Constraints on Photometric Calibration from Observations of High-Redshift Type Ia Supernovae

The good match of the type Ia supernova (SNIa) Hubble Diagram to the prediction of a not-unreasonable cosmological world model shows that measurements of standard stars and their comparison with point sources down to m = 25 mag is good to better than ±0.5 mag over an 11 mag range. It also shows that the true spectral energy distribution (SED) shapes of standard stars are known to better than ±0.5 mag over an octave in wavelength. On the other hand, the SNIa argument for an accelerating Universe assumes that the magnitude system is good to much better than ∼ 0.1 mag over the 11 mag range, and that SED shapes are known to much better than ∼ 10 percent over an octave in wavelength. There is no independent empirical evidence for these plausible assumptions. Subject headings: cosmology: observations — distance scale — standards — stars: fundamental parameters — supernovae: general — techniques: photometric 1. The type Ia supernovae results The type Ia supernova (SNIa) Hubble Diagrams established by the high-redshift SNIa search and photometry teams (Perlmutter et al 1997; Garnavich et al 1998; Schmidt et al 1998; Perlmutter et al 1999) stand as a great astronomical achievement of this decade. These studies provide a tremendous confirmation of the expanding Universe and big-bang cosmology. Along with massive searches for microlensing events (eg, Alcock et al 1998; Beaulieu et al 1995; Udalski et al 1994), they show that large, coordinated Hubble Fellow [email protected]