Physical cosmology

Physical cosmology uses physical laws and observations to study the universe’s origin, large-scale structure, evolution, and possible fate.

General relativity: A theory of gravity in which matter and energy shape spacetime, and spacetime guides their motion. Its equations describe how the universe’s expansion responds to matter and energy.
Big Bang: The hot, dense early phase from which the observable universe expanded and evolved. It names the universe’s early expansion, not an explosion into pre-existing space.
Hubble's law: The empirical relation between a galaxy’s distance and its recession velocity at low redshift. It established the observed relation between cosmic expansion and galaxy distances.
Steady-state theory: A cosmological model proposing that the universe expands while maintaining constant average density through continuous matter creation. It competed with the Big Bang model before observations favored an evolving universe.
Cosmic censorship hypothesis: A conjecture that singularities produced by gravitational collapse are hidden behind event horizons. Its unresolved status matters when extending physical laws to extreme cosmic conditions.
Cosmological principle: The assumption that, on sufficiently large scales, the universe is homogeneous and isotropic. It lets cosmologists model the universe with a small set of averaged quantities.
Cosmic inflation: A proposed brief period of extremely rapid expansion in the very early universe. Inflation offers an account of the universe’s large-scale uniformity and initial density fluctuations.
Type Ia supernova: A thermonuclear stellar explosion whose characteristic brightness makes it useful as a distance indicator. Their measured distances revealed that cosmic expansion is accelerating.
Lambda-CDM model: The standard cosmological model, combining a cosmological constant with cold dark matter and ordinary matter. It is the leading framework against which cosmological observations are interpreted.
Initial conditions of the universe: The physical state and boundary conditions from which the universe’s subsequent evolution developed. Cosmological models describe evolution more readily than why the universe began in its particular state.

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