Absent any indirect tests on the thermal history of the Universe prior to the
formation of light nuclear elements, it is legitimate to investigate situations
where, before nucleosyntheis, the sound speed of the plasma was larger than
c/3, at most equalling the speed of light c. In this plausible
extension of the current cosmological paradigm, hereby dubbed
Tensor-ΛCDM (i.e. TΛCDM) scenario, high-frequency gravitons are
copiously produced. Without conflicting with the bounds on the tensor to scalar
ratio stemming from the combined analysis of the three standard cosmological
data sets (i.e. cosmic microwave background anisotropies, large-scale structure
and supenovae), the spectral energy density of the relic gravitons in the
TΛCDM scenario can be potentially observable by wide-band
interferometers (in their advanced version) operating in a frequency window
which ranges between few Hz and few kHz.Comment: 15 pages, 8 figure