# Does the detection of primordial gravitational waves exclude low energy inflation?

## TL;DR

This paper demonstrates that detectable primordial gravitational waves can originate from low-energy inflation driven by gauge fields and axions, challenging the assumption that a high tensor-to-scalar ratio directly indicates high inflation energy scales.

## Contribution

It introduces a mechanism where low-energy inflation can produce observable gravitational waves via gauge fields and axions, decoupling $r$ from inflation energy scale.

## Key findings

- Detectable tensor-to-scalar ratio ($r \,\geq\, 10^{-3}$) can occur at low inflation energy scales.
- Gravitational waves originate from gauge field fluctuations, not quantum graviton fluctuations.
- The model remains consistent with curvature perturbation and backreaction constraints.

## Abstract

We show that a detectable tensor-to-scalar ratio $(r\ge 10^{-3})$ on the CMB scale can be generated even during extremely low energy inflation which saturates the BBN bound $\rho_{\rm inf}\approx (30 {\rm MeV})^4$. The source of the gravitational waves is not quantum fluctuations of graviton but those of $SU(2)$ gauge fields, energetically supported by coupled axion fields. The curvature perturbation, the backreaction effect and the validity of perturbative treatment are carefully checked. Our result indicates that measuring $r$ alone does not immediately fix the inflationary energy scale.