Axions Connect Two Cosmic Radio Mysteries ⚡ экспресс
The possibility of axion–photon conversion as a common source of two low-frequency anomalies—the isotropic radio excess ARCADE2 and the deep global 21-cm absorption trough EDGES—is investigated. Within the framework of axion action in the Friedmann–Lemaître–Robertson–Walker background metric with primordial magnetic fields (PMFs), a scale-dependent conversion probability is derived, accounting for plasma effects. Resonant conversion, when the axion mass equals the plasma effective photon mass, generates soft photons in the MHz–GHz range. Modeling stochastic PMFs with amplitude B₀ and spectral index n_B shows that axion-like particles with masses ~10⁻¹⁴–10⁻¹² eV and nearly scale-invariant PMFs at sub-nano-Gauss levels can explain both signals. Heating from PMF dissipation via ambipolar diffusion and turbulent decay reduces the 21-cm trough depth, shifting the allowed parameter region. The results are obtained from first principles and joint analysis of the radio excess and 21-cm signal, consistent with CMB constraints on PMFs and ΔN_eff. This opens avenues for axion searches through 21-cm cosmology.
In 2006, the ARCADE-2 instrument on a balloon detected extra radio noise in space. Later, the EDGES antenna showed that the signal from ancient hydrogen has an unexpectedly weak dip. Two independent mysteries have found a common explanation: the culprit is the axion—an ultralight dark matter particle that can turn into radio waves.
This burst explains the excess ARCADE-2 signal, and the released heat warms the hydrogen, making its radio signal appear weaker (EDGES). Thus, one particle solves two observational problems at once.
A surprising twist: such axion conversions happen constantly, and a stream of axion radio waves is passing through Earth right now. We don't hear them only because they get lost in the noise from our own instruments. This discovery gives astronomers a new method to search for dark matter—through spectral analysis of ancient hydrogen. Once, Vera Rubin first noticed hidden mass in galaxies, and Arno Penzias accidentally stumbled upon cosmic radio noise. Now, axions turn that noise from interference into a key to the universe's secrets.
🎯 The 21-centimeter hydrogen line is the most famous radio signal from space, which has allowed us to map our Galaxy and peer into the era of the first stars.