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We present a consistent one-loop calculation for the inflationary tensorpower spectrum in the presence of an excited spectator scalar field using thein-in formalism. We find that the super-horizon primordial power spectrum ofthe tensor mode can be scale-invariantly enhanced or reduced by the loopeffects of a subhorizon scalar field. Our calculation also includes thescalar-induced gravitational wave spectrum classically computed in the previousliterature, which is significant only near the scales where the scalar field isamplified. The super-horizon enhancement is a higher-order effect of theinteraction Hamiltonian, which can be understood as a Bogoliubov transformationintroduced by nonlinear interactions. On the other hand, the scale-invariantreduction of the tensor power spectrum may occur due to the fourth-orderscalar-scalar-tensor-tensor coupling. This phenomenon can be understood as theevolution of an anisotropic Bianchi type-I background in the separate universeapproach. Our result suggests that large-scale measurements may indirectly testthe dramatic effects of small-scale cosmological perturbations through loopcorrections. This possibility opens a new ground in probing the small-scalephysics of the primordial Universe through gravitational wave detectors ofcosmological scales.

One-loop tensor power spectrum from an excited scalar field during inflation