Cosmology is the study of the large-scale structure of the universe. It is concerned with fundamental questions about the formation, evolution, and future development of the universe. Specific cosmological questions of interest include the isotropy of the universe, the density of the universe, the equality of matter and anti-matter, and the origin of density fluctuations in galaxies. The most hotly sought after cosmological parameters of the present-day universe are the expansion rate, most commonly parameterized by the Hubble constant, the critical density (the density required for the universe to stop expansion and eventually collapse back onto itself), the related quantity, the deceleration parameter, the cosmological constant, the curvature of the universe and the expansion parameter. Many of these quantities are related under the assumptions of a given cosmological model.

Different cosmological models give different evolutions and therefore different ages for the universe based on the current rate of expansion. Let k be the curvature of the universe, Pcrit the critical density, and t be the age of the universe. Then the following table summarizes the most common cosmological models.

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