Cosmological origin of anomalous radio background

TL;DR

This paper explores a cosmological explanation for the anomalous radio background, proposing that late decays of particles into electrons produce synchrotron radiation, but faces constraints from CMB observations.

Contribution

It introduces a novel mechanism involving particle decays and magnetic fields to explain the radio excess, addressing previous observational challenges.

Findings

Decays of particles >~ 9 GeV can produce the observed radio excess.

Constraints from CMB anisotropy challenge simple decay scenarios.

Millicharged particles with dark magnetic fields offer a potential solution.

Abstract

The ARCADE 2 collaboration has reported a significant excess in the isotropic radio background, whose homogeneity cannot be reconciled with clustered sources. This suggests a cosmological origin prior to structure formation. We investigate several potential mechanisms and show that injection of relativistic electrons through late decays of a metastable particle can give rise to the observed excess radio spectrum through synchrotron emission. However, constraints from the cosmic microwave background (CMB) anisotropy, on injection of charged particles and on the primordial magnetic field, present a challenge. The simplest scenario is with a >~ 9 GeV particle decaying into e+e- at a redshift of z ~ 5, in a magnetic field of ~ 5 microgauss, which exceeds the CMB B-field constraints, unless the field was generated after decoupling. Decays into exotic millicharged particles can alleviate this tension, if they emit synchroton radiation in conjunction with a sufficiently large background magnetic field of a dark U(1)' gauge field.