Solution
= Solution
Differentiate the scalar-field <Friedmann equation> on the expanding branch $H>0$:
$$
2H\dot H=\frac{\dot\Phi}{3M_{\mathrm{Pl}}^2}(V'+\ddot\Phi).
$$
The scalar equation of motion gives $V'+\ddot\Phi=-3H\dot\Phi$. Canceling $H$ yields
$$
\boxed{2M_{\mathrm{Pl}}^2\dot H=-\dot\Phi^2.}
$$
Thus the <Hubble parameter> cannot increase for a canonical homogeneous scalar field. The mass scale here is the <reduced Planck mass>.