  Quintessence models with a dark energy generated by pseudo Nambu-Goldstone
bosons provide a natural framework in which to test the possibility that type
Ia supernovae luminosity distance measurements are at least partially due to an
evolution of the sources, since these models can have parameter values for
which the expansion of the Universe is decelerating as well as values for which
it is accelerating, while being spatially flat in all cases and allowing for a
low density of clumped matter. The results of a recent investigation
(astro-ph/0004138) of current observational bounds which allow for SNe Ia
source evolution are discussed. It is found that models with source evolution
still favour cosmologies with an appreciable amount of acceleration in the
recent past, but that the region of parameter space which is most favoured
shifts significantly.
