Solution
= Solution
The <constant-equation-of-state density scaling> gives
$$
\bar\rho_D\propto a^{-3(1+w)}=a^{-1}.
$$
For a spatially flat universe dominated by this component, the <Friedmann equation> gives $H^2\propto a^{-1}$. Integrating $\dot a/a\propto a^{-1/2}$ yields
$$
\boxed{a(t)\propto(t-t_*)^2,
\qquad H(t)=\frac{2}{t-t_*}}.
$$
This is accelerated expansion because $w=-2/3<-1/3$.