  Propagation of the strong shock in the flat expanding Friedman universe is
investigated using methods of dimension and similarity. Exact analytic solution
of self-similar equations is obtained, determining dependences of the radius
and velocity of the shock wave on time and radius. It is obtained, that in the
expanding medium the velocity of shock decreases as $\sim t^{-1/5}$, what is
slower than the shock velocity in the static uniform medium $\sim t^{-3/5}$.
The radius of the shock wave in the expanding self-gravitating medium increases
$\sim t^{4/5}$, more rapidly than the shock wave radius in the uniform
non-gravitating medium $\sim t^{2/5}$. So, the shock propagates in the
direction of decreasing density with larger speed, that in the static medium,
due to accelerating action of the decreasing density, even in the presence of a
self-gravitation.
