Solution
= Solution
As $\Omega_M\uparrow1$, use $\operatorname{arsinh}z\sim z$ near zero. Consequently
$$
\boxed{t_0\longrightarrow\frac2{3H_0}.}
$$
The <flat dust-and-vacuum expansion> tends to $a(t)=(3H_0t/2)^{2/3}$, the <Einstein-de Sitter universe>.