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

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.

26 pages, 4 figures, updated to match published version

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Cited by in corpus (15)

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• Exploring circular polarization in the CMB due to conventional sources of cosmic birefringence
• Detecting Magnetic Fields in Exoplanets with Spectropolarimetry of the Helium Line at 1083 nm
• Lyman-$\alpha$ Polarization Intensity Mapping
• Mean Energy Density of Photogenerated Magnetic Fields Throughout the Epoch of Reionization
• Primordial magnetic fields during the cosmic dawn in light of EDGES 21-cm signal
• Detecting primordial gravitational waves with circular polarization of the redshifted 21 cm line: I. Formalism
• A new probe of magnetic fields in the pre-reionization epoch: II. Detectability
• Magnetizing the Cosmic Web during Reionization
• Detecting primordial gravitational waves with circular polarization of the redshifted 21 cm line: II. Forecasts
• Magnetic Fields from Compensated Isocurvature Perturbations
• Magnetic fields from small-scale primordial perturbations