Parametrized Post-Friedmann Framework for Interacting Dark Energy

TL;DR

This paper introduces a new parametrized post-Friedmann framework to accurately model cosmological perturbations in interacting dark energy scenarios, avoiding large-scale instabilities and ensuring physical consistency.

Contribution

The authors develop a novel framework based on the parametrized post-Friedmann approach to correctly handle dark energy pressure perturbations in interacting models.

Findings

Successfully avoids large-scale instability in dark energy models

Provides well-behaved density and metric perturbations

Applicable to all interacting dark energy models

Abstract

Dark energy might directly interact with cold dark matter. However, in such a scenario, an early-time large-scale instability occurs occasionally, which may be due to the incorrect treatment for the pressure perturbation of dark energy as a nonadiabatic fluid. To avoid this nonphysical instability, we establish a new framework to correctly calculate the cosmological perturbations in the interacting dark energy models. Inspired by the well-known parametrized post-Friedmann approach, the condition of the dark energy pressure perturbation is replaced with the relationship between the momentum density of dark energy and that of other components on large scales. By reconciling the perturbation evolutions on the large and small scales, one can complete the perturbation equations system. The large-scale instability can be successfully avoided and the well-behaved density and metric perturbations are obtained within this framework. Our test results show that this new framework works very well and is applicable to all the interacting dark energy models.