Measuring deviations from a cosmological constant: a field-space parametrization

Most parametrizations of the dark energy equation of state do not reflect realistic underlying physical models. Here we develop a relatively simple description of dark energy based on the dynamics of a scalar field which is exact in the limit that the equation of state approaches a cosmological constant, assuming some degree of smoothness of the potential. By introducing just two parameters defined in the configuration space of the field, we are able to reproduce a wide class of quintessence models. We examine the observational constraints on these models as compared to linear evolution models and show how priors in the field space translate into priors on observational parameters.