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

TL;DR

This paper investigates how primordial magnetic fields influence the thermal and ionisation history of the early universe and affect the observable 21 cm hydrogen line signal, providing a potential method to constrain such fields.

Contribution

It presents a detailed analysis of the effects of primordial magnetic fields on the Dark Ages' thermal history and the 21 cm signal, highlighting the sensitivity of the signal to magnetic field parameters.

Findings

Heating from magnetic turbulence depends on PMF strength and spectral index.

Ambipolar diffusion's impact varies with spectral index but is subdominant for weak fields.

The 21 cm signal profile is highly sensitive to PMF parameters, enabling constraints.

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 ($30\le z\le300$), 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' $B_0$ and weakly depends on the spectral index of the initial power spectrum of the PMFs' $n_{\rm B}$. The heating function caused by ambipolar diffusion, in contrast, noticeably depends on the spectral index in the range $-3\lesssim n_{\rm B}\lesssim4$ but is subdominant in the Dark Ages epoch for PMF models with $B_0\lesssim0.5$ 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, $0.05\lesssim B_0\lesssim0.5$ nG, $-2.9\lesssim n_{\rm B}\lesssim4$, and show the essentially distinguished thermal evolution from one in the Lambda cold dark matter ($\Lambda$CDM) 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.