Detection of tensor-mode fluctuations at the largest cosmological scales is often regarded as a robust evidence of inflation and as the probe of the inflationary energy scale. Such a direct connection is however valid only if the gravitational waves (GWs) are originated from their own quantum fluctuations during inflation and are effectively decoupled from other energy contents. The speaker considers cases distinguished from this standard lore, where the GWs are sourced by particles produced during inflation. In the case of U(1) gauge fields as a source, their effects must respect the constraints from scalar perturbations, due to the higher-order nature of their sourcing mechanisms, which limits the amplitude of the sourced GWs. On the other hand, if an SU(2) gauge field acquires a background configuration that preserves the isotropy, its perturbations introduce the modes coupled to the GWs at the linearized level. The energy of the former, amplified during inflation, is then transferred to that of the latter through the linear interaction, resulting in the GW amplitudes substantially larger than the standard ones. In this talk, the speaker will discuss several potential constraints on the mechanism and demonstrate that detectable tensor mode signals are viable even for low energy inflation.
