The decoupling of scalar-modes from a linearly perturbed dust-filled Bianchi type I model

TL;DR

This paper investigates how scalar perturbations behave in a dust-filled Bianchi Type I universe, revealing that background shear degeneracy is key for decoupling these modes from others, unlike in FLRW models.

Contribution

It demonstrates the conditions under which scalar modes decouple in Bianchi Type I models, extending understanding beyond standard FLRW cosmologies.

Findings

Scalar modes decouple when background shear tensor is degenerate.

Coupling of modes differs from FLRW models due to anisotropy.

Decoupling condition is specific to shear tensor properties.

Abstract

We study linear perturbations about a dust filled Bianchi Type I model with the vorticity set to zero. In comparison to linear perturbations about FLRW models, modes of perturbations about Bianchi type I models are coupled. We find that the tensor that represents the background shear needs to be degenerate in order for the scalar-mode perturbations to decouple from the rest of the flow.