Revisiting primordial magnetic fields through 21-cm physics: Bounds and forecasts

TL;DR

This paper investigates how primordial magnetic fields influence 21-cm signals, deriving tighter constraints on their strength and forecasting detection prospects with SKA-Low, considering effects on IGM heating and star formation.

Contribution

It provides a unified analytical framework to incorporate magnetic heating and matter power spectrum effects, setting new upper bounds on primordial magnetic fields from 21-cm observations.

Findings

Upper bound of B_0 ≲ 10^{-2} nG on magnetic field strength.

Potential to constrain PMF parameters with <10% uncertainty at SKA-Low.

Detection of magnetized 21-cm power spectrum feasible with high significance.

Abstract

Primordial magnetic fields (PMFs) may significantly influence 21-cm physics via two mechanisms: (i) magnetic heating of the intergalactic medium (IGM) through ambipolar diffusion (AD) and decaying magnetohydrodynamic turbulence (DT), (ii) impact on the star formation rate density (SFRD) through small-scale enhancement of the matter power spectrum. In this analysis, we integrate both of these effects within a unified analytical framework and use it to determine upper bounds on the parameter space of a nearly scale-invariant non-helical PMF in the light of the global 21-cm signal observed by EDGES. Our findings reveal that the joint consideration of both effects furnishes constraints of the order $B_0\lesssim\mathcal{O}(10^{-2})$ nG on the present-day magnetic field strength, which are considerably tighter compared to earlier analyses. We subsequently explore the prospects of detecting such a magnetized 21-cm power spectrum at the upcoming SKA-Low mission. For the relevant parameters of the PMF ($B_0$ and $n_{\!_{B}}$) and the excess radio background ($\xi$), SNR estimation and Fisher forecast analysis indicate that it may be possible to constrain these three parameters with relative $1\sigma$ uncertainties $\lesssim10\%$ and an associated SNR $\gtrsim10$ at SKA-Low. This also leads to possible correlations among these three parameters, thus revealing intriguing trends of interplay among the various physical processes involved.