Distortions of the 3 K background radiation spectrum - Observational constraints on the early thermal history of the universe

The distortions of the cosmic microwave background radiation spectrum due to heat input prior to recombination is examined analytically and also by direct numerical solution of the time-dependent Kompaneets equation. The analytic approximations are found to be very good over a large range of thermal histories that are compatible with observation. The constraints on the cosmic thermal history imposed by observations of the Rayleigh-Jeans part of the spectrum are reevaluated. Subject headings: cosmic background radiation - cosmology