  Type Ia supernovae, with their remarkably homogeneous light curves and
spectra, have been used as standardizable candles to measure the accelerating
expansion of the Universe. Yet, their progenitors remain elusive. Common
explanations invoke a degenerate star (white dwarf) which explodes upon
reaching close to the Chandrasekhar limit, by either steadily accreting mass
from a companion star or violently merging with another degenerate star. We
show that circumstellar interaction in young Galactic supernova remnants can be
used to distinguish between these single and double degenerate progenitor
scenarios. Here we propose a new diagnostic, the Surface Brightness Index,
which can be computed from theory and compared with Chandra and VLA
observations. We use this method to demonstrate that a double degenerate
progenitor can explain the decades-long flux rise and size increase of the
youngest known Galactic SNR G1.9+0.3. We disfavor a single degenerate scenario.
We attribute the observed properties to the interaction between a steep ejecta
profile and a constant density environment. We suggest using the upgraded VLA
to detect circumstellar interaction in the remnants of historical Type Ia
supernovae in the Local Group of galaxies. This may settle the long-standing
debate over their progenitors.
