Nonlinear Genereation of Sound Waves in the Pre-Galactic Era and Absorption in the 21-cm Line

The structure of sound waves in baryonic gas generated during the evolution of spherically symmetric dark matter halos with a mass less than the Jeans mass has been calculated. In this case, the source of the gravitational field that creates the wave can be at both the linear stage (evolving perturbation in dark matter) and the nonlinear stage (separated and virialized object). Peculiar motions of baryons in a sound wave in the second order in speed cause the absorption of relict radiation in the 21 cm line. It is shown that this additional absorption on sound waves is a fraction of a percent at redshifts \(z\sim 15{-}20\) of the absorption value in the model of a homogeneous Universe, however, the additional absorption can be larger in the case of a non-standard spectrum of cosmological perturbations of matter density on small scales.