Sensitivity of the redshifted 21 cm signal from the Dark Ages to parameters of primordial magnetic fields

¹ Astronomical Observatory of Ivan Franko National University of Lviv, Kyryla i Methodia str. 8 Lviv 79005, Ukraine
² Bogolyubov Institute for Theoretical Physics of the NAS of Ukraine Metrolohichna Str. 14-b Kyiv 03143, Ukraine
³ Dipartimento di Fisica e Astronomia, Universitá di Bologna Via Gobetti 93/2 40122 Bologna, Italy

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Received: 17 June 2025
Accepted: 17 November 2025

Abstract

We analyse the impact of the decaying magnetic turbulence of primordial magnetic fields (PMFs) and ambipolar diffusion on the ionisation and thermal history of the Dark Ages Universe (30 ≤ z ≤ 300), and its imprint on the spectral profile of the global signal in the redshifted 21 cm hydrogen line. The heating function caused by decaying magnetic turbulence monotonically decreases after cosmological recombination; its amplitude strongly depends on the strength of the PMFs’ B₀ and weakly depends on the spectral index of the initial power spectrum of the PMFs’ n_B. The heating function caused by ambipolar diffusion, in contrast, noticeably depends on the spectral index in the range −3 ≲ n_B ≲ 4 but is subdominant in the Dark Ages epoch for PMF models with B₀ ≲ 0.5 nG. We computed the ionisation and thermal history of intergalactic gas from the cosmological recombination up to the end of the Dark Ages epoch for a range of PMF parameters, 0.05 ≲ B₀ ≲ 0.5 nG, −2.9 ≲ n_B ≲ 4, and show the essentially distinguished thermal evolution from one in the Lambda cold dark matter (ΛCDM) model. We also show that the profile of the redshifted 21 cm hydrogen line is very sensitive to the PMF parameters from this range and can be used for their constraints in conjunction with other observational data.