Upper left: tensor transfer functions for five example frequencies. The orange vertical shade indicates the horizon-crossing epoch of the corresponding frequency. Upper right: kination-amplified inflationary SGWB spectrum today, same as that indicated by the solid curves in the upper panels of Figure 1. The vertical dashed–dotted lines denote the example frequencies illustrated in the upper left panel. The gray vertical shaded areas indicate the sensitive frequency band of PTAs and that of Advanced LIGO/Virgo (R. Abbott et al. 2021), respectively. Lower left: evolution of the effective number of extra relativistic species due to the inflationary SGWB, ΔN_eff,GW. Lower right: expansion history described by the evolution of the EoS parameter. The vertical shaded areas indicate the horizon-crossing epochs of the frequencies in the upper left panel.