Abstract
The ARCADE 2 collaboration has reported asignificant excess in the isotropic radio background, whosehomogeneity cannot be reconciled with clustered sources. Thissuggests a cosmological origin prior to structure formation. Weinvestigate several potential mechanisms and show that injection ofrelativistic electrons through late decays of a metastable particlecan give rise to the observed excess radio spectrum through synchrotron emission. However, constraints from the cosmic microwavebackground (CMB) anisotropy, on injection of charged particles and on theprimordial magnetic field, present a challenge. The simplest scenariois with a ≳9 GeV particle decaying into e+e− at a redshiftof z ∼ 5, in a magnetic field of ∼ 5μG, which exceeds theCMB B-field constraints, unless the field was generated afterdecoupling. Decays into exotic millicharged particles can alleviatethis tension, if they emit synchroton radiation in conjunction with asufficiently large background magnetic field of a dark U(1)′ gaugefield.
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