Absorption in the 21 cm Hydrogen Line at $$\boldsymbol{z>10}$$ as a Sensitive Tool for the Construction of a Cosmological Model on Small Scales

The cosmic microwave background absorption intensity in the 21 cm line of neutral hydrogen in the presence of additional power in the form of a ‘‘bump’’ in the spectrum of cosmological density perturbations is calculated. The main absorption-amplifying effect is the earlier birth of the first stars forming an ultraviolet radiation background. This radiation reduces the spin temperature of neutral hydrogen and, thus, amplifies the absorption in the 21 cm line. A comparison of various cosmological models (with and without a bump in the density perturbation spectrum) shows that it is possible to determine the probable position of the bump in the perturbation spectrum and, thus, to reconstruct the spectrum of cosmological perturbations on scales $$k>1$$ Mpc $${}^{-1}$$ from the position of the absorption frequency profile.