Non-adiabatic perturbations in decaying vacuum cosmology

TL;DR

This paper explores a cosmological model with a decaying vacuum that produces matter, analyzing how non-adiabatic perturbations evolve and impact structure formation, with findings indicating negligible vacuum perturbations on relevant scales.

Contribution

It presents a detailed analysis of non-adiabatic perturbations in a decaying vacuum cosmology, reducing the complex dynamics to a single equation and quantifying perturbation contributions.

Findings

Vacuum perturbations are negligible on structure formation scales.

Dark-energy perturbations are small compared to dark-matter perturbations.

The ratio of dark matter to dark energy densities evolves as a^{-3/2}.

Abstract

We investigate a spatially flat Friedmann-Lema\^itre-Robertson-Walker cosmology in which a decaying vacuum term causes matter production at late times. Assuming a decay proportional to the Hubble rate, the ratio of the background energy densities of dark matter and dark energy changes with the cosmic scale factor as $a^{-3/2}$. The intrinsically non-adiabatic two-component perturbation dynamics of this model is reduced to a single second-order equation. Perturbations of the vacuum term are shown to be negligible on scales that are relevant for structure formation. On larger scales, dark-energy perturbations give a somewhat higher contribution but remain always smaller than the dark-matter perturbations.