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We analyze the formation of the redshifted hyperfine structure line 21-cm of hydrogen atoms in Dark Ages at 30 ≤ z ≤ 300 in different cosmologies. To study its dependence on the values of cosmological parameters and physical conditions in the intergalactic medium, the evolution of the global (sky-averaged) differential brightness temperature in this line was computed in standard and non-standard cosmological models with different parameters. The standard ΛCDM model with post-Planck parameters predicts the following value of the differential brightness temperature in the center of the absorption line: δTbr ≈ 35 mK at z ≈ 87. The frequency of the line in the absorption maximum is 16 MHz, the effective half-width of the line is 17 MHz. The depth of the line is moderately sensitive to Ωb and H0, weakly sensitive to Ωdm, and insensitive to other parameters of the standard ΛCDM model. But the line is very sensitive to additional mechanisms of heating or cooling of baryonic matter during the Dark Ages, so it can be a good test of non-standard cosmological models. In the models featuring decaying and self-annihilating dark matter, along with a primordial stochastic magnetic field, the temperature of baryonic matter in this period increases with higher densities of these dark matter components and stronger magnetic field strengths. The absorption line gradually becomes shallower, and eventually disappears, transitioning into emission when the values of the component parameters are lower than their upper limits, as determined by current observational data. Estimates show that such spectral features may be detected by radio telescopes in the decameter wavelength range in the near future. [ABSTRACT FROM AUTHOR]

We analyze the possibilities of detecting a signal in the hydrogen 21 cm line, which was formed in the early universe during the Dark Ages, using the Ukrainian radio telescopes UTR-2 and GURT of the National Academy of Sciences of Ukraine. As a result of cosmological expansion, this line is shifted to the decameter range of wavelengths (λobs ≈ 18 m, νobs ≈ 16 MHz) and is in the band of operating frequencies of these telescopes. The brightness temperature of the predicted sky-averaged global signal ranges from ∼ −0.08 to ∼ 0.02 K, depending on the cosmological model. Such a weak signal is a big challenge even for the world's largest radio telescope in the decameter wavelength range UTR-2, since the signal level of the synchrotron radiation of the Galaxy at these frequencies is 20 000-40 000 K. The paper highlights the peculiarities of spectroscopy at decameter waves, interfering factors of natural and instrumental origin and ways to eliminate them in order to reliably detect the signal in the 21 cm line, which can become an important source of information both about the environment in which the first stars and galaxies were born, and about the nature of dark matter particles and the magnitude of primordial magnetic fields. It was concluded that the detection of such a signal using the most sensitive radio telescopes of the decameter wavelength range is quite possible (with a frequency accumulation of 25 MHz, the detection time will be ∼ 50 days) and can be implemented in the coming years of peace in Ukraine. [ABSTRACT FROM AUTHOR]