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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