From loop quantum gravity to cosmology: the two-vertex model

We study the notion of volume and its dynamics in the loop-quantum-gravity truncation known as the two-vertex model. We also show that its U(N)-symmetry reduction provides the old effective dynamics of loop quantum cosmology with an arbitrary perfect barotropic fluid content. A suitable modification of the Poisson bracket structure of the U(N)-symmetric model leads to the loop quantum gravity improved dynamics.