  We present Chandra ACIS X-ray observations of the Galactic supernova remnant
Cassiopeia A taken in December 2007. Combining these data with previous
archival Chandra observations taken in 2000, 2002, and 2004, we estimate the
remnant's forward shock velocity at various points around the outermost shell
to range between 4200 and 5200 +/- 500 km/s. Using these results together with
previous analyses of Cas A's X-ray emission, we present a model for the
evolution of Cas A and find that it's expansion is well fit by a rho_ej ~
r^{-(7-9)} ejecta profile running into a circumstellar wind. We further find
that while the position of the reverse shock in this model is consistent with
that measured in the X-rays, in order to match the forward shock velocity and
radius we had to assume that ~ 30% of the explosion energy has gone into
accelerating cosmic rays at the forward shock. The new X-ray images also show
that brightness variations can occur for some forward shock filaments like that
seen for several nonthermal filaments seen projected in the interior of the
remnant. Spectral fits to exterior forward shock filaments and interior
nonthermal filaments show that they exhibit similar spectra. This together with
similar flux variations suggests that interior nonthermal filaments might be
simply forward shock filaments seen in projection and not located at the
reverse shock as has been recently proposed.
