We derive similarity solutions for the expansion of negative initial density perturbations \delta M/M \propto r^-s (s>0) in an Einstein-de Sitter universe filled with collisional baryonic gas and collisionless matter. The solutions are obtained for planar, cylindrical, and spherical perturbations. For steep perturbations, central cavities surrounded by over-dense shells expanding as t^{ 2(s+1)/3s} develop in both matter components. We also consider the case when baryonic shells are driven by internal pressure. Before redshift \sim 8-10 baryonic shells cool by inverse Compton scattering with the cosmic background radiation and can form bound objects of mass \sim 10^11\Omega_b^{ -1/2} M_{ \odot} (1+z)^{ -3/2}. Hot shocks of radius R cause a Compton y-distortion of order y=10^{ -5}\Omega_b (R/10 Mpc)^2 f_F, where f_F is the volume filling factor of these shells.
Categories: astro-ph