Dark Matter Tuning Fork: How Axions Tune the Radio Sky ✛ ✎

The ARCADE2 radiometer on a high-altitude balloon detected an excess radio emission, while the ground-based EDGES experiment found an anomalously deep trough in the hydrogen spectrum at 21 cm. Both signals do not fit the standard cosmological model and point to an unknown source of soft photons. Like a tuning fork set to the frequency of the expanding Universe, the axion resonates with the plasma of the early Universe, converting into photons and leaving a trace in the radio sky.

In the new work, axion-photon conversion is considered in a cosmological context: the authors derived the conversion probability from first principles in an expanding Universe with the Friedmann-Lemaitre-Robertson-Walker metric. Taking the plasma into account gives the photon an effective mass that depends on the electron density. Resonance occurs when this mass matches the axion mass. For an axion mass of 2×10⁻¹⁴ eV, resonance occurs at redshift z≈21.6, and for 2×10⁻¹³ eV, at z≈103.8. Simulations of stochastic primordial magnetic fields with amplitude B0 from 0.1 to 1 nG show that axion-like particles with mass ~10⁻¹⁴–10⁻¹² eV and nearly scale-invariant nano-Gauss fields explain both ARCADE2 and EDGES.

For a magnetic field amplitude B0=0.45 nG and a scale-invariant spectrum, the model reproduces the ARCADE2 radio excess: χ²_min≈48 for nB=−3 and nB=−2.75 with B0=0.44 nG and 0.58 nG, respectively, with a power-law slope β≈−2.34 (for χ²≲100, β∈[−2.6,−2.3]). To explain the EDGES trough, the contribution of axion photons to the radiation background must be ξ=5–20% for a scale-invariant spectrum and ξ=40–100% for a nearly scale-invariant one, due to heating from ambipolar diffusion and turbulent decay. Allowed parameters: axion mass 10⁻¹⁴–10⁻¹² eV, coupling constant g_φγ~5×10⁻¹³ GeV⁻¹, magnetic field B0~0.1–0.9 nG. The model is consistent with constraints on primordial magnetic fields from CMB (B0<4.4 nG) and on ΔN_eff (ΔN_eff≲0.17).

This work shows for the first time that global 21 cm observations can be sensitive to axions and primordial magnetism, opening a new window into physics beyond the Standard Model. In the coming years, experiments LEDA, PRIZM, REACH, and missions PIXIE, PRISM should refine the radio background spectrum and the 21 cm signal. A joint analysis of these data with constraints on CMB spectral distortions will test the model's predictions. The tuning fork is set: we just need to hear the clear tone.