Universe
Cosmology studies the largest possible physical system: the Universe as a whole, its expansion, history, contents and large-scale organization.
ExpansionObservations show systematic cosmological redshifts and an evolving large-scale Universe.
Early historyThe cosmic microwave background and light-element abundances preserve information about earlier epochs.
Structure formationGalaxies and clusters grew from earlier variations in matter density.
Observable UniverseWe see only regions whose signals have had time to reach us.
Open problemsDark matter, dark energy, the earliest physical regime and quantum gravity remain major research questions.