The cosmic microwave background as a probe of particle physics

Distortions from a blackbody spectrum of the cosmic microwave background radiation are most probably due, if real, to energy input into the early universe. Astrophysical sources, involving nuclear energy release, cannot easily account for the observed 10 percent energy excess relative to a 2.74 K blackbody. Particle physics motivated sources include decaying dark matter, superconducting cosmic strings or a weakly coupled scalar field, which heated up the intergalactic medium to a temperature such that Comptonization of the microwave photons occurs. The principal alternative to Comptonization photons, from decays χ → χ'+γ of weakly interacting particles at early epochs in the universe, can be ruled out because the related process γpl → χχ' or χχ' occurs in stars and would modify standard stellar evolution beyond acceptable bounds. (orig.)