The phenomenological approach to modeling the dark energy

TL;DR

This paper reviews the phenomenological approach to modeling dark energy, emphasizing how various models can be described using fluid parameters and discussing observational constraints and challenges.

Contribution

It provides a comprehensive overview of how to describe dark energy and modified gravity models phenomenologically, including generalizations and their physical implications.

Findings

Dark energy models can be effectively described using background and perturbation fluid quantities.

Phenomenological parameters like sound speed and anisotropic stress are linked to physical models.

Current cosmological data have limitations in constraining these phenomenological quantities.

Abstract

In this mini-review we discuss first why we should investigate cosmological models beyond LCDM. We then show how to describe dark energy or modified gravity models in a fluid language with the help of one background and two perturbation quantities. We review a range of dark energy models and study how they fit into the phenomenological framework, including generalizations like phantom crossing, sound speeds different from c and non-zero anisotropic stress, and how these effective quantities are linked to the underlying physical models. We also discuss the limits of what can be measured with cosmological data, and some challenges for the framework.