New simple and accurate quintessence approximations

Abstract

We derive new approximations for quintessence solutions that are simpler and an order of magnitude more accurate than anything available in the literature, which from an observational perspective \emph{makes numerical calculations superfluous}. For example, our tracking quintessence approximation yields $\sim 0.1\%$ maximum relative errors of $H(z)/H_0$ and $\hat{I}{\text{c◯}}_{\mathrm{m}}(z)$ for the observationally viable inverse power law scalar field potentials, and similarly for viable thawing quintessence models using two slow-roll parameters. The approximations are trivially computed from the scalar field potential and as an application we give \emph{analytic} expressions for the CPL parameters calculated from an arbitrary scalar field potential for thawing and tracking quintessence models.

36 pages, 12 figures. v2. Text improved, v3. Major re-writing, title slightly changed, matches final published version