Particle decay in the early Universe: predictions for 21 cm

The influence of ultra-high energy cosmic rays (UHECRs) and decaying dark matter particles on the emission and absorption characteristics of neutral hydrogen in 21 cm at redshifts z = 10-50 is considered. In the presence of UHECRs, 21 cm can be seen in absorption with the brightness temperature T b = -(5-10) mK in the range z = 10-30. Decaying particles can stimulate a 21-cm signal in emission with T b ∼ 50-60 mK at z = 50 and T b ≃ 10 mK at z ∼ 20. Characteristics of the fluctuations of the brightness temperature, in particular its power spectrum, are also calculated. The maps of the power spectrum of the brightness temperature on the plane wavenumber redshift are shown to be sensitive to the parameters of UHECRs and decaying dark matter. Observational possibilities to detect manifestations of UHECRs and/or decaying particles in 21 cm with the future radio telescopes (LOFAR, 21CMA and SKA), and to distinguish contributions from them, are briefly discussed.