What can we learn on the thermal history of the Universe from future CMB spectrum measures at long wavelengths?

Abstract

We analyse the implications of future observations of the CMB absolute temperature at centimeter and decimeter wavelengths, necessary to complement the accurate COBE/FIRAS data. Our analysis shows that forthcoming ground and balloon measures 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 improving the sensitivity up to ~10 times. Thus, we have studied the impact of very high quality data, such those in principle achievable with a space experiment like DIMES planned to measure the CMB absolute temperature at 0.5 - 15 cm with a sensitivity of ~0.1 mK, close to that of FIRAS. Such high quality data would improve by a factor ~50 the FIRAS results on the fractional energy exchanges associated to dissipation processes possibly occurred in a wide range of cosmic epochs, at intermediate and high redshifts (y_h > 1). The energy dissipation epoch could be also significantly constrained, while the limits on the energy possibly dissipated at late epochs can be improved by a factor ~2. In addition, such measures 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.