Signal in the Hyperfine Structure Line of the Ground State of Atomic Hydrogen from the Dark Ages as a Cosmological Test
We analyze the formation of the signal in the 21 cm hydrogen line of the Dark Ages ($30\le z\le300$) in different cosmological models and discuss the possibility of its detection by decameter-wavelength radio telescopes. To study the dependence of the intensity and profile of the line on the values of cosmological parameters and physical conditions in the intergalactic medium, the evolution of the global (averaged over the sky) differential brightness temperature in this line was calculated in standard and non-standard cosmological models with different parameters. The standard $\Lambda$CDM model with cosmological parameters predicts a value of the differential brightness temperature in the center of the absorption line $\delta T_{br}\approx-35$ mK at $z\approx87$. The frequency of the line at the absorption maximum is 16 MHz, the effective width of the line is $\approx$25 MHz. The depth of the line is moderately sensitive to $\Omega_b$ and $H_0$, weakly sensitive to $\Omega_{dm}$ and insensitive to other parameters of the standard $\Lambda$CDM model. However, the line is very sensitive to additional mechanisms of heating or cooling of baryonic matter during the Dark Ages, so it can be an effective test of non-standard cosmological models. In models with decaying and self-annihilating dark matter, as well as with an initial global stochastic magnetic field, the temperature of baryonic matter in this period is higher, the higher the density of these dark matter components and the magnetic field strength. The absorption line becomes shallower, disappears, and turns into emission at values of the component parameters lower than the upper bounds on them, which follow from observational data. The estimations show that such spectral features can be detected by decameter-wavelength radio telescopes in the near future.