On the Time Variation of Dust Extinction and Gas Absorption for Type~Ia
  Supernovae Observed Through Non-uniform Interstellar Medium
  For Type Ia supernovae (SNe Ia) observed through a non-uniform interstellar
medium (ISM) in its host galaxy, we investigate whether the non-uniformity can
cause observable time variations in dust extinction and in gas absorption due
to the expansion of the SN photosphere with time. We show that, owing to the
steep spectral index of the ISM density power spectrum, sizable density
fluctuation amplitudes at the length scale of typical ISM structures ($\gtrsim
\text{ 10 pc}$) will translate to much smaller fluctuations on the scales of a
SN photosphere. Therefore the typical amplitude of time variation due to
non-uniform ISM, of absorption equivalent widths and of extinction, would be
small. As a result, we conclude that non-uniform ISM density should not impact
cosmology measurements based on SNe Ia. We apply our predictions based on the
ISM density power law power spectrum to the observations of two highly reddened
SNe Ia, SN 2012cu and SN 2014J.
