 These equations reveal three basic features of these models. First, these are dynamical models: it is hard to arrange an unchanging universe, with \(\dot{R}(t) =0\). “Ordinary” matter has positive total stress-energy density, in the sense that \(\rho_{\textit{grav}}\coloneqq \rho + 3p &gt; 0\). From (\(\ref{eq:Ray}\)), the effect of such ordinary matter is to decelerate cosmic expansion, \(\ddot{R} &lt; 0\)—gravity is a force of attraction. This is only so for ordinary matter: a positive cosmological constant, or matter with negative gravitational-energy density \(\rho_{\textit{grav}}\) leads, conversely, to accelerating expansion, \(\ddot{R} &gt; 0\). Einstein was only able to construct a static model by delicately balancing the attraction of ordinary matter with a precisely chosen value of \(\Lambda\); he unfortunately failed to notice that the solution was unstable, and overlooked the dynamical implications of his own theory.