  If a sizeable fraction of the energy of supernova remnant shocks is channeled
into energetic particles (commonly identified with Galactic cosmic rays), then
the morphological evolution of the remnants must be distinctly modified.
Evidence of such modifications has been recently obtained with the Chandra and
XMM-Newton X-ray satellites. To investigate these effects, we coupled a
semi-analytical kinetic model of shock acceleration with a 3D hydrodynamic code
(by means of an effective adiabatic index). This enables us to study the
time-dependent compression of the region between the forward and reverse shocks
due to the back reaction of accelerated particles, concomitantly with the
development of the Rayleigh-Taylor hydrodynamic instability at the contact
discontinuity. Density profiles depend critically on the injection level eta of
particles: for eta up to about 10^-4 modifications are weak and progressive,
for eta of the order of 10^-3 modifications are strong and immediate.
Nevertheless, the extension of the Rayleigh-Taylor unstable region does not
depend on the injection rate. A first comparison of our simulations with
observations of Tycho's remnant strengthens the case for efficient acceleration
of protons at the forward shock.
