Chapter 13: Low-frequency Branch

For the present purposes, it is useful to consider the microwave background and its anisotropies as the largest electromagnetic detector of relic gravitons. The CMB is particularly sensitive in the aHz domain where the signal of the ΛCDM scenario dominates and, for this reason, it is appropriate to start the analysis of the phenomenological implications in the lowest frequency domain. After this step the intermediate and high-frequency ranges are examined, respectively, in Chapters 14 and 15. At higher frequencies the effects associated with unconventional scenarios are more important and may even represent the leading contribution in comparison with the signal of the concordance paradigm…