Scale-invariant enhancement of gravitational waves during inflation

TL;DR

This paper calculates 1-loop quantum corrections to primordial gravitational waves caused by an excited spectator scalar field during inflation, revealing scale-invariant enhancements on super Hubble scales that could be observed via CMB and gravitational wave experiments.

Contribution

It introduces a novel calculation of scale-invariant loop corrections to gravitational waves from an excited scalar field during inflation, contrasting previous peaked corrections.

Findings

Scale-invariant loop corrections on super Hubble scales.

Amplification of primordial gravitational waves due to scalar field excitation.

Potential for probing short-distance scalar field properties with cosmological observations.

Abstract

The inflationary 1-loop tensor power spectrum from an excited spectator scalar field is calculated. Recent studies on primordial black holes suggest that the inflationary curvature perturbation may be huge on small scales. An enhanced curvature perturbation may arise from a drastic enhancement of spectator scalar field fluctuations. In this letter, using the in-in formalism, we calculate 1-loop quantum corrections to primordial gravitational waves by such an excited spectator field with a sharp peak in momentum space. We find scale-invariant loop corrections in this full quantum setup, in contrast to the sharply peaked corrections in the previously calculated scalar-induced tensor modes. Especially, on super Hubble scales, the primordial gravitational waves are also amplified, which can be understood as a Bogolyubov transformation of the vacuum due to the excited scalar field. This mechanism allows us to probe the scalar field properties on extremely short-distance scales with the current and future cosmic microwave background and gravitational wave experiments, opening a novel window for inflationary cosmology.