  The youngest Galactic supernova remnant (SNR) G1.9+0.3, produced by a
(probable) SN Ia that exploded $\sim 1900$ CE, is strongly asymmetric at radio
wavelengths, much brighter in the north, but bilaterally symmetric in X-rays.
We present the results of X-ray expansion measurements that illuminate the
origin of the radio asymmetry. We confirm the mean expansion rate (2011 to
2015) of 0.58% per year, but large spatial variations are present. Using the
nonparametric "Demons" method, we measure the velocity field throughout the
entire SNR, finding that motions vary by a factor of 5, from 0.09" to 0.44" per
year. The slowest shocks are at the outer boundary of the bright northern radio
rim, with velocities $v_s$ as low as 3,600 km/s (for an assumed distance of 8.5
kpc), much less than $v_s = 12,000 - 13,000$ km/s along the X-ray-bright major
axis. Such strong deceleration of the northern blast wave most likely arises
from the collision of SN ejecta with a much denser than average ambient medium
there. This asymmetric ambient medium naturally explains the radio asymmetry.
In several locations, significant morphological changes and strongly nonradial
motions are apparent. The spatially-integrated X-ray flux continues to increase
with time. Based on Chandra observations spanning 8.3 years, we measure its
increase at 1.3% +/- 0.8% per year. The SN ejecta are likely colliding with the
asymmetric circumstellar medium ejected by the SN progenitor prior to its
explosion.
