  We submit that non thermalized support for the outer intracluster medium in
relaxed galaxy clusters is provided by turbulence, driven by inflows of
intergalactic gas across the virial accretion shocks. We expect this component
to increase briskly during the cluster development for z<1/2, due to three
factors. First, the accretion rates of gas and dark matter subside, when they
feed on the outer wings of the initial perturbations in the accelerating
Universe. Second, the infall speeds decrease across the progressively shallower
gravitational potential at the shock position. Third, the shocks eventually
weaken, and leave less thermal energy to feed the intracluster entropy, but
relatively more bulk energy to drive turbulence into the outskirts. The overall
outcome from these factors is physically modeled and analytically computed;
thus we ascertain how these concur in setting the equilibrium of the outer
intracluster medium, and predict how the observables in X rays and microwaves
are affected, so as to probe the development of outer turbulence over wide
cluster samples. By the same token, we quantify the resulting negative bias to
be expected in the total mass evaluated from X-ray measurements.
