Primordial Magnetic Fields at Cosmic Dawn: 21-cm Forecasts with HERA and SKA

Abstract

Primordial magnetic fields (PMFs) can enhance the abundance of low-mass halos during Cosmic Dawn by sourcing additional small-scale matter fluctuations. This enhanced small-scale power can accelerate early galaxy formation, shifting the timing of Lyman-$\alpha$ coupling, X-ray heating, and reionization toward earlier times and imprinting correlated signatures on the global and fluctuating 21-cm signals. We extend the fast analytic framework {\tt\string zeus21} to include a physically motivated PMF contribution to the linear matter power spectrum, including radiative damping before recombination and magnetic-pressure suppression below the magnetic Jeans scale. The implementation preserves the speed and modularity of {\tt\string zeus21}, enabling efficient exploration of PMF parameter space. For $n_B=-2.9$, we quantify the impact of PMFs on early structure formation and 21-cm observables across a range of fiducial magnetic amplitudes, and forecast detectability with \textit{HERA} and \textit{SKA}. Combining 21-cm forecasts with external CMB priors, we find that upcoming experiments can probe PMFs through their impact on small-scale structure, providing constraints complementary to existing cosmological probes.