Gravity Waves Signatures from Anisotropic pre-Inflation

TL;DR

This paper investigates how anisotropic pre-inflationary geometries amplify certain gravitational waves, potentially leaving observable imprints on the cosmic microwave background and constraining inflation's duration.

Contribution

It demonstrates the amplification of specific long-wavelength gravitational waves during anisotropic pre-inflation, linking GW signatures to inflationary parameters and initial conditions.

Findings

Amplification of certain GW polarizations during anisotropic expansion.

Potential observability of these GW signatures in CMB anisotropies.

Constraints on inflation duration based on GW amplitude limits.

Abstract

We show that expanding or contracting Kasner universes are unstable due to the amplification of gravitational waves (GW). As an application of this general relativity effect, we consider a pre-inflationary anisotropic geometry characterized by a Kasner-like expansion, which is driven dynamically towards inflation by a scalar field. We investigate the evolution of linear metric fluctuations around this background, and calculate the amplification of the long-wavelength GW of a certain polarization during the anisotropic expansion (this effect is absent for another GW polarization, and for scalar fluctuations). These GW are superimposed to the usual tensor modes of quantum origin from inflation, and are potentially observable if the total number of inflationary e-folds exceeds the minimum required to homogenize the observable universe only by a small margin. Their contribution to the temperature anisotropy angular power spectrum decreases with the multipole l as l^(-p), where p depends on the slope of the initial GW power-spectrum. Constraints on the long-wavelength GW can be translated into limits on the total duration of inflation and the initial GW amplitude. The instability of classical GW (and zero-vacuum fluctuations of gravitons) during Kasner-like expansion (or contraction) may have other interesting applications. In particular, if GW become non-linear, they can significantly alter the geometry before the onset of inflation.