The thermal history of the plasma and high-frequency gravitons

Possible deviations from a radiation-dominated evolution, occurring prior to the synthesis of light nuclei, impacted on the spectral energy density of high-frequency gravitons. For a systematic scrutiny of this situation, the Lambda CDM paradigm must be complemented by (at least two) physical parameters describing, respectively, a threshold frequency and a slope. The supplementary frequency scale sets the lower border of a high-frequency domain where the spectral energy grows with a slope which depends, predominantly, upon the total sound speed of the plasma right after inflation. While the infrared region of the graviton energy spectrum is nearly scale invariant, the expected signals for typical frequencies larger than 0.01 nHz are hereby analyzed in a model-independent framework by requiring that the total sound speed of the post- inflationary plasma be smaller than the speed of light. Current (e. g., low-frequency) upper limits on the tensor power spectra (determined from the combined analysis of the three large-scale data sets) are shown to be compatible with a detectable signal in the frequency range of wideband interferometers. In the present context, the scrutiny of the early evolution of the sound speed of the plasma can then be mapped onto a reliable strategy of parameter extraction including not only the well-established cosmological observables but also the forthcoming data from wideband interferometers.