Mapping the thermal history of the Universe with the new generation of CMB spectrum space experiments

We have studied the implications of the new generation of CMB spectrum space experiments for our knowledge of the thermal history of the Universe. The combination of two experiments with the sensitivity and the frequency coverage jointly forseen for DIMES and FIRAS II will be able to greatly change our vision of the capability of the CMB spectrum information to constrain physical processes at different cosmic ages. The limits on the energy dissipations at the all cosmic times accessible to CMB spectrum investigations (z < ∼ ztherm) could be improved by about two order of magnitudes and even dissipation processes with ∆ǫ/ǫi ∼ 10 −6 could be detected and possibly accurately studied. With a joint analysis of two dissipation processes occurring at different epochs, we demonstrated that the joint performances of DIMES and FIRAS II would allow to accurately recover the two amounts of energy exchanged in the primeval plasma and to constrain quite well also the epochs of the two processes even when possible imprints from free-free distortions are taken into account. All the three distortion parameters could be accurately reconstructed in this perspective: the sensitivity at 95 per cent CL is ≃ (5 − 9) × 10 for the two values of ∆ǫ/ǫi and of ≃ 10 −7 for the free-free distortion parameter yB. These results are possible because such levels of accuracy on a so wide frequency range allow to remove the approximate degeneracy both between free-free and Bose-Einstein (BE) like distortions and between Comptonization and BE-like distortions that remain in presence of future significant improvements only at λ > ∼ 1 cm or at λ < ∼ 1 cm, respectively. The sensitivity on ∆ǫ/ǫi, mainly determined by a FIRAS II-like experiment, improves by a factor ≃ 1.5 by adding the information from a DIMES-like experiment, while the sensitivity on yB, mainly determined by a DIMES-like experiment, improves by a factor ≃ 1.3− 2.6, by adding the information from a FIRAS II-like experiment. Finally, we discussed the different signatures imprinted on the CMB spectrum by some late astrophysical and particle decay models recently proposed in the literature and possibly related to the reionization of the Universe indicated by WMAP, and compared them with the sensitivity of such classes of CMB space spectrum experiments.