Chapter 17: Afterword: The Penultimate Questions

Throughout this book I discussed the relic gravitons and their potential interplay with the electromagnetic spectrum. As I already said at the beginning of this script, for me the most interesting of all sources has always been the primordial (or cosmological) stochastic background of gravitational waves since its detection promises important news about the early expansion history of Universe and these informations would be completely unaccessible otherwise. In the observed Universe we have at least two powerful electromagnetic detectors of relic gravitons namely the CMB itself and the pulsar timing arrays. These cosmological detectors complement the ground-based interferometers that operate in a much higher frequency range and while the astrophysical and astronomical interest of each frequency band cannot be disputed, the most intriguing physical aspect of the relic graviton backgrounds is their wide spectrum that covers, in principle, three decades of frequencies. It will be essential, in the years to come, to undertake a synergic approach encompassing the low-frequency region (where we might test the inflationary paradigm) and ultra-high-frequency domains (associated with various potential completions of the concordance scenario). In the future we might also investigate more directly the quantum mechanical nature of the relic gravitons and this is probably the most fundamental aspect of the problem. The primordial backgrounds of relic gravitons are also indirectly sensitive to the new physics beyond the standard lore of fundamental interactions and, on a more practical ground, the detection of diffuse backgrounds of gravitational radiation might represent a unique chance to improve the current bounds on the supplementary polarizations of gravitational waves as they arise in non-Einsteinian theories of gravity. All in all the considerations developed here seem to suggest that the relic gravitons really represent one of the possible bridges connecting the microworld of the standard model of fundamental interactions with the macroworld of gravity and cosmology…