When recent observational evidence and the GR+FRW+CDM model are combined we obtain the result that the Universe is accelerating, where the acceleration is due to some not-yet-understood “dark sector”. There has been a considerable number of theoretical models constructed in an attempt to provide an “understanding” of the dark sector: dark energy and modified gravity theories. The proliferation of modified gravity and dark energy models has brought to light the need to construct a “generic” way to parameterize the dark sector.

I will discuss our way of approaching this problem: constructing equations of state for perturbations. Our approach is inspired by that taken in particle physics, where the most general modifications to the standard model are written down for a given field content that is compatible with some assumed symmetries. Our emphasis is on constructing a theoretically motivated toolkit which can be used to meaningfully transcribe from experimentally obtained observations into well defined statements about the allowed properties of the dark sector. It is key to use meaningful models of the perturbed universe when analysing data sets which are sensitive to the clustering of the dark sector.

I will present our observational constraints (using e.g., Planck CMB temperature and lensing, CFHTLenS data) on the parameters in the equations of state for perturbations.