  We present a consistent one-loop calculation for the inflationary tensor
power spectrum in the presence of an excited spectator scalar field using the
in-in formalism. We find that the super-horizon primordial power spectrum of
the tensor mode can be scale-invariantly enhanced or reduced by the loop
effects of a subhorizon scalar field. Our calculation also includes the
scalar-induced gravitational wave spectrum classically computed in the previous
literature, which is significant only near the scales where the scalar field is
amplified. The super-horizon enhancement is a higher-order effect of the
interaction Hamiltonian, which can be understood as a Bogoliubov transformation
introduced by nonlinear interactions. On the other hand, the scale-invariant
reduction of the tensor power spectrum may occur due to the fourth-order
scalar-scalar-tensor-tensor coupling. This phenomenon can be understood as the
evolution of an anisotropic Bianchi type-I background in the separate universe
approach. Our result suggests that large-scale measurements may indirectly test
the dramatic effects of small-scale cosmological perturbations through loop
corrections. This possibility opens a new ground in probing the small-scale
physics of the primordial Universe through gravitational wave detectors of
cosmological scales.
