We consider the interaction of the decay photons of an unstable relic particle species with the microwave background radiation. The radiative decays of these particles delay recombination and serve as an energy source for the resultant plasma. Nonrelativistic Compton scattering by these electrons couples the decay photons to the microwave background, producing submillimeter distortions. If the decay products close the Universe, they must decay with a radiative branching ratio larger than 2.5×${10}^{\mathrm{-}5}$ in order to produce recently observed excess submillimeter background radiation. To be consistent with measurements of the uv background, their mass m≫114 keV and their decay redshift $Z_d$≫5200.

• Received 7 April 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.117