Observable gravitational waves from inflation with small field excursions

TL;DR

This paper explores how non-monotonic evolution of the slow-roll parameter epsilon in small-field inflation models can produce observable primordial gravitational waves, challenging previous bounds and predicting unique spectral features.

Contribution

It introduces a mechanism for small-field inflation to generate detectable gravitational waves by evading the Lyth bound through non-monotonic epsilon evolution.

Findings

Small-field models can produce r ≥ 0.05 with current constraints.

Non-monotonic epsilon evolution leads to enhanced small-scale power.

The model predicts scale-dependent running at CMB scales.

Abstract

The detection of primordial gravitational waves, or tensor perturbations, would be regarded as compelling evidence for inflation. The canonical measure of this is the ratio of tensor to scalar perturbations, r. For single-field slow-roll models of inflation with small field excursions, the Lyth bound dictates that if the evolution of the slow-roll parameter epsilon is monotonic, the tensor-to-scalar ratio must be below observationally detectable levels. We describe how non-monotonic evolution of epsilon can evade the Lyth bound and generate observationally large r, even with small field excursions. This has consequences for the scalar power spectrum as it necessarily predicts an enhancement in the spectrum at very small scales and significant scale-dependent running at CMB scales. This effect has not been appropriately accounted for in previous analyses. We describe a mechanism that will generically produce the required behaviour in epsilon and give an example of this mechanism arising in a well-motivated small-field model. This model can produce r\geq0.05 while satisfying all current observational constraints.