[You may work in units of the speed of light, so that $c=1$.]

By considering a spherical distribution of matter with total mass $M$ and radius $R$ and an infinitesimal mass $\delta m$ located somewhere on its surface, derive the Friedmann equation describing the evolution of the scale factor $a(t)$ appearing in the relation $R(t)=R_{0} a(t) / a\left(t_{0}\right)$ for a spatially-flat FLRW spacetime.

Consider now a spatially-flat, contracting universe filled by a single component with energy density $\rho$, which evolves with time as $\rho(t)=\rho_{0}\left[a(t) / a\left(t_{0}\right)\right]^{-4}$. Solve the Friedmann equation for $a(t)$ with $a\left(t_{0}\right)=1$.