Constraints on the thermal history of the universe from the cosmic microwave background spectrum

We have carried out an extensive numerical analysis of distortions of the microwave background spectrum that can arise over a broad interval of cosmic times. The range of validity of existing analytic approximation formulae, holding for large and very small values of the Comptonization parameter y, has been assessed and the shape of distortions that can arise for intermediate values of y has been determined. From a comparison with existing spectral data we have derived upper limits to the amount of energy that could have been dissipated at each cosmic time. The effectiveness of future measurements in tightening such constraints is discussed.