Stable Large-Scale Perturbations in Interacting Dark-Energy Model

TL;DR

This paper addresses large-scale instabilities in interacting dark energy models by proposing a new covariant framework that stabilizes curvature perturbations and analyzes the growth of non-adiabatic modes.

Contribution

It introduces a covariant model for dark energy and cold dark matter interaction that avoids large-scale instabilities present in previous models.

Findings

Large-scale instabilities are eliminated in the new model.

Non-adiabatic modes grow with a power-law exponent around unity.

The model remains stable when including radiation and baryons.

Abstract

It is found that the evolutions of density perturbations on the super-Hubble scales are unstable in the model with dark-sector interaction $Q$ proportional to the energy density of cold dark matter (CDM) $\rho_m$ and constant equation of state parameter of dark energy $w_d$. In this paper, to avoid the instabilities, we suggest a new covariant model for the energy-momentum transfer between DE and CDM. Then we show that the the large-scale instabilities of curvature perturbations can be avoided in our model in the universe filled only by DE and CDM. Furthermore, by including the additional components of radiation and baryons, we calculate the dominant non-adiabatic modes in the radiation era and find that the modes grow in the power law with exponent at the order of unit.