  Using Monte-Carlo computer simulations, we study the impact of matter fields
on the geometry of a typical quantum universe in the CDT model of lattice
quantum gravity. The quantum universe has the size of a few Planck lengths and
the spatial topology of a three-torus. The matter fields are multi-component
scalar fields taking values in a torus with circumference $\delta$ in each
spatial direction, which acts as a new parameter in the CDT model. Changing
$\delta$, we observe a phase transition caused by the scalar field. This
discovery may have important consequences for quantum universes with
non-trivial topology, since the phase transition can change the topology to a
simply connected one.
