The phenomenological approach to modeling the dark energy
  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.
