  We present new Chandra ACIS-S3 observations of Cassiopeia A which, when
combined with earlier ACIS-S3 observations, show evidence for a steady ~
1.5-2%/yr decline in the 4.2-6.0 keV X-ray emission between the years 2000 and
2010. The computed flux from exposure corrected images over the entire remnant
showed a 17% decline over the entire remnant and a slightly larger (21%)
decline from regions along the remnant's western limb. Spectral fits of the
4.2-6.0 keV emission across the entire remnant, forward shock filaments, and
interior filaments indicate the remnant's nonthermal spectral powerlaw index
has steepened by about 10%, with interior filaments having steeper powerlaw
indices. Since TeV electrons, which give rise to the observed X-ray synchrotron
emission, are associated with the exponential cutoff portion of the electron
distribution function, we have related our results to a change in the cutoff
energy and conclude that the observed decline and steepening of the nonthermal
X-ray emission is consistent with a deceleration of the remnant's ~5000 km/s
forward shock of ~10--40 km/s/yr
