Matter-driven change of spacetime topology

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.