A Back-reaction Induced Lower Bound on the Tensor-to-Scalar Ratio

TL;DR

This paper establishes a fundamental lower limit on the tensor-to-scalar ratio in cosmological models by analyzing the back-reaction effects of scalar perturbations, impacting theories like string-inspired inflation.

Contribution

It derives a lower bound on the tensor-to-scalar ratio considering back-reaction effects, constraining models with small gravitational wave signals.

Findings

Tensor-to-scalar ratio cannot be less than approximately 10^{-8} at recombination.

Back-reaction of scalar modes induces a minimum gravitational wave background.

Higher-order effects cause logarithmic growth of r during radiation domination.

Abstract

There are large classes of inflationary models, particularly popular in the context of string theory and brane world approaches to inflation, in which the ratio of linearized tensor to scalar metric fluctuations is very small. In such models, however, gravitational waves produced by scalar modes cannot be neglected. We derive the lower bound on the tensor-to-scalar ratio by considering the back-reaction of the scalar perturbations as a source of gravitational waves. These results show that no cosmological model that is compatible with a metric scalar amplitude of $\approx 10^{-5}$ can have a ratio of the tensor to scalar power spectra less than $\approx 10^{-8}$ at recombination and that higher-order terms leads to logarithmic growth for r during radiation domination. Our lower bound also applies to non-inflationary models which produce an almost scale-invariant spectrum of coherent super-Hubble scale metric fluctuations.