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A standardizable candle has a predictable absolute magnitude after correcting measured source properties, rather than identical intrinsic brightness for every object. Type Ia supernova cosmology uses light curve shape and colour for this purpose.

Type Ia supernova cosmology uses standardized supernova luminosities to measure the shape of the luminosity-distance--redshift relation. In a spatially flat universe,An unknown common absolute luminosity is degenerate with the overall distance scale but does not erase the redshift dependence.

The useful observable is a relative luminosity distance at measured redshift, rather than an object's brightness alone. Spectroscopy supplies $z={\lambda }_{\mathrm{obs}}/{\lambda }_{\mathrm{rest}}-1$; repeated photometry supplies a light curve. A Type Ia supernova is a standardizable candle: a fitted relation between light curve shape and luminosity replaces the incorrect assumption that every explosion has identical peak absolute magnitude. Nearby objects establish the relation and its common intercept; distant objects test its continuation and the shape of the Hubble diagram.