Hierarchical structure formation

Hierarchical structure formation is the growth of cosmic structure as small gravitationally bound systems merge and accrete matter to form larger systems, from galaxies to clusters.

Gravitational instability: The growth of density perturbations when gravity amplifies small differences in matter density. It turns tiny early-universe density variations into the seeds of bound structures.
Cosmic microwave background: The relic radiation released when the early universe became transparent, carrying a map of primordial density variations. Its temperature fluctuations constrain the initial seeds of later structure.
Millennium Simulation: A large cosmological N-body simulation that traced dark matter structure formation in a representative universe volume. It visualized halo growth and merger histories across cosmic time.
Top-down structure formation: A model in which large cosmic structures form first and fragment into smaller systems. It contrasts with the small-to-large assembly characteristic of hierarchical growth.
Galaxy merger: The interaction and eventual coalescence of two or more galaxies under gravity. Galaxy mergers are visible outcomes of the assembly of their host halos.
Dark matter halo: A gravitationally bound concentration of dark matter that surrounds and helps assemble galaxies. Halos are the main units that grow, merge, and host galaxies.
Primordial density perturbations: Small spatial variations in density in the early universe that later grow under gravity. Their initial spectrum determines which structures can form first.
Illustris simulation: A cosmological hydrodynamical simulation that models dark matter, gas, stars, and feedback in galaxy formation. It shows how baryonic processes modify galaxies within growing halos.
Hot dark matter: Dark matter that was moving relativistically when primordial structures began to grow. Its free streaming erases small-scale fluctuations and favors larger-first formation.
Galaxy morphology: The structure and shape of a galaxy, including features such as disks, bars, and spheroids. Mergers can transform disks into spheroids and reshape stellar distributions.

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