Simplifying Assumptions
Dark fluid theory is not treated like a standard fluid mechanics model, because many of the fluid mechanics equations are too difficult to solve completely. A formalized fluid mechanical approach, like the generalized Chaplygin gas model, would be an ideal method for modeling this theory, but it requires too many observational data points to work properly, and there aren't enough such data points available to cosmologists yet. So a simplification step was undertaken by modeling the theory through scalar field models instead, as is done in other alternative approaches to dark energy and dark matter.
Parameters must be chosen in dark fluid theory to satisfy the conditions and constraints of Big Bang nucleosynthesis, parameterized post-Newtonian formalism, and causality.
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