# Probing the interaction between dark energy and dark matter with the parametrized post-Friedmann approach

## TL;DR

This paper develops a parametrized post-Friedmann framework to analyze potential interactions between dark energy and dark matter, addressing perturbation instabilities and enabling observational testing of such couplings.

## Contribution

It introduces an effective theory based on dark energy properties to stabilize perturbations in interacting dark energy models within the PPF framework.

## Key findings

- Addresses divergence issues in dark energy perturbations
- Provides a method to explore the full parameter space of IDE models
- Enables testing of dark sector interactions with observational data

## Abstract

There possibly exists some direct, non-gravitational coupling between dark energy and dark matter. This possibility should be seriously tested by using observations, which requires us to understand such a scenario from the aspects of both expansion history and growth of structure. It is found that once calculating the perturbations in the interacting dark energy (IDE) scenario, for most cases the curvature perturbation on superhorizon scales is divergent, which is a catastrophe for the IDE cosmology. We found a solution to this issue, which is to establish an effective theory to treat the dark energy perturbations totally based on the basic facts of dark energy. This scheme generalizes the parametrized post-Friedmann framework of uncoupled dark energy and can be used to cure the instability of the IDE cosmology. The whole parameter space of IDE models can henceforward be explored by observational data. The IDE scenario can thus be tested or falsified with current and future observational data by using the PPF approach. We expect that the future highly accurate observational data would offer the certain answer to the question whether there is a direct coupling between dark energy and dark matter.