Spectral distortions to the cosmic background radiation due to massive unstable relic particles which decayed during the thermalization epoch are calculated. We impose stringent constraints on such particles from recent observations. Early energy injection of this type generates characteristic spectral distortions which could potentially serve to measure the baryon density of the Universe.

• Received 14 December 1992

DOI:https://doi.org/10.1103/PhysRevLett.70.2661