= Observational cosmology
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<Observational cosmology> tests models of the Universe through <observations> such as <galaxy> <redshifts>, distance indicators, element abundances and the <cosmic microwave background>. Each observable constrains a combination of cosmological parameters through an explicit model.
For example, a <galaxy> <redshift> measures a <wavelength> ratio directly; converting it to a distance requires an expansion model and assumptions about peculiar motion. Combining several kinds of <observation> can break degeneracies that one dataset cannot resolve. Instrument response, selection effects and foreground signals therefore belong in the inference alongside the cosmological equations.
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