  The observation of SN 1997ff at redshift 1.7 has been claimed to refute
alternative models such as grey dust or evolution for the faintness of distant
supernovae, leaving only an accelerating Universe as a viable model. However, a
very simple one parameter evolution model, with the peak luminosity varying as
an exponential function of cosmic time, converts the flux vs. distance law of
the critical density matter-dominated model into that of the concordance
Omega_matter = 0.3 flat vacuum-dominated model with an error no larger than
0.03 mag over the range 0-2 in redshift. A grey dust model that matches this
accuracy can easily be contrived but it still fails by overproducing the far-IR
background or distorting the CMB. Models that involve oscillation between
photons and axions could emulate an exponential function of cosmic time without
violating these background constraints. Clearly a better and well-tested
understanding of the Type Ia supernova explosion mechanism and the origin of
the correlation between the decay rate and luminosity is needed before any
effort to reduce statistical errors in the supernova Hubble diagram to very
small levels.
