= Supernova calibration and population tests
The <High-Z Supernova Search Team> analysis fitted multicolour <light curves> and <interstellar extinction>, and compared a second decline-rate method. Similar local and distant <spectroscopy> supported, but did not establish, population stability. Nearly colourless attenuation remained a qualified alternative.<Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant by Adam Riess and others>[Sections 3.2, 5.1–5.2: multicolour fits, evolution, grey dust]{ref}
The <Supernova Cosmology Project> compared low- and high-<redshift> colours, removed red outliers in its primary fit, and tested <Malmquist bias> with artificial-source recovery.<Measurements of Omega and Lambda from 42 High-Redshift Supernovae by Saul Perlmutter and others>[Sections 4.1–4.2: colours, subsamples, artificial-source selection tests]{ref}
The inference can be organized as a chain of independently testable failure modes. A <magnitude zero point> offset between nearby and distant observations imitates a <distance modulus> offset; a common offset instead changes the intercept. An erroneous <photometric passband> response changes a <K correction> in a <redshift>-dependent way because different emitted <wavelengths> cross the detector band. Residual host-<galaxy> light changes the recovered <radiative flux>. <Color excess> constrains wavelength-dependent <interstellar extinction> only after intrinsic colour is modeled; grey attenuation is not bounded by colour alone. Changes in <stellar population> or <metallicity> could shift standardized <absolute magnitude> without large changes in the chosen shape parameter. These are logical degeneracies, so increasing the sample size alone cannot remove them.
A useful control strategy therefore compares subsets with different host environments, fits consistent rest-frame <photometric passbands>, and checks artificial-source recovery through the same image processing and selection. A <Malmquist bias> tends to select brighter detectable sources, but the relative bias between the nearby and distant samples is what matters. <Peculiar velocity> mainly perturbs the nearby <redshift> anchor; <weak gravitational lensing> changes individual brightnesses and the selection distribution. Neither can be replaced by an arbitrary extra scatter term when its mean or selection dependence is uncertain.
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