Structure formation
Structure formation is the growth of initially small density variations into galaxies, galaxy clusters, and the cosmic web, driven chiefly by gravity.
Cosmological perturbation theory: A framework for calculating how small deviations from a uniform universe evolve under gravity and cosmic expansion. It describes the early growth of density fluctuations before structures become nonlinear.
Cosmic microwave background: Relic radiation from the early universe, released when photons decoupled from matter. Its temperature anisotropies reveal the small primordial density variations that seed later structure.
Large-scale structure of the universe: The distribution of galaxies and matter across vast cosmic distances, including filaments, clusters, and voids. It is the broadest observable outcome of structure formation.
Galaxy formation: The assembly of galaxies from gas, stars, and dark matter within gravitationally bound systems. It is the luminous, complex outcome of matter collapsing inside dark-matter halos.
Hot dark matter: Dark matter whose particles moved relativistically when cosmic structures began to form. Its free streaming erases small-scale seeds, unlike cold dark matter’s bottom-up growth.
Jeans instability: The gravitational growth of density perturbations when self-gravity overcomes pressure support. It identifies when gas fluctuations can collapse rather than disperse.
Primordial density fluctuation: A small early-universe variation in matter density relative to the cosmic average. These are the initial seeds amplified during structure formation.
Galaxy cluster: A gravitationally bound system containing many galaxies, hot gas, and dark matter. Clusters are among the largest bound objects produced by cosmic growth.
Baryon acoustic oscillations: A preferred comoving scale in the clustering of matter, inherited from sound waves in the early universe. Their imprint in galaxy clustering traces cosmic expansion and the growth of structure.
Top-down formation: A structure-formation scenario in which large systems form first and fragment into smaller ones. It contrasts with the small-halo-first hierarchy favored by cold dark matter.