  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.
