fully reconcile early, surprisingly massive high-redshift galaxy observations with standard hierarchical formation timescales — active areas of ongoing observational and theoretical research.
Concept Relationship Map
Requires: dark matter, cosmic microwave background
Enables: Milky Way [PENDING FULL ENTRY], large-scale structure [PENDING FULL ENTRY]
Commonly confused with: assuming large galaxies formed whole from the start, rather than hierarchically through mergers of smaller structures over billions of years
Sources
- White, S.D.M., Rees, M.J. (1978). Monthly Notices of the Royal Astronomical Society, 183(3), 341–358.
- Vogelsberger, M., et al. (2014). "Introducing the Illustris Project." Monthly Notices of the Royal Astronomical Society, 444(2), 1518–1547.