What can we learn on the thermal history of the Universe from future cosmic microwave background spectrum measurements at long wavelengths?

We analyse the implications of future observations of the cosmic microwave background (CMB) absolute temperature at centimetre and decimetre wavelengths, where both ground, balloon and space experiments are currently under way to complement the accurate COBE /FIRAS data available at lambdaless than or similar to 1 cm. Our analysis shows that forthcoming ground and balloon measurements will allow a better understanding of free-free distortions but will not be able to significantly improve the constraints already provided by the FIRAS data on the possible energy exchanges in the primeval plasma. The same holds even for observations with sensitivities up to similar to10 times better than those of forthcoming experiments. Thus, we have studied the impact of very high-quality data, such as those, in principle, achievable with a space experiment such as the Diffuse Microwave Emission Survey (DIMES ) planned to measure the CMB absolute temperature at 0.5 less than or similar tolambdasimilar to 15 cm with a sensitivity of similar to0.1 mK, close to that of FIRAS. We have demonstrated that such high-quality data would improve by a factor of similar to50 the FIRAS results on the fractional energy exchanges, Deltaepsilon/epsilon (i) , associated with dissipation processes possibly occurred in a wide range of cosmic epochs, at intermediate and high redshifts (y (h) greater than or similar to 1), and that the energy dissipation epoch could also be significantly constrained. By jointly considering two dissipation processes occurring at different epochs, we demonstrated that the sensitivity and frequency coverage of a DIMES -like experiment would allow one to accurately recover the epoch and the amount of energy possibly injected into the radiation field at early and intermediate epochs even in the presence of a possible late distortion, while the constraints on the energy possibly dissipated at late epochs can be improved by a factor of similar or equal to2. In addition, such measurements can provide an independent and very accurate cross-check of FIRAS calibration. Finally, a DIMES -like experiment will be able to provide indicative independent estimates of the baryon density: the product Omega(b) H (2)(0) can be recovered within a factor of similar to2-5 even in the case of (very small) early distortions with Deltaepsilon/epsilon (i) similar to (5-2) x 10(-6). On the other hand, for Omega(b) (H (0) /50)(2) less than or similar to 0.2, an independent baryon density determination with an accuracy at similar to per cent level, comparable to that achievable with CMB anisotropy experiments, would require an accuracy of similar to1 mK or better in the measurement of possible early distortions but up to a wavelength from similar to few x dm to similar to7 dm, according to the baryon density value.