# Tensor to scalar ratio from single field magnetogenesis

## TL;DR

This paper derives exact evolution equations for scalar and tensor modes during inflation with gauge fields, showing the tensor-to-scalar ratio cannot be below 0.001 in single-field magnetogenesis scenarios.

## Contribution

It provides a rigorous derivation of evolution equations and establishes a lower bound on the tensor-to-scalar ratio in single-field inflationary magnetogenesis.

## Key findings

- Tensor-to-scalar ratio bounded from below by adiabatic dominance
- Ratio cannot be smaller than 0.001 in single inflaton field scenarios
- Exact evolution equations derived for scalar and tensor modes

## Abstract

The tensor to scalar ratio is affected by the evolution of the large-scale gauge fields potentially amplified during an inflationary stage of expansion. After deriving the exact evolution equations for the scalar and tensor modes of the geometry in the presence of dynamical gauge fields, it is shown that the tensor to scalar ratio is bounded from below by the dominance of the adiabatic contribution and it cannot be smaller than one thousands whenever the magnetogenesis is driven by a single inflaton field.