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Parameterizing dark sector perturbations

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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


We will discuss our new way of approaching this problem,
looking at linearised 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
symmetry (which we take to be isotropy of the background spatial sections). Our
emphasis is on constructing a theoretically motivated toolkit which can be used
to extract meaningful information about the dark sector (such as its field
content). We find, for example, that the observational impact of very broad
classes of theories can be encoded by a very small (less than 5) number of
parameters. It is these parameters which we hope to measure with observational