The spectral energy density of the relic gravitons is illustrated the context of the ΛCDM scenario and for different values of r
(see Eqs. (1.2)-(1.3)). In the plot at the right the tensor spectral index nT does depend upon the frequency (αT follows from Eq. (1.6) with the ΛCDM choice of parameters). The plot at the left corresponds to αT = 0.