Chapter 15: The High-frequency Domain

Since well above the Hz the potential signals coming from the concordance paradigm remain very small and gradually disappear, the detection of relic gravitons for frequencies larger than the kHz would suggest that either the post-inflationary expansion history is not exactly the one of the concordance paradigm or that the early evolution does not coincide with the conventional inflationary hypothesis. Both conclusions are relevant for a wide range of suggestions examined in this chapter. Furthermore, there are little doubts that a careful analysis of the relic graviton spectra is the only way of pushing our potential observations in a range that is otherwise unreachable. The high-frequency domain ranges, approximately, from few Hz to 100 GHz and it encompasses not only the operating window of wide-band interferometers but also the regions potentially accessible to the electromechanical detectors. This broad domain can be partitioned into a high-frequency region (between the Hz and the MHz) eventually supplemented by an ultra-high-frequency range (between the MHz and the THz)…