Axions Connect Two Cosmic Radio Mysteries ✛ ✎

Original: "Axion-Photon Conversion in FLRW with Primordial Magnetic Fields: Explaining the Radio Excess"

· Setabuddin, Md Riajul Haque, Rajesh Karmakar, Supratik Pal

arXiv:2509.09472 · 2025-09-11 · arXiv non-exclusive · independent analysis · 1 min

Axions turning into radio waves explain the excess cosmic radio noise and the mysterious dip in the hydrogen signal.

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.

The mechanism is like a swing: if you push it at the right frequency, even weak efforts cause a strong oscillation. Similarly here — when the axion mass matches the effective photon mass in the hot gas of the early universe, a resonant burst of radio emission occurs.

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.

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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.

P(\eta, \Delta l) = \frac{g_{\phi\gamma}^2}{4} \int_{\eta_0}^{\eta} d\eta_1 \int_{\eta_0}^{\eta} d\eta_2 \int \frac{d^3k}{(2\pi)^3} e^{ik\cos\theta \Delta l} P_B(k) \left( 1 + \frac{iH(\eta_1)}{\omega_1} - \frac{iH(\eta_2)}{\omega_2} \right) e^{-i \int_{\eta_2}^{\eta_1} d\eta_3 [\Delta_{pl}(\eta_3) - \Delta_{\phi}(\eta_3)]}

The probability of axion-to-photon conversion depends on the magnetic field power spectrum and the difference between the effective masses of the photon and the axion.

T_b(\omega_0) = \frac{\pi^4}{15} \frac{T_0^4}{\omega_0^3} \frac{\Omega_{\phi}^0}{\Omega_{\gamma}^0} P_{\text{tot}}(\omega_0)

The present-day brightness temperature is determined by the CMB temperature, the ratio of axion and photon densities, and the total conversion probability.

Scientists

Fritz Zwicky Vera Rubin Frank Wilczek Roberto D. Peccei Helen Quinn R. Brent Tully

Concepts

axion photon coupling x ray photon axion dark matter X-ray radiation ultralight dark matter tuning brown dwarf phenomena microelectronvolt mass range alfven wave Compton effect fast radio burst magnetic radiation effects microwave photon soft x ray temperature fluctuation axion dark matter search coronal mass ejection

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Axions Connect Two Cosmic Radio Mysteries ✛ ✎