Excess radiation from axion-photon conversion

Two notable anomalies in radio observations—the excess radiation in the Rayleigh-Jeans tail of the cosmic microwave background, revealed by ARCADE-2, and the twice-deeper absorption trough of the global 21-cm line, identified by EDGES—remain unresolved. These phenomena may have a shared origin, as the enhancement of the 21-cm absorption trough could arise from excess heating. We investigate this scenario through the framework of axionlike particles (ALPs), showing that the resonant conversion of ALPs into photons can produce a photon abundance sufficient to resolve both anomalies simultaneously. Our model naturally explains the observed radio excess between 0.4 and 10 GHz while also enhances the 21-cm absorption feature at 78 MHz. Furthermore, it predicts a novel power-law scaling of the radio spectrum above 0.5 GHz and an additional absorption trough below 30 MHz, which could be verified through cross-detection in upcoming experiments.