Phase diagram of four-dimensional dynamical triangulations with a boundary

We report on simulations of DT simplicial gravity for manifolds with the topology of the 4-disk. We find evidence for four phases in a two-dimensional parameter space. In two of these the boundary plays no dynamical role and the geometries are equivalent to those observed earlier for the sphere S⁴. In another phase the boundary is maximal and the quantum geometry degenerates to a one-dimensional branched polymer. In contrast we provide evidence that the fourth phase is effectively three-dimensional. We find discontinuous phase transitions at all the phase boundaries. (C) 2000 Published by Elsevier Science B.V.