21-cm fluctuations from primordial magnetic fields

TL;DR

Primordial magnetic fields induce small-scale density fluctuations that accelerate early star formation and reionization, significantly impacting 21 cm signals and allowing future experiments to detect magnetic fields as weak as a picogauss.

Contribution

This study models the effects of primordial magnetic fields on 21 cm signals using semi-numerical simulations, providing new constraints and forecasts for magnetic field detection.

Findings

Enhanced small-scale structure leads to earlier reionization.

Interferometers like HERA can detect magnetic fields of order pG.

Altered 21 cm signals reflect primordial magnetic field influence.

Abstract

The fluid forces associated with primordial magnetic fields (PMFs) generate small-scale fluctuations in the primordial density field, which add to the $\mathrm{\Lambda CDM}$ linear matter power spectrum on small scales. These enhanced small-scale fluctuations lead to earlier formation of galactic halos and stars and thus affect cosmic reionization. We study the consequences of these effects on 21 cm observables using the semi-numerical code 21cmFAST v3.1.3. We find the excess small-scale structure generates strong stellar radiation backgrounds in the early Universe, resulting in altered 21 cm global signals and power spectra commensurate with earlier reionization. We restrict the allowed PMF models using the CMB optical depth to reionization. Lastly, we probe parameter degeneracies and forecast experimental sensitivities with an information matrix analysis subject to the CMB optical depth bound. Our forecasts show that interferometers like HERA are sensitive to PMFs of order $\sim \mathrm{pG}$, nearly an order of magnitude stronger than existing and next-generation experiments.