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Quintessence models based on a scalar field with an inverse power law potential display simple tracking behavior at early times, when the quintessence energy density _ is subdominant. At late times, when _ becomes comparable to the matter density ₌, the evolution of diverges from its scaling behavior. We calculate the first-order departure of from its tracker solution at low redshift. Our results for the evolution of, _, _, and w are surprisingly accurate even down to z=0. We find that w and _ are related linearly to first order, and derive a semianalytic expression for w (z) which is accurate to within a few percent. Our analytic techniques are potentially applicable to any quintessence model in which the quintessence component comes to dominate at late times.
Watson et al. (Mon,) studied this question.