We examine the class of barotropic fluid models of dark energy, in which the pressure is an explicit function of the density, $p=f(\rho )$. Through general physical considerations we constrain the asymptotic past and future behaviors and show that this class is equivalent to the sum of a cosmological constant and a decelerating perfect fluid, or “aether,” with $w_{\mathrm{AE}}\ge 0$. Barotropic models give substantially disjoint predictions from quintessence, except in the limit of $\Lambda \mathrm{CDM}$. They are also interesting in that they simultaneously can ameliorate the coincidence problem and yet “predict” a value of $w\approx -1$.

• Received 21 November 2008

DOI:https://doi.org/10.1103/PhysRevD.80.023008