Excess Radiation from Axion-Photon Conversion

TL;DR

This paper proposes that axion-like particles converting into photons can simultaneously explain the excess radio background and the deep 21cm absorption feature, predicting new spectral signatures for future detection.

Contribution

It introduces a model where ALP-photon conversion accounts for both anomalies and predicts distinctive spectral features for observational verification.

Findings

Resonant ALP-photon conversion can produce sufficient photon abundance.

The model explains the radio excess between 0.4 and 10 GHz.

It predicts an additional absorption trough below 30 MHz.

Abstract

Two notable anomalies in radio observations -- the excess radiation in the Rayleigh-Jeans tail of the cosmic microwave background, revealed by ARCADE2, and the twice-deeper absorption trough of the global 21cm line, identified by EDGES -- remain unresolved. These phenomena may have a shared origin, as the enhancement of the 21cm absorption trough could arise from excess heating. We investigate this scenario through the framework of axion-like 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 10GHz while also enhances the 21cm absorption feature at 78MHz. Furthermore, it predicts a novel power-law scaling of the radio spectrum above 0.5GHz and an additional absorption trough below 30MHz, which could be verified through cross-detection in upcoming experiments.