Cosmological parameters

Cosmological parameters are numerical quantities that specify a model of the universe, including its expansion rate, matter and energy densities, and spatial curvature.

Hubble constant: The present-day rate at which the universe’s expansion increases the distance between widely separated galaxies. It sets the current expansion rate in the standard cosmological parameter set.
Friedmann equations: Einstein’s equations specialized to a homogeneous, isotropic universe, governing its expansion over time. They translate density and curvature parameters into an expansion history.
Planck space observatory: A European Space Agency mission that mapped the cosmic microwave background from 2009 to 2013. Its measurements produced a precise parameter estimate within the Lambda-CDM model.
Parameter degeneracy: A situation in which different parameter combinations produce similar observable predictions. It limits how uniquely observations can determine individual cosmological parameters.
Density parameter: A dimensionless ratio of a component’s density to the critical density required for a spatially flat universe. It expresses matter, radiation, and dark energy densities in comparable units.
Lambda-CDM model: The standard cosmological model, combining a cosmological constant with cold dark matter and ordinary matter. It gives cosmological parameters a specific, widely tested model context.
Type Ia supernova: A thermonuclear stellar explosion whose calibrated brightness makes it a standardizable distance indicator. Supernova distances constrain the universe’s expansion history and dark-energy parameters.
Bayesian inference: A statistical framework that updates probability distributions using data and prior assumptions. It makes explicit how data and parameter priors shape cosmological estimates.
Spatial curvature: A measure of whether space is geometrically flat, positively curved, or negatively curved. It is a geometric parameter constrained alongside the cosmic energy densities.
Cosmic microwave background: The relic radiation released when the early universe became transparent, now observed as microwave light. Its temperature and polarization patterns encode several cosmological parameters.

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