Back-Reaction of Gravitational Waves Revisited

TL;DR

This paper revisits the impact of gravitational waves on early universe dynamics, deriving constraints on their spectral properties by analyzing their back-reaction effects on the cosmological energy budget.

Contribution

It provides new bounds on gravitational wave spectral parameters by examining both super-Hubble and sub-Hubble mode back-reactions in early cosmology.

Findings

Sub-Hubble modes contribute radiation-like energy density.

Super-Hubble modes induce negative energy density contributions.

Constraints on tensor spectral slope n_T and tensor-to-scalar ratio r.

Abstract

We study the back-reaction of gravitational waves in early universe cosmology, focusing both on super-Hubble and sub-Hubble modes. Sub-Hubble modes lead to an effective energy density which scales as radiation. Hence, the relative contribution of such gravitational waves to the total energy density is constrained by big bang nucleosynthesis. This leads to an upper bound on the tensor spectral slope n_T which also depends on the tensor to scalar ratio r. Super-Hubble modes, on the other hand, lead to a negative contribution to the effective energy density, and to an equation of state of curvature. Demanding that the early universe is not dominated by the back-reaction leads to constraints on the gravitational wave spectral parameters which are derived.