New probe of magnetic fields in the prereionization epoch. I. Formalism

TL;DR

This paper introduces a novel method to detect extremely weak primordial magnetic fields in the early universe by analyzing the Larmor precession effects on the 21-cm hydrogen line fluctuations during the epoch of reionization.

Contribution

It develops a formalism for measuring weak magnetic fields via hyperfine transition physics, including detailed microphysics calculations and an analytic formula for brightness temperature fluctuations.

Findings

Sensitivity to magnetic fields as weak as 10^{-19} G at redshift ~20

Method can infer magnetic field direction and lower bounds on strength

Provides a comprehensive theoretical framework for future observational tests

Abstract

We propose a method of measuring extremely weak magnetic fields in the intergalactic medium prior to and during the epoch of cosmic reionization. The method utilizes the Larmor precession of spin-polarized neutral hydrogen in the triplet state of the hyperfine transition. This precession leads to a systematic change in the brightness temperature fluctuations of the 21-cm line from the high-redshift universe, and thus the statistics of these fluctuations encode information about the magnetic field the atoms are immersed in. The method is most suited to probing fields that are coherent on large scales; in this paper, we consider a homogenous magnetic field over the scale of the 21-cm fluctuations. Due to the long lifetime of the triplet state of the 21-cm transition, this technique is naturally sensitive to extremely weak field strengths, of order $10^{-19}$ G at a reference redshift of $\sim 20$ (or $10^{-21}$ G if scaled to the present day). Therefore, this might open up the possibility of probing primordial magnetic fields just prior to reionization. If the magnetic fields are much stronger, it is still possible to use this method to infer their direction, and place a lower limit on their strength. In this paper (Paper I in a series on this effect), we perform detailed calculations of the microphysics behind this effect, and take into account all the processes that affect the hyperfine transition, including radiative decays, collisions, and optical pumping by Lyman-$\alpha$ photons. We conclude with an analytic formula for the brightness temperature of linear-regime fluctuations in the presence of a magnetic field, and discuss its limiting behavior for weak and strong fields.