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

TL;DR

This paper calculates the one-loop tensor power spectrum during inflation with an excited scalar field, revealing potential scale-invariant enhancements or reductions in primordial gravitational waves due to quantum effects.

Contribution

It provides the first consistent one-loop calculation of tensor spectra with an excited spectator scalar field during inflation, including both super-horizon and subhorizon effects.

Findings

Super-horizon tensor spectrum can be enhanced or reduced by scalar field loops.

Scalar-induced gravitational waves are significant near amplified scalar scales.

Large-scale measurements could indirectly probe small-scale primordial physics.

Abstract

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.