  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.
