Non-adiabatic perturbations in Ricci dark energy model

TL;DR

This paper investigates the growth of non-adiabatic perturbations in Ricci dark energy models, revealing potential instabilities and their impact on cosmic microwave background observations.

Contribution

It demonstrates that non-adiabatic perturbations can grow and cause instabilities in Ricci dark energy, especially near the end of matter era, and explores conditions under which these effects are mitigated.

Findings

Non-adiabatic perturbations grow on superhorizon and subhorizon scales.

Instabilities occur when dark energy's equation of state approaches -1.

The model predicts modifications to the CMB power spectrum due to dark energy perturbations.

Abstract

We show that the non-adiabatic perturbations between Ricci dark energy and matter can grow both on superhorizon and subhorizon scales, and these non-adiabatic perturbations on subhorizon scales can lead to instability in this dark energy model. The rapidly growing non-adiabatic modes on subhorizon scales always occur when the equation of state parameter of dark energy starts to drop towards -1 near the end of matter era, except that the parameter \alpha\ of Ricci dark energy equals to 1/2. In the case where \alpha\ = 1/2, the rapidly growing non-adiabatic modes disappear when the perturbations in dark energy and matter are adiabatic initially. However, an adiabaticity between dark energy and matter perturbations at early time implies a non-adiabaticity between matter and radiation, this can influence the ordinary Sachs-Wolfe (OSW) effect. Since the amount of Ricci dark energy is not small during matter domination, the integrated Sachs-Wolfe (ISW) effect is greatly modified by density perturbations of dark energy, leading to a wrong shape of CMB power spectrum. The instability in Ricci dark energy is difficult to be alleviated if the effects of coupling between baryon and photon on dark energy perturbations are included.