Quintessence

Quintessence is dynamical dark energy with negative pressure, conventionally $-1<w<-1/3$.

For $w=-2/3$, zero cosmological constant, and negative curvature, $a(t)=\sqrt{\beta }t+\gamma t^2/4$ with density and curvature parameters $\gamma ,\beta >0$.

For $w=-2/3$, a quintessence-dominated scale factor grows quadratically and $tH$ approaches two.